WO2014027348A1

WO2014027348A1 - Process for manufacturing phosphoric acid with lowered cadmium content

Info

Publication number: WO2014027348A1
Application number: PCT/IL2013/050686
Authority: WO
Inventors: Malka NIR; Uri Sasson COHEN; Shimon ISRALOVITZ
Original assignee: Rotem Amfert Negev Ltd
Current assignee: Rotem Amfert Negev Ltd
Priority date: 2012-08-16
Filing date: 2013-08-12
Publication date: 2014-02-20
Anticipated expiration: 2015-02-16

Description

PROCESS FOR MANUFACTURING PHOSPHORIC ACID

WITH LOWERED CADMIUM CONTENT

Field of the Invention

The present invention relates to a process of purifying phosphoric acid and providing a product with lowered levels of cadmium.

Background of the Invention

Phosphoric acid is consumed in huge amounts in chemical industry, mainly in manufacturing fertilizers, detergents, and food products. The world production is about 30 million tons per year (as P2O5), most of which employs so called wet process, in which sulfuric acid reacts with naturally occurring phosphate rocks. The main reaction of the wet process is the formation of the desired product and calcium sulfate. The side product, calcium sulfate, may be found in three polymorphs differing in the amount of crystal water, as dihydrate, hemihydrate, and anhydride; the amount of water bound in calcium sulfate is important for the course and balance of the wet process. Said dihydrate is called gypsum, but the term is often used for all calcium sulfate forms, and the same simplification is used throughout the present text. The phosphate rocks comprise calcium phosphate, less of fluorides and silicates, and many other admixtures. Said admixtures comprise cadmium, of which level in the marketed products is regulated still more strictly, especially when the product could contaminate the environment, but particularly strictly when the product is used in food industry. It is therefore an object of the invention to provide a process for manufacturing phosphoric acid with lower content of cadmium.

It is another object of this invention to provide a process for extracting phosphoric acid from phosphate rocks without an increased demand on the filtration capacity. Other objects and advantages of the present invention will appear as description proceeds.

Summary of the Invention

The present invention provides a process for manufacturing low-cadmium phosphoric acid comprising the steps of i) providing a mass of raw phosphoric acid having a concentration of between 45 and 55 % P2O5, and containing up to 50 ppm cadmium; ii) admixing to said phosphoric acid of step i) concentrated sulfuric acid to reach a concentration of between 4 and 12% SO4"² in the mixture; iii) admixing to said phosphoric acid of step i) a naturally occurring phosphate rock in an amount corresponding to between 5 and 15% of said mass; iv) reacting said rock with said sulfuric acid at a temperature of between 80 and 90°C to obtain a mixture comprising phosphoric acid and suspended particles of calcium sulfate; and v) filtering the suspension of step iv) at a temperature of at least 80°C; thereby obtaining a filtration cake of calcium sulfate with bound cadmium ions, and a product of low-cadmium phosphoric acid having a concentration of at least 46 % P2O5 and containing at most 5 ppm cadmium. In a preferred embodiment, the present invention provides a process for manufacturing a low-cadmium and low-sulfate phosphoric acid product. When relating to concentrations in the present text, the term "percent" or symbol "%" means weight percent. In any stage of the process, water can be added to adjust the concentrations to desired values. Water may be removed from the system as condensed steam. The process of the invention preferably comprises the steps of i) providing a mass of phosphoric acid having a concentration of at least 50 % P2O5, and containing up to 40 ppm cadmium; ii) admixing to said phosphoric acid of step i) concentrated sulfuric acid to reach a concentration of at least 7 % SO₄" ² in the mixture; iii) admixing to said mixture of step ii) a naturally occurring phosphate rock in an amount corresponding to at least the mass of said SO₄ in the mixture; iv) reacting said rock with said sulfuric acid at a temperature of between 80 and 90°C; and v) filtering the suspension of step iv); thereby obtaining a product of low-cadmium phosphoric acid having a concentration of at least 52 % P2O5, and containing at most 0.2 % S0₄ and at most 4 ppm cadmium. Said concentrated sulfuric acid has preferably a concentration of at least 95%. In a preferred embodiment of the invention, said step v) comprises adding water to the filtered suspension or onto the filter. The process according to the invention provides a low-cadmium product of phosphoric acid, which in addition has a phosphoric acid concentration higher than the initial raw material, and further it has less sulfates than said raw phosphoric acid. When using the term sulfates in this context, intended are all molecular forms comprising SO4"², whereas their total amount is characterized as weight percent of S0₄. The phosphoric acid concentration is characterized, as is usual, by weight percent of P2O5. The process steps may be separated in the space and time in various ways. In one embodiment, the process according to the invention is performed in more reactors. At least one of the reactors is a temperature-controlled reactor. In a preferred embodiment, said steps i) and ii) are performed in a first stirred reactor, and said steps iii) and iv) in a second stirred reactor. When employing two reactors in the instant process, said raw phosphoric acid and said concentrated sulfuric acid may be continuously fed to a first stirred reactor, the mixture from said first reactor may be continuously transferred to a second stirred reactor, and the mixture from said second reactor may be continuously transferred either to the filter or to a collection tank. In a preferred embodiment of the process according to the invention, one part of the reaction mixture from said second reactor is transferred to said filter or tank, and another part of the reaction mixture from said second reactor is recirculated back to said first reactor. Water may be added to various stages of the process, according to the concentration needs. In one embodiment of the invention, water is continually added either onto said filter or into said tank.

