RU2800449C1 - Integrated method of profitable and industrial utilization of calcium sulfate with obtaining rare earth elements in the production of phosphoric acid - Google Patents

Abstract

FIELD: chemical industry.

SUBSTANCE: method of the production of sulfuric acid and cement clinker and the production of rare earth elements using calcium sulfate formed and separated as a solid by-product in the production of phosphoric acid by reacting phosphate raw materials with sulfuric acid to form phosphoric acid is proposed. The calcium sulfate separated from the phosphoric acid is treated with an inorganic acid to obtain a slurry containing purified calcium sulfate. Then the purified calcium sulfate in solid form is separated from the liquid phase of the resulting suspension and treated with water or an aqueous solution containing a salt and/or a chelate ligand to leach rare earth elements (REE) from calcium sulfate. The additionally purified calcium sulfate is separated in solid form from the liquid phase of the slurry with one or more REE present in the liquid phase. Purified calcium sulfate is mixed with additives, which are raw materials or compounds, including one or more oxides selected from oxides of Si, Al, Fe, and optionally Ca, or their precursors, and reducing agents, which are carbon and/or hydrocarbons, and/or elemental sulfur to obtain raw meal for the production of cement clinker. The raw meal is calcined to produce cement clinker, producing sulfur dioxide as an off-gas, which is fed as a raw material to the sulfuric acid plant to produce sulfuric acid.

EFFECT: invention makes it possible to reduce the impact of deposits and/or burials of phosphogypsum on the environment, providing its complex processing and extraction of REE with high yield and purity.

12 cl, 3 dwg, 12 ex

Description

The invention relates to an integrated method for the production of sulfuric acid and cement clinker, and for the production of rare earth elements using calcium sulfate formed during the production of phosphoric acid, and to a plant for carrying out this process.

Calcium sulfate is a waste product of phosphoric acid production and is obtained in the form of a dihydrate and/or hemihydrate when phosphate ores are exposed with sulfuric acid. Calcium sulfate, which is formed during the production of phosphoric acid, is also called phosphogypsum.

Due to the presence of impurities such as phosphorus, fluorine, radioactive components and heavy metals in calcium sulfate, which is formed during the production of phosphoric acid, the possibility of using calcium sulfate, for example, as a cement additive, fertilizer, mortar or road construction material in are currently limited and for this reason only 15% of the world's annual production of calcium sulfate from the production of phosphoric acid is used. The remaining 85% of the annual production of calcium sulfate is either stockpiled or dumped into the ocean.

Due to the current environmental situation, phosphogypsum dump storage facilities or phosphogypsum dumping in the ocean are either only permitted under stricter regulations or not permitted at all.

The quality of phosphogypsum achieved during the production of phosphoric acid depends on such factors as the phosphate raw material used and the process conditions for opening the phosphate raw material with sulfuric acid.

The processed phosphate raw material can be used for the production of phosphogypsum by the following processing operations using possible methods for the production of phosphoric acid: 1. calcination, 2. flotation, 3. processing into monocalcium phosphate or dicalcium phosphate using inorganic acids. These processes are known to those skilled in the art and are described, for example, in Physical and thermal treatment of phosphate ores - An overview; Int. J. Miner. process. 85 (2008) 59-84; Abdel-Zaher M. Abouzeid; The Fertilizer Manual 3rd Edition; published by UNIDO & IFDC, 1998 edition; ISBN: 0792350324 9780792350323; and Pyroprocessing for the minerals industry; Thyssenkrupp Polysius; identifier: 1625/D (1.0 12.11 Stu) Printed in Germany.

For every metric ton of phosphoric acid produced, 4-5 tons of calcium sulfate is produced according to the production method. Due to the naturally varying quality/composition of the phosphate ores and the different technological parameters for the production of phosphoric acid, calcium sulphates of varying quality are also produced, respectively.

Increased handling requirements for the resulting calcium sulfate or phosphogypsum create difficult problems for the production of phosphoric acid. The estimate for indefinite storage of phosphogypsum for manufacturing companies reaches $25 per ton of gypsum.

One example of a possible commercial and industrial use of calcium sulfate/phosphogypsum that has been discussed many times in the literature is the conversion of phosphogypsum to cement clinker or cement and SO ₂ or sulfuric acid. The conversion of gypsum and phosphogypsum to cement and sulfuric acid by the Müller-Kuhne or OSW-Krupp process has long been known; see, for example, patent specification AT 284701 B or AT 292539 B.

DE 32 22 721 C2 describes an improved process based on the Müller-Kühne process, resulting from the primary partial dehydration of gypsum by centrifugation followed by a two-stage drying, in which the SO ₂ -containing off-gas from the first drying stage is introduced directly into the scrubber of a sulfuric acid plant. The method described does not report anything about the quality of the phosphogypsum with regard to specific components, nor about the quality of the off-gas SO ₂ for said downstream sulfuric acid plant.

In Kandil et al., Journal of Radiation Research and Applied Sciences; 10; 2017, a process for the production of ammonium sulfate from phosphogypsum is described, in which sulfuric acid is used in the first stage to leach P ₂ O ₅ , F, lanthanides and other components. The described process is not related to the conversion of phosphogypsum into cement clinker and sulfuric acid.

EP 0041761 A1, in connection with the conversion of phosphogypsum to cement clinker and gaseous SO ₂ , refers exclusively to the recrystallization of calcium sulfate dihydrate to calcium sulfate anhydrite. You can achieve the content of R ₂ About ₅ less than 0.01% of the mass. and a fluorine content of less than 0.05 wt%. in calcium sulfate anhydrite. The calcium and sulfur dioxide components can be obtained at high temperatures from the anhydrite thus obtained. The possibility of reusing spent sulfuric acid in the process of obtaining phosphoric acid is mentioned, but such influencing factors as the size of gypsum grains and the ratio of solid and liquid phases (S/L), which directly affect the water consumption in the phosphoric acid complex, are not taken into account.

GB 120991 describes the production of cement by the Müller-Kuhne process based on calcium sulfate hemihydrate, which is obtained by recrystallization of calcium sulfate dihydrate in the production of phosphoric acid. This type of process is now known as the DHH (dihydrate-hemihydrate) process. Additionally, the production of sulfuric acid and Portland cement based on hemihydrate is described. There is no description of the treatment of calcium sulfate with acid after separation from phosphoric acid.

US 3547581 relates to a method for processing filtered gypsum from the production of phosphoric acid, in which the gypsum, after adding silicon dioxide and sulfuric acid, is heated to temperatures of 200-400°C and calcined to hemihydrate or anhydrite. The resulting product is washed with water, which reduces the content of phosphates and fluorides in gypsum. Due to the high temperatures required, this is an energy-intensive process that involves several process steps. In addition, the method according to US 3547581 requires heating and the addition of silicon dioxide in comparison with the process concept of the invention presented here. There is no description of the preparation of a suspension by adding acid to calcium sulfate.

DE 1671215 relates to a process for the production of sulfuric acid and cement from waste calcium sulfate such as phosphogypsum, which discusses the production of raw meal.

DE 1912183 describes a process for the production of cement clinker with a low fluorine content from phosphogypsum, in which the fluorine content of phosphogypsum is selectively reduced by heating gypsum waste in a finely divided state to a temperature of 500-900°C.

US 4,415,543 describes a process in which the fluoride and phosphate content of phosphogypsum is reduced by washing with water followed by heating.

In general, the relevant individual processes are simply described in the literature, such as the processing of phosphogypsum, the thermal decomposition of phosphogypsum to cement clinker and sulfur dioxide. There are only sporadic references to possible links to upstream and/or downstream processes.

In addition, it was found that significant amounts of rare earth elements are present in calcium sulfate.

Rare earth elements (usually abbreviated as R3) is understood as a collective term for the elements: scandium (atomic number 21), yttrium (39) and lutetium (71), as well as fourteen lanthanides: lanthanum (57), cerium (58), praseodymium (59) , neodymium (60), promethium (61), samarium (62), europium (63), gadolinium (64), terbium (65), dysprosium (66), holmium (67), erbium (68), thulium (69) and ytterbium (70), formerly known as the lanthanides. Rare earth elements - scandium, yttrium and lutetium are in the 3rd group of the Periodic table and belong to the transition metals. Lanthanides are the group name for lanthanum and the 13 rare earth metals that follow lanthanum with atomic numbers 58 to 70.