In one aspect, the invention provides a continuous process for manufacturing low-cadmium phosphoric acid, comprising the steps of i) providing raw phosphoric acid containing up to 40 ppm cadmium and up to 8% sulfate as S0₄"²; ii) reacting said phosphate rock in the mixture with said sulfuric acid and said raw phosphoric acid at a temperature of between 80 and 90°C; and iii) filtering the reaction mixture; thereby obtaining low-cadmium and low- sulfate phosphoric acid having a P2O5 concentration higher than said raw phosphoric acid.

The invention relates to a process for reducing cadmium amounts in phosphoric acid, simultaneously increasing the concentration of P2O5 in said phosphoric acid. Preferably, some stages of the process are performed in a continuous mode; more preferably the process is a continuous process. In a preferred embodiment, the process steps are performed at a temperature between 80 and 90°C. The product has preferably a concentration of P2O5 of 54% or more.

The invention relates to an apparatus for reducing the cadmium level in phosphoric acid, comprising i) at least one stirred and temperature-controlled reactor enabling to keep the reaction mixture between 80 and 90°C; ii) a unit for feeding raw phosphoric acid comprising at least 45 % P2O5 and cadmium up to 50 ppm; iii) a unit for feeding concentrated sulfuric acid; iv) a unit for feeding phosphate rock; v) pumps and transferring means for transporting solutions or suspensions between the units and reactors; and vi) at least one filtration unit for removing calcium sulfate from suspension and providing a filtrate of phosphoric acid having at most 5 ppm cadmium.

Brief Description of the Drawings

The above and other characteristics and advantages of the invention will be more readily apparent through the following examples, and with reference to the appended drawing, wherein: Fig. 1. is a schematic presentation of a process in accordance with one embodiment of the invention, comprising two stirred, flow-through reactors.

Detailed Description of the Invention

It has now been found that phosphoric acid containing unacceptably high amounts of cadmium, called raw acid in this text throughout, may be efficiently purified by inducing the formation of calcium sulfate in the raw acid at a temperature of between 80 and 90°C followed by filtration. The mechanism is not entirely clear, but without wishing to be bound by any theory, the inventors believe that, under certain conditions, calcium sulfate formed in the mixture binds cadmium which is removed from the solution during the filtration step. Said calcium sulfate may probably be in the anhydrous form, or partially as a hemihydrate. Said special conditions comprise certain temperature range for both the reacting step and the filtration step, a relatively high P2O5 concentration, and the presence of sulfuric acid in the reaction stage. It was observed by the inventors that, under certain conditions, cadmium is removed during the filtration, but the filtration rate was very low. It was found that the kinetics of anhydrite formation affects the structure of the formed calcium sulfate, while leading to much better filterability of the sulfate cake under certain reaction conditions. Another important factor taken onto consideration is the P2O5 concentration, as dilution of phosphoric acid during the purification would increase the process cost. The process according to the invention addresses both the problem of undesired impurities and the problem of undesired dilution. The conditions of the process enable formation of anhydrite from hemihydrate, without involving calcium sulfate dihydrate, so avoiding the excessive water release and product dilution.