Rare earth elements are used in many key technologies. Europium was required, for example, in cathode ray tubes and plasma screens for the red component in the RGB color space. Neodymium is used, among other things, in an alloy with iron and boron to produce permanent magnets, and these neodymium magnets are used in permanently excited electric motors, wind turbine generators, as well as electric motors such as hybrid power plants for cars. Lanthanum is required, for example, for alloys in batteries. Rare earth metals are also used in glazes, in the manufacture of special eyeglasses, and in medical radiological diagnostics as an additive to the contrast agent in MRI examinations.

In the context of further processing of the obtained calcium sulfate and considering the importance of rare earth elements, there is a need to extract rare earth elements from calcium sulfate.

The recovery of rare earth elements from calcium sulfate obtained in the production of phosphoric acid is known in principle.

For example, CN 101186284 B describes a process for extracting rare earth elements from phosphogypsum in a phosphoric acid plant. It involves leaching phosphogypsum from a phosphoric acid plant with 5-30% sulfuric acid (corresponding to ~0.5-3.7 M H ₂ SO ₄ ) and at a temperature of 20-90°C. The sulfuric acid solution is then mixed with potassium, sodium or ammonium ions and an "inorganic acid salt" to produce sparingly soluble rare earth double salts, which can then be filtered and further processed.

CN 101597687 A describes a process in which phosphogypsum from a phosphoric acid plant is leached with sulfuric acid. After filtration, the filtrate is concentrated to 35-40% (~4.5-5 M) by additional addition of concentrated sulfuric acid. The resulting lanthanide-containing solution is then mixed with about a 100-fold excess of CaCl ₂ to bring the lanthanides into readily soluble chlorides into solution or leave them in solution.

EP 2455502 A2 describes a combined method consisting of the recovery of rare earth elements with a mixture of sulfuric and nitric acids while using ultrasound. The suspension of phosphogypsum and acid solution is then filtered, and the rare earth elements are recovered using ion exchangers.

In the method described in RU 2487834, phosphogypsum is mixed with dilute sulfuric acid and stirred at low temperatures. After filtration, the resulting filtrate is mixed with oxalic acid and the pH is adjusted to 0.9-2.5 using an ammonia solution. This precipitates the lanthanide oxalates as a salable product.

In the method described in RU 2610186, a mixture of phosphoric acid (25%) and sulfuric acid (15%) is mixed with phosphogypsum and stirred. After 1 hour, the suspension is filtered and wet phosphogypsum is mixed with water, stirred again for 15 minutes. and filter. The filtrates are then passed together through a combination of cation and anion exchangers.

US Pat. No. 4,312,842 A describes a process for the production of phosphoric acid, including isolation of by-products.

EP 2455502 A2 describes a process for extracting rare earth elements from phosphogypsum.

WPI Database, Week 201002, Thomson Scientific, London, GB: AN 2009-S54348 XP002802306. CN 101597687 A, 09/12/2009, discloses a method for converting rare earth sulfates from phosphogypsum.

US 9631258 B2 discloses a process for the extraction of rare earth elements from phosphogypsum hemihydrate.

These known methods have the disadvantage that the rare earth elements are leached directly in the first stage, with the yield of lanthanides being described as moderate or poor after leaching with sulfuric acid or sulfuric acid-nitric acid mixtures.

In contrast to known methods, in the present method rare earth elements are first converted into sparingly soluble salts in the first stage, removed in solid form, and only leached in the second stage.

One of the objectives of the present invention is to provide a method for using calcium sulfate (phosphogypsum) generated during the production of phosphoric acid to produce valuable products that can be used on an industrial scale to meet environmental and economic requirements.

Another object of the present invention is to provide a method for producing rare earth elements from calcium sulfate (phosphogypsum) formed during the production of phosphoric acid, which is characterized by the maximum yield of rare earth elements with maximum purity.

This goal is achieved in accordance with the invention using the method stated in paragraph 1 of the claims. More specifically, the object is achieved by a method for producing sulfuric acid and cement clinker and producing rare earth elements using calcium sulfate, which is formed and separated as a solid by-product in the production of phosphoric acid by reacting phosphate raw materials with sulfuric acid to form phosphoric acid, in which the method includes the following steps:

a) calcium sulfate separated from phosphoric acid is treated with acid to obtain a suspension containing purified calcium sulfate,

b) separating the purified calcium sulfate in solid form from the liquid phase of the resulting suspension,

b1) the purified calcium sulfate obtained in step b) is treated with water or an aqueous solution containing a salt and/or a chelating ligand to leach the rare earth elements from the calcium sulfate,

b2) the further purified calcium sulfate is separated in solid form from the liquid phase of the suspension obtained in step b1), with one or more rare earth elements present in the liquid phase,

c) the isolated purified calcium sulfate is mixed with additives and reducing agents to obtain raw meal for the production of cement clinker,

d) the raw meal is calcined to produce cement clinker with the formation of sulfur dioxide as off-gas,

e) the resulting sulfur dioxide is fed as a raw material to the production of sulfuric acid to produce sulfuric acid, while the resulting sulfuric acid can be used as a starting material in the production of phosphoric acid and / or sent to another use.

The method of the invention is in particular an integrated process. An integrated approach, as a general concept from phosphogypsum to cement clinker and sulfur dioxide in an integrated process, allows you to balance the dependencies / influencing factors for optimal process selection, combined with the appropriate flexibility of the technological regime with the possibility of incorporation into existing complexes of plants for the production of phosphoric and sulfuric acid. Thus, it is possible to achieve a mutual balance in all technological parameters and technological stages of the overall complex to achieve a specific product quality, based on the variable characteristics of the reagents, for example, the characteristics of the composition of apatite used in the production of phosphoric acid, or phosphogypsum of current production or from stock. So far, this approach has not been considered. The combination of features of the complex method of the invention cannot be obtained from the prior art.

In this regard, the expressions "complex" and "installation" are used interchangeably in the following. The separated liquid phase obtained in step b) can be introduced as raw material into a reaction plant for the production of phosphoric acid.

The specific advantages of the method according to the invention, especially the integrated method, may include the following points:

1. Using the resulting calcium sulfate / phosphogypsum as a raw material for the production of valuable products.

2. Converting calcium sulfate/phosphogypsum into cement clinker as raw material for cement production and sulfur dioxide as raw material for sulfuric acid production. 3. Reducing the impact of sedimentation and / or burial of calcium sulfate / phosphogypsum on the environment.

4. Recovery and circulation of sulfuric acid used in the production of phosphoric acid, and/or its further use, simply causes the need for additional sulfur to compensate for losses in the overall process. In the latter case, it is necessary to export sulfuric acid, since the supply of phosphogypsum from external sources, especially from stocks, leads to an excess of sulfur in the overall process. In the latter case, it is necessary to export sulfuric acid, since the supply of phosphogypsum from external sources, especially from warehouses, leads to an excess of sulfur in the overall process. Therefore, it is necessary to balance the balance of sulfur, especially with regard to the circulation of sulfuric acid.

5. Increasing the yield of phosphorus from phosphate raw materials and, consequently, more efficient use of phosphate raw materials used for the production of phosphoric acid.

6. Reducing _CO2 emissions by using calcium sulfate/phosphogypsum as a raw material for cement clinker production.

7. Comprehensive use of the existing units of the complex for the production of phosphoric and sulfuric acids and, consequently, reduction of additional capital costs.

The integrated process approach described in the application can be summarized in 6 categories of manufacturing processes:

- production of phosphogypsum of suitable quality for the cement clinker manufacturing process in steps a) and b of the process according to the invention,

- production of raw meal, especially for the cement clinker production process, in step c) of the process according to the invention,

- production of cement clinker in step d) of the process according to the invention and, optionally, further processing of cement clinker into cement,

- production of gaseous sulfur dioxide with a quality suitable for the process for producing sulfuric acid in step d) of the process according to the invention, in which case, if necessary, gaseous sulfur dioxide can be subjected to an off-gas treatment before being fed to step e),

- production of sulfuric acid in step e) of the process according to the invention,

- extraction of rare earth elements with high yield and high purity.

The method of the invention for the production of sulfuric acid and cement clinker using calcium sulfate is preferably integrated into the production of phosphoric acid. The production of phosphoric acid is a conventional wet method in which the phosphate raw material is reacted/decomposed with sulfuric acid to form phosphoric acid (decomposed phosphoric acid).

The phosphate raw materials used are sedimentary and igneous phosphate rocks, which are usually treated prior to use. Processing usually involves grinding and concentrating the phosphate content. For processing, the phosphate ore or phosphate rock, especially an apatite-containing ore, can be subjected, for example, to calcination, flotation and/or conversion to monocalcium phosphate or dicalcium phosphate with inorganic acids. The production of phosphoric acid by the wet process is well known to those skilled in the art. A general description of the method can be found, for example, in Ullmann's Encyclopedia of industrial chemistry, 5th edition, 1991, vol. A19, "Phosphoric Acid and Phosphate", p. 465-505.

The phosphoric acid plant preferably includes a plant for processing phosphate ore, a reaction plant for reacting crude phosphate with sulfuric acid, and a filtration plant for removing calcium sulfate. Typically, the phosphoric acid plant further includes a sulfuric acid plant, resulting in a phosphoric acid-sulfuric acid production complex.

For the production of cement clinker and sulfuric acid by the method according to the invention, for example, calcium sulfate or phosphogypsum can be used from the following existing processes, which are known to those skilled in the art of phosphoric acid production: 1. dihydrate process (DH), 2. hemihydrate process (HH), 3 a dihydrate-hemihydrate (DHH) process, 4. a hemihydrate-dihydrate (HDH) process, and 5. a dihydrate-hemihydrate decomposition-filtration (DA-HF) process.

For the present invention, one of the five processes listed above should be used, with methods 3 to 5 (DHH/HDH/DA-HF) recrystallizing calcium sulfate, which is not required for the method of the invention.

In the method of the invention in step a), calcium sulfate, which has been obtained, for example, by one of the five processes mentioned above, after being removed from the process for the production of phosphoric acid, is treated with an acid which increases the yield of P ₂ O ₅ .

Thus, the calcium sulfate removed from the phosphoric acid production process is preferably not subjected to any recrystallization before being fed to step a).

During the production of phosphoric acid, the reaction of phosphate raw materials with sulfuric acid to form phosphoric acid results in the formation of calcium sulfate as a solid by-product, which is separated from phosphoric acid. Separation can be carried out, for example, by centrifugation, filtration, settling or concentration, mechanical separation being preferred. The solid calcium sulfate is preferably separated from the phosphoric acid by filtration. The separation comprises or consists of separating the calcium sulfate formed from the phosphoric acid (acid product) (first separation), preferably by filtration. Depending on the method, the solid calcium sulfate (preferably a filter cake) separated in the first separation is optionally subjected to one or more washes with a liquid, in particular water, followed by removal of the liquid, preferably by filtration.

The calcium sulfate separated from the phosphoric acid used in step a) may be the calcium sulfate obtained immediately after the first separation from the phosphoric acid, since additional washing is not necessarily required. This makes it possible to reduce the filter area during the production of phosphoric acid. But the calcium sulfate separated from the phosphoric acid used in step a) can also be calcium sulfate which, after the first separation from the phosphoric acid, has been subjected to one or more washes with a liquid, preferably water, before being sent to step a).

Alternatively, calcium sulfate accumulated during the production of phosphoric acid can be used as the calcium sulfate used in step a). It may be necessary to subject the stored calcium sulfate to further pre-machining prior to use in step a). It is also possible to wash the accumulated calcium sulfate before use in step a) with one or more washes with a liquid, preferably water.

The reaction of phosphate raw materials with sulfuric acid takes place in the complex for the production of phosphoric acid in the reaction plant of the plant for the production of phosphoric acid. The resulting reaction mixture is then sent to a separation unit, preferably a filtration unit of a phosphoric acid plant, where the phosphoric acid is separated/filtered from the calcium sulfate formed in one or more separation steps. The calcium sulfate to be used in step a) is taken from the separation plant or filtration plant, preferably from the first separation stage, and/or the calcium sulfate used in step a) is the calcium sulfate stock in the production of phosphoric acid.

The separated calcium sulfate which is used in step a) is preferably calcium sulfate in the form of a dihydrate, a hemihydrate or a combination thereof. In step a) of the process according to the invention, the calcium sulfate recovered from the production of phosphoric acid is treated with acid. The acid is added to the calcium sulfate to carry out the treatment. Thus, after treatment, a suspension is obtained, including purified calcium sulfate. The resulting slurry may be, for example, a slurry or a slurry. The treatment can reduce the content of impurities in calcium sulfate, which adversely affect the subsequent cement clinker production process and cement quality, to the level required for the subsequent cement clinker production process.

For the treatment of calcium sulfate in step a), an acid is added. The acid is preferably a dilute acid. Diluted acids are acids diluted with water (hydrous acids). The acid, preferably a dilute acid, is preferably an inorganic acid, i.e. hydrochloric acid, nitric acid, sulfurous acid and/or sulfuric acid, with particular preference given to sulfurous acid and/or sulfuric acid.

In step a), the acid is preferably added in such an amount that the solid to liquid ratio (S/L ratio) of the suspension is in the range of 1/10 to 1/1, more preferably 1/5 to 1/1, more preferably 1/5 to 1/1.3, even more preferably 1/4 to 1/2. The solid to liquid ratio (S/L ratio) refers to the mass of the solid in kilograms and the volume of liquid in liters at a standard temperature of 20°C.

The concentration of the acid used here is chosen such that the concentration of the acid resulting from the treatment in step a) is, for example, in the range of 1-12 molar, preferably in the range of 3-10 molar, more preferably in the range of 5-8 molar acid, and the acid is an aqueous acid. By 1 molar acid in the description is meant 1 mol of acid per 1 liter of solution at 20°C. Since the calcium sulfate to be treated may contain water, the concentration of added acid may be higher than the concentration of acid formed after the addition. Generally, the molarity of the added acid is preferably within the ranges given above and below for the resulting acid.

It has been found that the yield of rare earth elements at these acid concentrations, especially sulfurous acid and/or sulfuric acid, is particularly favorable, since the rare earth elements in step a) form sparingly soluble salts and can be isolated in solid form. This increases the possible yield of rare earth leaching in step b2).

The acid obtained from the treatment after step a) is preferably 1-12 molar, preferably 5-8 molar sulfuric acid or 1-12 molar, preferably 5-8 molar sulfuric acid. Therefore, the added acid is preferably sulfurous acid or sulfuric acid.

The treatment in step a) is carried out, for example, at a reaction temperature in the range from ambient temperature to the boiling point of the reaction mixture, for example, at a temperature in the range of 15-100°C, preferably 30-80°C, more preferably 45-75°C .

The treatment time in step a) or the reaction time for stabilizing the impurity content of the acid is, for example, 5-120 minutes, preferably 15-100 minutes, in particular 15-100 minutes, in particular 15-90 minutes, particularly preferably 20-60 minutes.

During the treatment in step a), the slurry or reaction mixture for adjusting the impurity content is preferably set in motion, for example by circulation, agitation or gas purge.

For step a), calcium sulfate from phosphoric acid production is fed from a separation plant, preferably a filtration plant from phosphoric acid production, to a calcium sulfate reaction plant, or calcium sulfate is taken from stock in phosphoric acid production. Calcium sulfate is treated by adding an acid to it. The calcium sulfate reactor may be a simple stirring device such as a stirred vessel. In step b) of the method according to the invention, the purified calcium sulfate, after treatment in solid form, is separated from the liquid phase of the suspension obtained. The resulting slurry may be a slurry. The separation of solids or purified calcium sulfate from the liquid phase of the suspension is in particular a mechanical separation and can be carried out, for example, by centrifugation or filtration, preferably filtration.

The purified calcium sulfate obtained in step b) obtained after separation may be a dihydrate, hemihydrate, anhydrite or a mixture of at least two of these components, preference is given to calcium sulfate in the form of anhydrite. The desired calcium sulfate composition depends on the desired (cement/clinker manufacturer) and/or desired (phosphoric acid manufacturer) recovery of P ₂ O ₅ . The desired composition of calcium sulfate depends on the required and/or desired degree of release P ₂ O ₅ ; the cement/clinker manufacturer determines the desired P ₂ O ₅ release, while the phosphoric acid manufacturer determines the desired release. Acid treatment and subsequent separation from the liquid phase can especially reduce the content of fluorine and phosphorus in calcium sulfate present as impurities. The liquid phase to be removed is typically an acidic aqueous solution including, inter alia, water and P ₂ O ₅ .

According to the invention, the purified calcium sulfate isolated in stage b) contains at least 5% of the mass. anhydrite, preferably at least 30% wt., according to the invention, at least 50% of the mass. anhydrite in terms of dry calcium sulfate.

The D _v (50) of the particle size distribution of the calcium sulfate obtained in step b), especially calcium sulfate anhydrite, after purification is, for example, 0.5-100 µm, particularly preferably 1-50 µm, in particular 2-30 µm. Dv(50) is defined as the value at which 50% vol. particles have a larger diameter than the specified value.