The present process enables to purify phosphoric acid from various sources and of various purity levels, and containing various impurities; particularly, the instant process enables to lower unacceptably high levels of cadmium. Preferably, cadmium decreases to 5 ppm or less, preferably to 4 ppm or less, and still more preferably to 3 ppm or less, for example to 2 ppm or less. The process of cadmium removal according to the invention does not lead to the dilution of phosphoric acid, but surprisingly even provides a P2O5 higher than in the raw acid. Advantageously, the process also enables to remove excessive sulfate in the raw acid. The level of sulfates, even if present in amounts of for example 5%, is lowered to less than 1%, preferably to less than 0.5%, still preferably to at most 0.2%. The method of the invention comprises step i) of providing a mass of raw phosphoric acid having a concentration of between 45 and 55 % P2O5, preferably at last 50 %, and containing up to 50 ppm cadmium, preferably up to 40 ppm, or up to 30 ppm. Further, in step ii) concentrated sulfuric acid is admixed to said phosphoric acid of step i) to reach a concentration of between 4 and 12% SO4"² in the mixture, preferably from 6 to 10%; if the raw acid contained sulfates, such as in an amount of 4%, sulfuric acid is added to increase the SO4"² level by only about 4 or 5 %, making totally 8 or 9% SO4"²; excess of sulfuric acid increases the amounts of solids and accelerates sulfate dehydratation, but may lower the concentration of P2O5. In step iii), a naturally occurring phosphate rock is admixed to said phosphoric acid of step i) in an amount corresponding to between 5 and 15% of said raw acid mass, but preferably in such an amount so as to enable to said rock to completely react with sulfuric acid. The reaction of said rock with said sulfuric acid, step iv), occurs between 80 and 90°C, dehydratation runs best at 90°C, but in many cases it is optimal to keep the mixture at 87°C. Provided is a mixture comprising phosphoric acid and suspended particles of calcium sulfate, wherein the conditions are believed to lead to calcium sulfate forms without crystal water. The reaction time, or retention time in a flow-through reactor, may be several hours, for example 1.5 hour. Step v) includes filtering the suspension of step iv) at a temperature of at least 80°C; thereby obtaining a filtration cake of calcium sulfate which, in its spatial structure, binds a greater part of the cadmium atoms, whereas the filtrate provides the desired product of low-cadmium phosphoric acid. The temperature should preferably be kept below 85°C during the filtration step; the concentration of monocalcium phosphate (MCP) is preferably 2.5% or more. Perlite may be added in the mixture to improve the use of phosphate rock which is not finely milled, or its admixture, may be helpful. The concentration of the components is controlled in all stages, during the whole process. The product is at least 46 % P2O5, preferably at least 48%, more preferably at least 50%, and still more preferably at least 52%, for example at least 54% but preferably lower than 56.0%, whereas cadmium level is at most 5 ppm, or preferably at most 4 ppm, for example 3 ppm or less, such as 2 ppm or less.

In another aspect, the invention provides an apparatus for reducing the cadmium level in phosphoric acid, comprising at least one stirred and temperature-controlled reactor, a raw acid feeder unit, sulfuric acid feeder unit, phosphate rock feeder unit, pumps, a filtration unit, and transferring means for transporting solutions or suspensions. In one embodiment, the apparatus has two stirred reactors. A schematic view of one of possible arrangements is shown in Fig. 1. Number symbols (1), (2), (3), and (4) represent stocks of raw phosphoric acid, concentrated sulfuric acid, phosphate rock, and water, respectively. The materials (1) and (2) are fed to reactor Rl, whereas rock (3) is fed to reactor R2. The arrows symbolize material transport; filtration cake (7) is removed from filter F, liquid streams comprising (1), (2), (4) (5), (6), (8), and the stream between reactors Rl and R2 are driven preferably by pumps. In some embodiments, water vapor is removed from the apparatus as a condensate. In special cases, some of the liquid streams may be driven by gravity. In some embodiments, parts of the process may be performed in a batch mode. Said filtration cake may be removed periodically or continually, eventually washing out acid residues and returning it to the reactors. Preferably, the apparatus works essentially in a continuous mode. The material transport velocities are selected to ensure equilibrium of the system, including constant volumes of the mixtures in reactors. The velocity of the recirculation stream (5) may contribute to the mixing efficiency of the reaction mixture, it may be, for example, about a half of the feeding velocity of (1). The retention time, affected by the ratio of feeding velocity and the reactor volume, may be, for example, between 1 and 5 hours. A skilled person will select the initial values based on small batch tests, knowing that the values will be affected by the composition of the reagents, including the processed raw phosphoric acid and the used phosphate rock, and later will adjust the values to obtain the highest yield.

The process of the invention may incorporate further steps aiming at removing additional undesired component in a raw material, including other toxic elements, or halogens, etc.

The invention will be further described and illustrated in the following examples.

Example

Phosphoric acid (52.5% P2O5, 5.0% SO4"², 20ppm Cd) was fed at a velocity of 2220 g/hr (vl) to a first, heated, stirred reactor of about 10 liters. Sulfuric acid (97.5%) was fed to the same reactor at a velocity of 82.0 g/hr (v2). The mixture was pumped from said first reactor to a second reactor of the same volume, to which phosphate rock was continuously fed at a velocity of 230.0 g/hr (v3). The stirred suspension from the second rector was pumped simultaneously to the filter unit at a velocity of about 2500 g/hr (v6) and back to the first reactor at a velocity of about 1100 g/hr. The velocities were kept in balance so that the volumes of the mixtures in both reactors be kept constant (for example, vl+v2+v3 approximately equals v6 if there are no losses). Water was pumped to the filter unit by a velocity onto the filter at a velocity of about 304 ml/hr. The filtrate was obtained at a velocity of 2190 ml/hr (54.9% P2O5, 0.192 SO4-², 3.65 ppm Cd). About 640 g/hr calcium sulfate with other reagent residues was obtained. The total losses of phosphoric acid were 3%, the total yield of the phosphoric acid production was 97%.