In a preferred embodiment in step b), the liquid phase obtained after the separation of the solids is used as a feedstock for the production of phosphoric acid. The liquid phase or filtrate can be used in an existing phosphoric acid plant, especially an existing phosphoric acid-sulfuric acid plant, as a "recycled acid" that can be fed to a phosphoric acid reaction plant for reaction with the phosphate feedstock.

For step b), the slurry/slurry present in the calcium sulfate reaction unit is transferred to a calcium sulfate recovery unit in which the liquid and the resulting solids (purified calcium sulfate) can be separated from each other. The separation plant can be, for example, a filtration plant or a centrifugation plant, with a filtration plant being preferred.

The method according to the invention is preferably characterized in that the aqueous solution used for leaching the rare earth elements from calcium sulfate in step b1) comprises a salt and/or a chelate ligand at a concentration of 1-50% by weight, preferably 1-20% by weight, particularly preferably 5-10% wt.

It is also possible that the rare earth elements in step b1) are leached from the calcium sulfate by adding water.

The method of the invention is preferably characterized in that the salt used for leaching rare earth elements from phosphogypsum in step b1) comprises chloride and/or nitrate and/or in step b1) a chelating ligand is used.

The chelate ligand in the application is not limited in principle, but includes all chelate ligands that form a soluble complex with at least one of the rare earth elements.

Preferred chelating ligands include ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA) and/or citric acid and/or their respective salts.

Suitable salts are in particular the sodium, ammonium, calcium, magnesium and potassium salts with chloride, nitrate, acetate, bromide, bromate, perchlorate and iodide anions. It has also been found that in some cases it may be advantageous to use ammonium sulfate.

In this case, the nitrate can also be introduced in step b1) in the form of preferably dilute nitric acid.

The method of the invention is characterized in that the leaching time of rare earth elements from phosphogypsum in step b1) is from 20 minutes to 80 hours, preferably 1-76 hours or 2-72 hours, particularly preferably 3-70 hours and very preferably 4-68 hours, in according to the invention 5-68 hours, or preferably 5-64 hours.

The reaction mixture is preferably kept in motion in step b1) during the reaction.

The reaction mixture is preferably kept in motion in step b1) during the reaction by circulating, stirring or blowing gas.

The reaction is preferably carried out in step b1) at a temperature of 10-100°C, preferably 15-80°C, particularly preferably 20-75°C and in particular at a temperature of 20-60°C.

The reaction in step b1) can preferably be carried out even at room temperature without additional heating or cooling.

The method according to the invention is preferably characterized in that the weight ratio of solid to liquid (ratio S/L) for leaching rare earth elements from phosphogypsum in step b1) is 1:1-1:10 and preferably in the range of 1:2-1:5.

The method according to the invention is preferably characterized in that the separation of the further purified phosphogypsum in solid form from the liquid phase comprising one or more rare earth elements in step b2) comprises centrifugation, filtration, settling and/or evaporation of the liquid phase.

The process according to the invention is preferably characterized in that the rare earth elements are recovered by solvent extraction, ion exchange methods, precipitation as hydroxides, oxalates, carbonates and/or phosphates from the solution containing one or more rare earth elements obtained in step b2). Suitable ion exchange methods here particularly include cation exchange methods or a combination of a cation exchanger and an ion exchanger.

The method according to the invention is preferably characterized in that the phosphogypsum isolated in step b2) has a moisture content of 0-60% by weight, preferably 5-50% by weight. and especially preferably 10-40% of the mass. and sent for processing into raw meal in the clinker production plant.

However, process modes are also possible in which the calcium sulfate slurry or sludge is obtained in step b2) and processed further.

The purified calcium sulfate isolated in step b) can be used directly as the isolated purified and processed calcium sulfate. Alternatively, it is possible that the purified calcium sulfate obtained in step b) is subjected to one or more further purification steps, especially to remove rare earth elements and optionally to remove radioactive elements and/or heavy metals, for example Cd, Pb, Hg, before than purified calcium sulfate will be sent to stage c). Optional additional purification steps are explained below.

The purified calcium sulfate used in step c) preferably has a P ₂ O ₅ content of less than 0.5 wt%, preferably less than 0.25 wt%, more preferably less than 0.05 wt%. and the fluoride content is less than 0.5% wt., preferably less than 0.25% wt., more preferably less than 0.15% wt. and even more preferably less than 0.05% of the mass. Purified calcium sulfate having the purity level indicated in relation to P ₂ O ₅ and fluorine is preferably obtained immediately after step b). The purification step of steps a) and b) and any purification step to remove rare earth elements can also completely or partially remove unwanted heavy metals and/or radioactive elements, especially radioactive elements that are present in calcium sulfate.

In step c) of the process according to the invention, the separated purified and processed calcium sulfate is mixed with additives and reducing agents to obtain a raw meal for the production of cement clinker. Calcium sulfate is mixed in the appropriate ratio with the necessary additives for the required quality of the cement clinker. As already mentioned, the Müller-Kuhne and OSW-Krupp processes for the reaction of calcium sulfate with additives and reducing agents have long been known to produce cement clinker and SO ₂ or sulfuric acid.

Further details in this regard can be found in specialist literature, for example Ullmann's Encyclopedia of Industrial Chemicals, 5th edition, 1986, vol. A5, "Cement and Concrete", p. 489-537, or patent specification AT 292539 B. As a rule, cement clinker is obtained from a mixture of raw materials, called raw meal, including calcium oxide, silicon dioxide and oxides of aluminum and iron.

Raw meal components in step c) can be purified phosphogypsum/calcium sulfate and clinker composition additives known to those skilled in the art, which are mixed with calcium sulfate to form a calcium component entirely from purified calcium sulfate converted into raw meal. "Totally" here does not rule out the possibility of calcium being present in small amounts or as impurities in supplements. "Totally" here means that at least 95%, more preferably at least 99%, even more preferably at least 99.9% of the calcium present in the cement clinker comes from purified precipitated calcium sulfate. . As a result, the additives used do not contain or practically do not contain calcium. The additives are, in particular, Al carriers such as bauxites, clays and thinners, shales, etc., Fe carriers, such as iron ores, laterites, etc., and Si carriers, such as sands, sandstones, quartzites. , clays and thinners, shales, etc., i.e. raw materials or compounds comprising one or more oxides selected from oxides of Si, Al, Fe and optionally Ca or their precursors, and one or more reducing agents, for example, a carbon and/or hydrocarbon carrier, i.e. charcoal, coke, petroleum coke or charcoal/coke derived from renewable raw materials (biocharcoal/coke) and/or elemental sulphur.

Obviously, the type and amount of additives are chosen so that, together with calcium sulfate, they give a raw meal having a chemical composition suitable for cement clinker.

Suitable fuels are solid fuels such as charcoal, coke, petroleum coke, solid secondary fuels, etc., and liquid fuels, such as petroleum, fuel oil, liquid secondary fuels, etc. or gaseous fuels such as natural gas, biogas, hydrogen, or combinations of the above fuels.

Similarly, additives and/or mineralizers may be added to accelerate the decomposition of purified calcium sulfate and/or to improve the formation of clinker minerals.

The raw meal for the production of cement clinker is a mixture of substances, including calcium sulfate, all additives common to cement, and additives/mineralizers, including reducing agents.

For the preparation of the raw meal in step c), the purified calcium sulfate is preferably dried to the desired residual moisture content, for example in a drum dryer and/or a moving flow dryer and/or in a fluidized bed; the water content in the dried calcium sulfate is, for example, less than 22 wt. -%, preferably 12-14 wt. -%, more preferably less than 5 wt. -%, even more preferably less than 1 wt. The water content here refers exclusively to free water; any water of hydration present is ignored.

Dried calcium sulfate and all conventional additives are sent to a conventional tank and/or mixing device. Suitable examples that are used for this purpose include a longitudinal mixing bed and/or a rounded mixing bed and/or a trough tank and/or an open tank and/or conventional bins such as a tangential mixing bin, multi-section bin, conical bin or multi-chamber bunker. For example, such a mixing bed is described in detail in DE 10252585 or such types of hoppers are described in DE 10118142 or DE 10344040.

Raw materials (calcium sulfate and additives) are crushed separately or together to the fineness required for the production of clinker. This grinding can be carried out as a mill drying operation using waste heat from the process (eg, preheated gases) and/or specially supplied heat (eg, waste gases from a hot gas generator).

The grinding equipment used for grinding is equipment such as at least one vertical roller mill and/or roller press and/or agitated ball mill and/or ball mill and/or vertical roller mill and/or rod mill. mill and/or magnetic mill. Appropriate equipment is known, for example, from DE 102012111217, DE 102014108334 or DE 102017114831.

The crushed raw material is fed together or separately to one or more bins for intermediate storage and/or homogenization.

The chemical composition of the raw meal is constantly monitored before being fed into the clinker production process and, in case of deviations from the set values, is corrected by changing the dosages.

The raw meal thus pretreated is fed into the clinker plant in a controlled manner, optionally together with a reducing agent. The reducing agent contributes to the removal of SO ₂ during thermal decomposition of purified calcium sulfate. The reducing agent is crushed or ground separately to the required fineness. This grinding can be carried out as a combined drying and grinding. The reducing agent thus prepared is fed together with the pretreated raw meal into the clinker plant and/or separately at one or more points in the clinker plant.

Alternatively, the raw meal can be fed at a liquid content of 9-22% by weight of the mixture, preferably 12-14% by weight, into the agglomerate molder. These agglomerates can be dried and temporarily stored in a separate device. The raw meal thus agglomerated is then fed into the preheater and the roasting process. The agglomerates have a size of over 250 µm, preferably over 500 µm, up to a range of several mm. The forming equipment used may, for example, be a press and/or a bowl and/or a drum and/or a mechanical fluidized bed reactor and/or a forming mixer.

In step d) of the process according to the invention, the raw meal obtained in step c) is converted into cement clinker by a thermal process, especially in a cement clinker plant, with the formation of sulfur dioxide as exhaust gas. The cement clinker obtained in step d) can be used to produce cement.

In step d) the raw meal is preferably preheated to a temperature of up to 800° C. with process gases from the roasting or cooling process to reduce energy costs, which removes adhering surface moisture and at least partially calcines the additives.

With further preheating above 800°C, most of the calcium sulfate under the action of the reducing agent decomposes with the formation of SO ₂ .

The preheating and decomposition of the raw meal can be carried out, for example, in a heat exchanger, preferably a moving flow heat exchanger. The SO ₂ -containing process gas is preferably separated by a separation system downstream of the preheater and sent to a disposal process, such as a sulfuric acid plant.

The next stage is the final decomposition of raw meal and the subsequent formation of cement clinker (clinker firing); process gases from this stage are sent to the heat exchanger. This roasting of the raw flour is carried out in a kiln, preferably a rotary kiln. The firing temperature in the production of cement clinker may, for example, be in the range of 1200-1600°C, preferably at a temperature of 1200-1500°C. The firing time may, for example, be 5-60 minutes.

In a preferred embodiment, the raw meal is calcined with oxygen-enriched process air containing more than 21% vol., preferably more than 45% vol., more preferably more than 60% vol. and up to 100% vol., oxygen. Appropriate process air can easily be obtained by mixing with oxygen. This is a particularly preferred implementation of the oxygen-fuel process. Details of the oxygen-fuel process are described, for example, in EP 2449328 A1, EP 1037005 B1, WO 2019-211196 A1, WO 2019-211202 A1 or JP 2007-126328 A and the present invention refers to them.

The use of oxygen-enriched process air can increase the concentration of SO ₂ in the process gas; this facilitates the extraction of sulfuric acid from gases containing SO ₂ .

The above process air may have been used in advance to cool the calcined material. This process air can also be fed directly into the furnace. Also, the process gas may at least partially be diverted to the bypass system and/or from the moving flow reactor, then enriched with oxygen and returned to the roasting process.

The clinker exiting the kiln is preferably cooled by a suitable cooling system to a temperature below 120°C, preferably below 100°C. The cooler exhaust air may be fed at least in part to the roasting process and/or the preheating process and/or the production of raw meal and/or the drying of the calcium sulfate. The process air supplied to the cooling system may be at least partially enriched with oxygen. It would also be possible to use at least some of the colder exhaust air to generate electricity. The material cooled during the cooling process, after possible further technological operations, is sent to the tank equipment and / or to the packaging shop.

The cement clinker thus obtained can be further processed into cement. Cement clinker or cement produced by this method has a lower CO ₂ emission than conventional clinker or cement produced from carbonate rocks. The processes mentioned above, such as the use of phosphogypsum as a _CO2 neutral feedstock and the use of an oxygen-fuel process, reduce _CO2 emissions in a cement plant. In addition, the use of at least partially renewable energy to operate plant components can further reduce CO ₂ emissions.

In step e) of the process according to the invention, the sulfur dioxide formed in step d) is fed as a raw material into the production of sulfuric acid for the production of sulfuric acid, for example in a one-way or two-way process. The production of sulfuric acid based on SO ₂ is well known to those skilled in the art. Details can be found in the technical literature, for example Ullmann's Encyclopedia of Industrial Chemicals, 5th edition, 1994, vol. A25, "Sulfuric Acid and Sulfur Trioxide", p.635-705 or Handbook of Sulfuric Acid Manufacturing, by Douglas K. Louie, 2nd edition 2008, published by DKL Engineering.

The sulfuric acid obtained in step e) can be used, for example, in the production of phosphoric acid as a starting material for the reaction with phosphate raw materials. Alternatively, the resulting sulfuric acid can be sent to further or other uses as needed. The treated sulfur dioxide gas may optionally be fed to a sulfuric acid plant existing in the production of phosphoric acid or to another sulfuric acid plant.

In a preferred embodiment, the sulfur dioxide which is removed in step d) as off-gas from the cement clinker plant is sent to a sulfur dioxide off-gas treatment before being fed to the sulfuric acid plant in step e). The off-gas treatment is preferably an off-gas scrubbing operation to remove particulate matter and residual moisture from the SO ₂ gas.

In an optional embodiment of the method according to the invention, the method includes an additional step of purifying the calcium sulfate obtained after step b), which is carried out before mixing the calcium sulfate with additives and reducing agents in step c). In this additional purification step, all rare earth metals present as an impurity can be washed out of the calcium sulfate.

In an optional additional purification step to remove rare earth elements from calcium sulfate, before being fed to step c), the purified calcium sulfate obtained in step b) is preferably treated with a liquid, preferably water or an aqueous solution containing a salt and/or a chelating ligand. After treatment, the calcium sulfate is separated in solid form from the liquid phase of the suspension thus obtained. The calcium sulfate, further purified and thus separated, is then fed to step c). The liquid phase contains one or more rare earth elements in the form of dissolved ions and can be sent for further processing to recover the rare earth elements. For example, this processing operation includes the conversion of sparingly soluble sulfates of rare earth elements into soluble forms.

The method of the invention may also serve to remove radioactive elements and/or heavy metals. The purified calcium sulfate obtained after step b) under suitable adjustment of the reaction conditions (S/W ratio, reaction time, acid concentration and temperature) shows a marked recovery of radioactive elements, eg thorium and uranium. In an additional optional purification step for the recovery of rare earth elements, heavy metals (and other radioactive components), such as cadmium and lead, can be further removed by appropriate selection of the processing step. Thus, the purified calcium sulfate may have a reduced content of phosphorus and fluorine and possibly rare earths, radioactive elements and/or heavy metals before being fed to step c). Obviously, the reduced content of the corresponding element refers to the comparison with the content of the corresponding element in the calcium sulfate, which is served in stage a).

The complex approach or integrated method of the invention is particularly characterized by the following features or advantages:

- the process for the production of cement clinker or cement, taking into account the initial quality of phosphogypsum, can be carried out flexibly and in combination with the process for obtaining phosphoric acid,

- sulfuric acid obtained from the waste gas SO ₂ can be introduced into the general complex and used, as well as obtained and, for example, returned to the cycle,

- the following process steps can be combined, for example, in whole or in part to produce cement clinker/cement and sulfuric acid from calcium sulfate:

○ Chemical and thermal modification of the phosphoric acid production process to produce phosphogypsum of a certain quality suitable for the cement clinker production process.

○ Mechanical removal of phosphogypsum from the existing phosphoric acid production process, eg by centrifugation, filtration, evaporation or sedimentation/sludge sedimentation of calcium sulfate/phosphogypsum.

○ Chemical-thermal and mechanical separation of phosphorus from phosphogypsum, eg by chemical conversion, recrystallization, precipitation, flocculation.

○ Chemical-thermal and mechanical separation of fluorine from phosphogypsum, eg by chemical conversion, recrystallization, precipitation, flocculation.

○ Chemical-thermal and mechanical separation of radioactive components from phosphogypsum, for example, by chemical conversion with heating/cooling, recrystallization, precipitation, flocculation.

○ Chemical-thermal and mechanical separation of heavy metals from phosphogypsum, for example, by chemical conversion with heating/cooling, recrystallization, precipitation, flocculation.