While this invention has been described in terms of some specific examples, many modifications and variations are possible. It is therefore understood that within the scope of the appended claims, the invention may be realized otherwise than as specifically described.

Claims

1. A process for manufacturing low-cadmium and low-sulfate phosphoric acid, comprising the steps of

i) providing a mass of raw phosphoric acid having a concentration of between 45 and 55 % P2O5, and containing up to 50 ppm cadmium; ii) admixing to said phosphoric acid of step i) concentrated sulfuric acid to reach a concentration of between 4 and 12% SO4"² in the mixture;

iii) admixing to said phosphoric acid of step i) a naturally occurring phosphate rock in an amount corresponding to between 5 and 15% of said mass;

iv) reacting said rock with said sulfuric acid at a temperature of between 80 and 90°C to obtain a mixture comprising phosphoric acid and suspended particles of calcium sulfate; and

v) filtering the suspension of step iv) at a temperature of at least 80°C;

thereby obtaining a filtration cake of calcium sulfate with bound cadmium ions, and a product of low-cadmium and low-sulfate phosphoric acid having a concentration of at least 46 % P2O5 and containing at most 5 ppm cadmium.

2. A process for manufacturing low-cadmium and low-sulfate phosphoric acid according to claim 1, comprising the steps of

i) providing a mass of phosphoric acid having a concentration of at least 50 % P2O5, and containing up to 40 ppm cadmium; ii) admixing to said phosphoric acid of step i) concentrated sulfuric acid to reach a concentration of at least 7 % SO4"² in the mixture; iii) admixing to said mixture of step ii) a naturally occurring phosphate rock in an amount corresponding to at least the mass of said SO4 in the mixture; iv) reacting said rock with said sulfuric acid at a temperature of between 80 and 90°C; and

v) filtering the suspension of step iv);

thereby obtaining a product of low-cadmium and low-sulfate phosphoric acid having a concentration of at least 52 % P2O5, and containing at most 0.2 % SO4 and at most 4 ppm cadmium.

3. A process according to claim 1, wherein said concentrated sulfuric acid has a concentration of at least 95%.

4. A process according to claim 1, wherein said step v) comprises adding water to the filtered suspension or onto the filter.

5. A process according to claim 1, wherein said low-cadmium phosphoric acid has higher concentration of P2O5 and lower concentration of SO4 than said raw phosphoric acid.

6. A process according to claim 1, wherein said steps i) and ii) are performed in a first stirred reactor, and said steps iii) and iv) in a second stirred reactor.

7. A process according to claim 1, wherein said raw phosphoric acid and said concentrated sulfuric acid are fed to a first stirred reactor, the mixture from said first reactor is continuously transferred to a second stirred reactor, and the mixture from said second reactor is continuously transferred either to the filter or to a collection tank.

8. A process according to claim 7, wherein a first part of the reaction mixture from said second reactor is transferred to said filter or tank, and a second part of the reaction mixture from said second reactor is recirculated back to said first reactor.

9. A process according to claim 7, comprising continually adding water either to said filter or to said tank.

10. A continuous process for manufacturing low-cadmium phosphoric acid according to claim 1, comprising the steps of

i) providing raw phosphoric acid containing up to 40 ppm cadmium and up to 8% sulfate as S0₄-²;

ii) reacting said phosphate rock in the mixture with said sulfuric acid and said raw phosphoric acid at a temperature of between 80 and 90°C; and

iii) filtering the reaction mixture;

thereby obtaining low-cadmium and low-sulfate phosphoric acid having a P2O5 concentration higher than said raw phosphoric acid.

11. A process according to claim 1, being a continuous process for reducing cadmium and phosphate amounts in phosphoric acid while increasing the concentration of P2O5 in said phosphoric acid.

12. An apparatus for reducing the cadmium level in phosphoric acid according to the process of claim 1, comprising

i) at least one stirred and temperature-controlled reactor enabling to keep the reaction mixture between 80 and 90°C;

ii) a unit for feeding raw phosphoric acid comprising at least 45 % P2O5 and cadmium up to 50 ppm;

iii) a unit for feeding concentrated sulfuric acid;

iv) a unit for feeding phosphate rock;

v) pumps and transferring means for transporting solutions or suspensions between the units and reactors; and

vi) at least one filtration unit for removing calcium sulfate from suspension and providing a filtrate of phosphoric acid having at most 5 ppm cadmium.