○ Chemical-thermal and mechanical extraction of rare earth elements from phosphogypsum, for example, by chemical conversion with heating/cooling, recrystallization, precipitation, flocculation.

○ Chemical and/or thermal and/or mechanical dehydration of phosphogypsum, eg by heating/cooling, flocculation and settling, filtration.

○ Mechanical processing of dry phosphogypsum, eg by grinding, screening.

○ Mixing phosphogypsum with raw meal additives to achieve specific and marketable qualities of cement clinker/cement.

○ Thermal and mechanical separation of sulfur dioxide from raw meal during the roasting process in the production of cement clinker.

○ Thermal and chemical conversion of raw meal into cement clinker.

○ Chemical and mechanical conversion of cement clinker into commercial grade cement.

○ Dry and wet chemical cleaning of off-gases containing sulfur dioxide, eg electrofiltration, absorption.

○ Chemical and thermal modification of the sulfuric acid production process to incorporate and use the resulting sulfur dioxide in sulfuric acid plants.

○ Chemical conversion of the treated off-gas containing sulfur dioxide into commercial grade sulfuric acid, which can be reused in the phosphoric acid production process, for example in a one-way or two-way process.

Depending on the available quality of the calcium sulphate used, production processes must be chosen flexibly and coordinated with each other in terms of process selection and parameters. The quality of the calcium sulfate produced in the phosphoric acid production process depends on factors such as the phosphate feedstock used and the process conditions in the decomposition of the phosphate feedstock with sulfuric acid.

The degree of purification, especially with regard to the reduction of phosphorus and/or fluorine, calcium sulfate in step a) can be adjusted depending on the impurities present in the additives used and the indicative values for fluorine and phosphorus that must be observed for cement clinker. Approximate values for phosphorus are preferably not more than 1.0 wt. -%, more preferably not more than 0.5% of the mass. and even more preferably not more than 0.1% of the mass. P ₂ O ₅ and/or reference values for fluorine preferably do not exceed 0.5% wt., more preferably not more than 0.25% wt. and even more preferably not more than 0.1% of the mass. F. This is possible through a combined approach, as all stages of the process are considered together, taking into account their respective requirements. The degree of purification can be set by adjusting the parameters in step a), eg S/W ratio, acid concentration, temperature and treatment time.

The liquid consumption, especially the water consumption, in the production of phosphoric acid changes only slightly, if at all, in the integrated process, especially since the liquid consumption, especially the water consumption, for the purification of calcium sulfate in the process in step a) can be associated with the liquid consumption, especially water consumption in the production of phosphoric acid. The liquid flow rate or water flow rate in the production of phosphoric acid includes the liquid or water supplied to the production and the liquid or water discharged from the production. Fluid flow rate changes, especially water flow rate, have a significant impact on the efficiency of process steps in the production of phosphoric acid.

The invention also relates to a plant for the production of sulfuric acid and cement clinker using calcium sulfate, which is formed and separated as a solid by-product in the production of phosphoric acid from the reaction of phosphate raw materials with sulfuric acid to form phosphoric acid, and the plant includes the following devices:

a) a calcium sulfate reaction plant suitable for treating calcium sulfate separated from phosphoric acid with acid to form a slurry comprising purified calcium sulfate,

b) 1st calcium sulphate separation unit suitable for separating the purified calcium sulphate in solid form from the liquid phase of the resulting slurry,

b1) a rare earth recovery unit suitable for bringing the rare earths present in calcium sulfate into solution,

b2) 2nd calcium sulfate separation unit, suitable for separating purified calcium sulfate in solid form from a liquid phase containing rare earth elements,

c) a raw meal mixing plant suitable for mixing purified separated calcium sulphate with additives and reducing agents to obtain raw meal for the production of cement clinker,

d) plant for the production of cement clinker, for example, a combination of a preheater, a calciner and at least one cooling plant, which are suitable for preheating, calcining and cooling a mixture of raw meal in order to produce cement clinker with the formation of sulfur dioxide as exhaust gases, and

e) a sulfuric acid plant, which is supplied as a raw material for the production of sulfuric acid with off-gas sulfur dioxide produced in a cement clinker plant and optionally purified, where the sulfuric acid plant may be part of a phosphoric acid plant, or an external sulfuric acid plant and/or for other uses.

The installation can be used for the above method of the invention. For detailed information relating to individual devices or units, reference is made to their above details. The above process details apply accordingly to the installation.

Preferably, in the plant according to the invention, the calcium sulfate separation plant is fluidly connected, for example, by a pipeline, to the phosphoric acid production plant, so that the liquid phase obtained in the calcium sulfate separation plant can be introduced as a raw material for the production of phosphoric acid.

Preferably, in the plant according to the invention, the calcium sulfate separated in step a) can be supplied by a separation plant in the production of phosphoric acid, which separates phosphoric acid from calcium sulfate, or from a stock of calcium sulfate from the production of phosphoric acid.

The invention is described below with the help of working examples, which are explained in detail by the figures. The working examples are in no way intended to limit the scope of the claimed invention.

Fig. 1 is a process flow diagram for the utilization of calcium sulfate in the production of phosphoric acid using an integrated complex for the production of cement clinker and sulfuric acid.

Fig. 2 is a process flow diagram for the utilization of calcium sulfate in the production of phosphoric acid using an integrated complex for the production of cement clinker and sulfuric acid according to FIG. 1 further showing alternative or additional process steps.

Fig. 3 is a process flow diagram for the recovery of rare earth elements from calcium sulfate obtained from the production of phosphoric acid.

In FIG. 1 shows the process flow diagram for the production of phosphoric acid (operating complex) and the integrated process for the production of cement clinker and sulfuric acid from calcium sulfate formed during the production of phosphoric acid (integrated complex). Phosphate ore is processed in the processing plant 1 to obtain phosphate raw materials. The phosphate feed reacts in reaction unit 2 of the phosphoric acid plant with sulfuric acid coming from the sulfuric acid plant to form phosphoric acid and solid calcium sulfate as a by-product. Calcium sulfate generated during the production of phosphoric acid is separated from phosphoric acid in the filtration unit 3 of the phosphoric acid plant and sent to the reaction unit 5 to produce calcium sulfate. There, calcium sulfate is treated with acid, so that after treatment, for example, 1-12 molar acid, especially 1-12 molar sulfuric acid, is obtained. For example, 1-12 molar sulfuric acid can be added for treatment. The treatment can be carried out after the addition of the acid, for example at a temperature of 15-100° C. for 5-120 minutes, preferably while the resulting suspension is in motion, for example with stirring. This reduces the level of impurities in calcium sulfate, which adversely affect the subsequent cement clinker production process and cement quality, to the level required for the cement clinker production process. In the calcium sulfate separation unit 6, which is preferably a filtration unit, the liquid and the resulting solids are separated from each other. The liquid, especially the filtrate, can be used in an existing phosphoric and sulfuric acid plant. The treated calcium sulfate is sent to the raw meal preparation plant 7 before the cement clinker production process. Calcium sulfate in it is mixed with the necessary additives for the required quality of cement clinker in the appropriate ratio. The prepared cement clinker raw meal is used to charge the cement clinker plant 8, preferably with the raw meal preheated in a heat exchanger before being fed into the plant 8 (not shown). In the cement clinker plant 8, sulfur dioxide is separated from calcium sulfate and supplied as off-gas from the cement clinker plant to the off-gas sulfur dioxide purification plant 9 . The treated gaseous sulfur dioxide may optionally be fed to the existing plant 4 for the production of sulfuric acid. Alternatively, the treated sulfur dioxide gas may optionally be fed to a new sulfuric acid plant (see reference 13 in FIG. 2). The calcium remaining in the cement clinker plant reacts with the additives, for example to form cement clinker. The firing temperature for the production of cement clinker may, for example, be in the range of 1200-1600° C. and the firing time is 5-60 minutes. The cement clinker thus obtained is cooled and can be further processed into cement.

In FIG. 2 shows a process flow diagram for the utilization of calcium sulfate in the production of phosphoric acid using an integrated complex for the production of cement clinker and sulfuric acid according to FIG. 1 further showing alternative or additional process steps. What follows is a discussion of alternative or additional process steps; otherwise, reference is made to the explanations relating to FIG. 1. In FIG. 2 shows an alternative source of calcium sulfate used in step a). Instead of calcium sulfate from the filtration unit 3 of the phosphoric acid plant, calcium sulfate from the calcium sulfate storage 10 used in step a) can be used, which is the stored calcium sulfate from the phosphoric acid production. In addition, FIG. 2 shows an optional processing step for removing rare earth elements, which includes a reaction unit 11 for extracting rare earth metals from calcium sulfate and a calcium sulfate recovery unit 12 for separating the liquid phase from purified calcium sulfate. In FIG. 2 also shows that the sulfur dioxide obtained from the SO ₂ gas treatment 9 can be used to produce sulfuric acid in an existing sulfuric acid plant 4 and/or in a new sulfuric acid plant 13.

Fig. 3 is a process flow diagram for the recovery of rare earth elements from calcium sulfate obtained from the production of phosphoric acid. The calcium sulfate generated during the production of phosphoric acid is fed from the filtration unit 3 to the calcium sulfate reaction unit 5 of the phosphoric acid plant. There, calcium sulfate is treated with acid. This reduces the level of impurities in calcium sulfate, which adversely affect the extraction of rare earth elements, as well as the subsequent clinker production process and cement quality. In the 1st calcium sulfate separation unit 6, which is preferably a filtration unit, the liquid and the resulting solids are separated from each other. The liquid, in particular the filtrate, can be used in an existing complex for the production of calcium sulfate and sulfuric acid. In the calcium sulfate and rare earth recovery unit 14, the purified calcium sulfate is treated with water or an aqueous solution containing a salt and/or a chelating ligand to bring the rare earths into solution. In the 2nd calcium sulfate separation unit 15, the solution containing rare earth elements is separated from the remaining calcium sulfate. The calcium sulfate is preferably sent to the clinker manufacturing process.

Examples

Some examples of recovery of rare earth elements and removal of radioactive elements and heavy metals are described below. The content of rare earth elements, radioactive elements and heavy metals in gypsum samples is determined using ICP-OES on a SPECTRO ARCOS device with a radial view of the plasma manufactured by SPECTRO Analytical Instruments. To do this, in each case, about 0.4 g of the appropriate gypsum sample is subjected to microwave decomposition at 190°C (heating time 8 minutes and holding time 6 minutes) in a mixture of 10 ml of nitric acid and 3 ml of hydrochloric acid, and then diluted with water - in accordance with the expected content - and analyze. The results of the analysis are compared with a standard calibration with an almost identical matrix. The filtrates - in accordance with the expected content - are diluted with water and similarly evaluated using a standard calibration. The content of rare earth elements in gypsum and filtrate samples is indicated hereinafter in total.

Example 1

In the first stage, 75 g of PG A gypsum are mixed with 150 ml (S/L=0.5) of 8 molar sulfuric acid at 60° C. for 20 minutes using a precision glass stirrer. After the time has elapsed, the suspension is rapidly filtered and washed twice with 86.3 ml of water at room temperature. The filtrate together with the wash water (total 320.9 d) is collected and stored for further analysis. 1 g of treated gypsum, dried at 50°C for at least 24 hours, is mixed in the 2nd stage with 10 ml (S/W=0.1) of 10% NaNO ₃ solution and stirred at room temperature for 18 hours. the filtrate from step 2 is separated. Gypsum samples and filtrates are analyzed for rare earth elements using ICP-OES. The concentration of rare earth elements (in total) in the original gypsum PG A is P3=430 mg/kg; which corresponds to 0.430 mg of rare earth elements in 1 g of the original gypsum. After the 1st stage for the treated gypsum, the concentration of rare earth elements was determined = 548 mg/kg, which corresponds to 0.548 mg of rare earth elements in 1 g of treated gypsum. The increase in concentration in the treated gypsum is due to the almost complete conversion of the dihydrate to anhydrite and the associated loss of water of crystallization in the treated gypsum. In the filtrate (total amount 320.9 d) after the 1st stage, the concentration of 0.9 mg/kg was determined, which corresponds to 0.004 mg per g of gypsum. Thus, at the first stage, only 0.9% of rare earth elements enter the solution. At the 2nd stage, the total amount of rare earth elements 0.194 mg was found in the filtrate. This corresponds to a yield of 35.5%.

Example 2

The same gypsum from example 1, pre-treated in accordance with stage 1, is processed in the second stage in the following experiment. 1 g of pre-treated gypsum in the second stage is mixed with 10 ml (S/L=0.1) of 10% NH solution₄NO₃ and stirred at room temperature for 64 hours. Upon completion, the filtrate of stage 2 is separated. At the 2nd stage, the total amount of rare earth elements of 0.373 mg was found in the filtrate. This corresponds to a yield of 68.1%.

Example 3

The same gypsum from example 1, pre-treated in accordance with stage 1, is processed in the second stage in the following experiment. 1 g of pretreated gypsum in the 2nd stage is mixed with 10 ml (S/L=0.1) of 10% NH ₄ NO ₃ solution and 0.1 M nitric acid solution and stirred at room temperature for 64 hours. Upon completion, the filtrate from stage 2 is separated. At the 2nd stage, a total of 0.414 mg of rare earth elements was found in the filtrate. This corresponds to a yield of 81.3%.

Example 4

The same gypsum from example 1, pre-treated in accordance with stage 1, is processed in the second stage in the following experiment. 1 g of pre-treated gypsum in the 2nd stage is mixed with 10 ml (S/L=0.1) of 10% Na ₂ EDTA solution and stirred at room temperature for 3 hours. Upon completion, the filtrate from stage 2 is separated. At the 2nd stage, the total amount of rare earth elements of 0.135 mg was found in the filtrate. This corresponds to a yield of 24.7%.

Example 5

To check the possibility of extracting rare earth elements directly in the 2nd stage, even without pre-treatment (1st stage), 1 g of PG A gypsum is mixed directly with 10 ml (S/L=0.1) of 10% NH ₄ NO ₃ . solution and stirred at room temperature for 18 hours. Upon completion, the filtrate is separated. In total, 0.001 mg of rare earth elements were found in the filtrate. This corresponds - based on gypsum PG A - to a yield of only 0.1% and shows that the pre-treatment according to step 1 is absolutely necessary.

Example 6

In the first stage, 75 g of PG B gypsum are mixed with 150 ml (S/L=0.5) of 8 molar sulfuric acid at 60° C. for 20 minutes using a precision glass stirrer. After the time has elapsed, the suspension is rapidly filtered and washed twice with 86.3 ml of water at room temperature. The filtrate together with the wash water (total 346.7 d) is collected and stored for further analysis. 4 g of treated gypsum, dried at 50° C. for at least 24 hours, are mixed with 40 ml (S/L=0.1) of water in the 2nd stage and stirred at room temperature for 5 hours. Upon completion, the filtrate from step 2 is separated. Using ICP-OES, gypsum samples and filtrates are analyzed for the yield of rare earth elements. The concentration of rare earth elements (total parameter) in the original gypsum PG B is РЗ=5.82 g/kg, which corresponds to 5.82 mg of rare earth elements in 1 g of the original gypsum. After the 1st stage for the treated gypsum found the concentration of REE=6.26 g/kg, which corresponds to 6.26 mg of rare earth elements in 1 g of treated gypsum. The increase in concentration in the treated gypsum is due to the almost complete conversion of the dihydrate to anhydrite and the associated loss of water of crystallization in the treated gypsum. In the filtrate (total amount 346.7 d) after the 1st stage, the concentration of 69.9 mg/kg was determined, which corresponds to 0.323 mg/g of gypsum. Thus, at the first stage, only 5.5% of rare earth elements enter the solution. At the 2nd stage, the total amount of rare earth elements of 2.24 mg was found in the filtrate. This corresponds to a yield of 9%.

Example 7

The same gypsum from example 6, pre-treated in accordance with stage 1, is processed in the second stage in the following experiment. 4 g of pre-treated gypsum in the 2nd stage is mixed with 40 ml (S/L=0.1) of 10% NaCl solution and stirred at room temperature for 5 hours. Upon completion, the filtrate from stage 2 is separated. At the 2nd stage, the total amount of rare earth elements of 4.39 mg was found in the filtrate. This corresponds to a yield of 17.5%.

Example 8

The same gypsum from Example 6, pre-treated in accordance with stage 1, is processed in the second stage in the following experiment. 4 g of pre-treated gypsum in the 2nd stage is mixed with 40 ml (S/L=0.1) 5% solution of NH ₄ NO ₃ and stirred at room temperature for 5 hours. Upon completion, the filtrate from stage 2 is separated. At the 2nd stage, the total amount of rare earth elements of 4.03 mg was found in the filtrate. This corresponds to a yield of 16.1%.

Example 9

The same gypsum from Example 6, pre-treated in accordance with stage 1, is processed in the second stage in the following experiment. 4 g of pre-treated gypsum in the 2nd stage are mixed with 20 ml (S/L=0.1) of 10% NH ₄ NO ₃ solution and stirred at room temperature for 5 hours. Upon completion, the filtrate from stage 2 is separated. At the 2nd stage, a total amount of rare earth elements of 4.40 mg was found in the filtrate. This corresponds to a yield of 17.6%.

Example 10

The same gypsum from example 6, pre-treated in accordance with stage 1, is processed in the 2nd stage in the following experiment. 4 g of pre-treated gypsum in the 2nd stage is mixed with 20 ml (S/L=0.2) of 10% NH ₄ NO ₃ solution and stirred at room temperature for 16 hours. Upon completion, the filtrate from stage 2 is separated. At the 2nd stage, the total amount of rare earth elements of 7.26 mg was found in the filtrate. This corresponds to a yield of 29.0%.

Example 11

To determine the efficiency in terms of removing radioactive components in the 1st stage of the described process, the PG A gypsum pretreated in example 1 after the 1st stage, the original PG A gypsum and the collected filtrate after pretreatment, are analyzed for the content of thorium and uranium. In the original gypsum PG A, the concentrations of thorium and uranium were found to be 1.04 mg/kg and 10.0 mg/kg, respectively, which, taking into account the used 75 g of gypsum, corresponds to a total amount of 0.078 mg and 0.750 mg of thorium and uranium, respectively. After pre-treatment of the gypsum, both values were below the limit of establishment. In the filtrate (320.9 d), the concentration of 0.208 mg/kg and 1.71 mg/kg of thorium and uranium, respectively, was found, which corresponds to a total amount of 0.067 mg and 0.549 mg of thorium and uranium, respectively, or an efficiency of 85.8% and 72.8% respectively.

Example 12

To determine the removal efficiency of heavy metals such as Pb in step 2 of the described process, the PG A gypsum pre-treated according to example 2 after the 1st step, the original PG A gypsum and the filtrate after the 2nd step are analyzed for lead. In the original PG A gypsum, a lead concentration of 6.94 mg/kg was found. After pre-treatment of the gypsum, the lead content was 7.56 mg/kg, which corresponds to 7.56 µg per g of treated gypsum. The increase in concentration in the treated gypsum is due to the almost complete conversion of the dihydrate to anhydrite and the associated loss of water of crystallization in the treated gypsum. Traces of lead in the filtrate were found only after the 1st stage. In the filtrate from the 2nd stage, a lead concentration of 0.71 mg/l was found, which corresponds to a total lead of 7.1 µg or an efficiency of 94.2%.

Position number list

1 - Plant for the processing of phosphate rock or phosphate ore

2 - Phosphoric acid plant reaction plant

3 - Phosphoric Acid Plant Filtration Plant

4 - Plant for the production of sulfuric acid (operating)

5 - Calcium sulfate treatment plant

6 - 1st calcium sulfate separation unit

7 - Raw meal preparation plant

8 - Plant for the production of cement clinker

9 - Purification of sulfur dioxide off-gases

10 - Calcium sulfate from stock (from phosphoric acid production)

11 - Extraction of rare earth elements from calcium sulfate

12 - Calcium sulfate extraction unit

13 - Sulfuric acid plant (new)

14 - Plant for the extraction of calcium sulfate / rare earth elements

15 - 2nd calcium sulfate separation unit.

Claims (19)

1. A method for producing sulfuric acid and cement clinker and producing rare earth elements using calcium sulfate, which is formed and separated as a solid by-product in the production of phosphoric acid, by reacting phosphate raw materials with sulfuric acid to form phosphoric acid, the method including the following steps : a) calcium sulfate separated from phosphoric acid is treated with an inorganic acid to obtain a suspension containing purified calcium sulfate, b) separating the purified calcium sulfate in solid form from the liquid phase of the resulting suspension, b1) the purified calcium sulfate obtained in step b) is treated with water or an aqueous solution containing a salt and/or a chelating ligand to leach the rare earth elements from the calcium sulfate, b2) the further purified calcium sulfate is separated in solid form from the liquid phase of the suspension obtained in step b1), with one or more rare earth elements present in the liquid phase, c) purified calcium sulfate isolated in step b2) is mixed with additives, which are raw materials or compounds comprising one or more oxides selected from oxides of Si, Al, Fe and optionally Ca, or their precursors, and reducing agents, which are is carbon and/or hydrocarbons and/or elemental sulfur for the production of raw meal for the production of cement clinker, d) the raw meal is calcined to produce cement clinker, producing sulfur dioxide as off-gas, and e) the resulting sulfur dioxide is fed as a raw material to the production of sulfuric acid to obtain sulfuric acid, while the resulting sulfuric acid is suitable for use as a starting material in the production of phosphoric acid and / or direct it to another use, characterized by that the mass ratio of solid to liquid (ratio S/L) for the leaching of rare earth elements from phosphogypsum in step b1) is 1:1-1:10, and the separated purified calcium sulfate obtained in step b) contains at least 50% wt. anhydrite based on dry calcium sulfate and wherein the reaction time for leaching rare earth elements from phosphogypsum in step b1) is 5-68 hours. 2. The method according to claim 1, characterized in that the calcium sulfate used in step a) is calcium sulfate obtained in a separation unit in the production of phosphoric acid, preferably using solid calcium sulfate obtained after the first separation from phosphoric acid, preferably calcium sulfate filter cake, from the production of phosphoric acid directly or after one or more washes with a liquid, preferably water, and/or is calcium sulfate accumulated during the production of phosphoric acid. 3. The method according to claim 1 or 2, characterized in that in step a) the acid is added in such an amount that the mass ratio of solid to liquid (S/L ratio) in the suspension is in the range of 1/10-1/1, preferably 1/5-1/1 and/or the acid obtained from the treatment after step a) is an acid with a molar concentration of 1-12 moles, preferably 3-10 moles, more preferably 5-8 moles, and/or wherein the acid is selected from hydrochloric acid, nitric acid, sulfurous acid and/or sulfuric acid, and/or wherein the treatment in step a) is carried out at a temperature in the range of 15-100°C, preferably 30-80°C, more preferably 45 -75°C, and/or in which the duration of treatment in step a) is in the range of 5-120 minutes, preferably 15-90 minutes, more preferably 20-60 minutes. 4. The method according to any one of paragraphs. 1-3, characterized in that the D _v (50) of the particle size distribution of calcium sulfate obtained in step b), in particular calcium sulfate anhydrite, is in the range of 0.5-100 µm, preferably 1-50 µm, especially 2- 30 µm. 5 The method according to any one of paragraphs. 1-4, characterized in that the aqueous solution used for leaching rare earth elements from phosphogypsum in step b1) comprises a salt and/or a chelate ligand at a concentration of 1-50 wt%, preferably 1-20 wt%, particularly preferably 5 -10% wt. 6. The method according to any one of paragraphs. 1-5, characterized in that the salt used to leach rare earth elements from phosphogypsum in step b1) includes chloride and/or nitrate and/or chelate ligand used in step b1) includes EDTA and/or citrate. 7. The method according to any one of paragraphs. 1-6, characterized in that the leaching time of rare earth elements from phosphogypsum in step b1) is 5-68 hours, preferably 5-64 hours, preferably the reaction mixture in step b1) is kept in motion during the reaction, in particular by circulating, mixing or purging gas. 8. The method according to any one of paragraphs. 1-7, characterized in that the mass ratio of solid to liquid (ratio S/L) for leaching rare earth elements from phosphogypsum in step b1) is 1:2-1:5. 9. The method according to any one of paragraphs. 1-8, characterized in that the separation of additionally purified phosphogypsum in solid form from a liquid phase comprising one or more rare earth elements in step b2) includes centrifugation, filtration, precipitation and/or evaporation of the liquid phase. 10. The method according to any one of paragraphs. 1-9, characterized in that rare earth elements are obtained by liquid extraction, ion exchange methods, precipitation in the form of hydroxides, oxalates, phosphates and/or carbonates from a solution containing one or more rare earth elements obtained in step b2). 11. The method according to any one of paragraphs. 1-10, characterized in that the cement clinker obtained in step d) is suitable for the production of cement. 12. The method according to any one of paragraphs. 1-11, characterized in that the degree of purification of calcium sulfate in stage a) is determined taking into account the values of phosphorus and fluorine that must be observed for cement clinker, which are preferably not more than 1.0 wt. -%, more preferably not more than 0.5 % wt., even more preferably not more than 0.1% wt. R ₂ O ₅ and/or preferably not more than 0.5% wt., more preferably not more than 0.25% wt., even more preferably not more than 0.1% wt. F.