WO2001077021A1

WO2001077021A1 - Production of strontium carbonate from celestite

Info

Publication number: WO2001077021A1
Application number: PCT/TR2000/000020
Authority: WO
Inventors: Sidika KOCAKUŞAK; Raşit TOLUN; Hacer Dogan; Murat Koral; Kani Akcay; Hansu Julide Koroglu; Hayrettin Yuzer; Fehim Isbilir; Omer Tunc Savasci; Turhan Ayok; Handan Karakale; Alaiddin Yilmaz
Original assignee: Tuebitak-Marmara Research Center
Current assignee: Tuebitak-Marmara Research Center
Priority date: 2000-04-12
Filing date: 2000-04-12
Publication date: 2001-10-18
Anticipated expiration: 2002-10-12
Also published as: TR200103265T1

Abstract

The main and commercial minerals of strontium are celestite and strontiyanite. The most common of these two minerals is celestite, which is strontium sulfate. The other mineral is mainly strontium carbonate and its occurrence is rare. Therefore pure strontium is mostly produced from celestite which is then used in production of glass suitable for production of color television tubes, in electro ceramic industry and in production of electrolytic metals. There are two main processes for economical production of strontium carbonate from celestite in industrial scale. These processes are reduction process (Black Ash). BA and direct conversion to carbonate and/or double decomposition process, DD. Although there are some processes using ammonium carbonate for double decomposition, the majority of the DD processes use sodium carbonate for conversion to carbonate. In the novel process developed, celestite is converted to strontium carbonate via double decomposition with ammonium carbonate. The obtained strontium carbonate contains some impurities and is called as raw strontium carbonate. To purify the raw strontium carbonate is then calcinated to obtain strontuim oxide, which is then converted to strontium hydroxide solution by reaction with water. Strontium hydroxide solution is filtered to get rid of the insoluble impurities. From the filtered and purified strontium hydroxide solution, pure strontium is precipitated out by reacting the solution with carbondioxide. The precipitated strontium carbonate is then separated and dried as powder form. The solution obtained from double decomposition contains both ammonium carbonate and ammonium sulfate. By increasing the temperature of this solution to a suitable value, ammonium carbonate is decomposed to ammonia and carbondioxide to be recycled to the process. Since the remaining solution is ammonium carbonate free, ammonium sulfate is crystallized from it as a commercial product or is reacted with milk of lime to recover ammonia.

Description

DESCRIPTION

PRODUCTION OF STRONTIUM CARBONATE FROM CELESTITE

Mineral Properties and Mineral Purification: Celestite mineral obtained from the mines at Akkaya village of Sivas province of Turkey by Barit Maden Turk A.§. is first dressed to prepare a commercial grade product and then marketed with following composition. SrSO₄ 96,2 %

BaSO₄ 1,0 %

CaSO₄ 2,0 % Fe₂O₃ 0,5 %

PbS 0,1 %

Others 0,2 %

This product is converted to strontium carbonate (SrCO₃) by double decomposition method with ammonium carbonate. However, due to formation of ferrites which stick to the surface of the calcination kiln during the calcination process which is carried out to purify the SrCO₃ product, iron content of the above mentioned commercial grade celestite is reduced by magnetic separation.

Particle size of the celestite to be further purified by magnetic separation, must be same as the particle size of the product which is used for production of SrCO₃ by double decomposition method. This is to eliminate further grinding after magnetic separation, which results in increase in iron content originating from the grinding equipment. The desired particle size is below 50 μ. Therefore commercial grade celestite is first grinded to particle size of below 50 μ , then 20 % grinded celestite containing sludge is prepared by addition of water. Iron is separated from this sludge by magnetic separation, which results in 1-2 % celestite loss alongside with iron removal. It is also found that cleaned celestite by magnetic separation for double decomposition reaction retains about 10% water.

Double Decomposition Reaction: The main equation of the double decomposition reaction is shown below:

SrSO₄ + (NH₄)₂CO_{3 ►} SrCO₃ + (NH₄)₂SO₄ (solid) (solution) (solid) (solution)

The above reaction has been investigated by various researchers and numbers of patents have been issued. Chinese Patents (CN 1043482A, 1990 and 88105880, 1988), Russian Patents (565877,1977) and Spanish Patents (ES-2 006338, 1989 and WO 92/01630, 1992) can be given as some examples. However in all these patents for reactions where reaction temperatures are above 85 °C, reactors are pressure vessels to meet the pressure requirements of the processes and sodium carbonate is used instead of ammonium carbonate. Only the Spanish patent covers an atmospheric reaction, which is carried out in three reactors in series. However, since a satisfactory conversion could not be reached, at the last stage, ammonium carbonate is used with reaction temperatures of 80-100 °C and reaction pressures of 0.5-1.5 arm. A Japanese patent (SHO 4851895, 1973) covers production of 97 % pure SrCO₃ by using ammonium carbonate. However to reach 99 % purity, the reaction temperature is increased to 90 °C which also indicate that the reaction is carried out under pressure. In the patent literature purification of the SrCO₃ obtained by double decomposition is achieved by two main approaches. In the first approach the impure SrCO₃ is dissolved in an acid, impurities are filtered out and from the impurity clean solution, SrCO₃ is precipitated once again. In the second approach, as it is the patent of Kaiser Aluminum & Chemical Corp. (US 3743691,1973), the SrCO₃ obtained from double decomposition with sodium carbonate, is converted into strontium oxide by calcination, dissolved in water, filtered from impurities and from the impurity clean solution SrCO₃ is reprecipitated again either with sodium carbonate or with carbondioxide. In the novel process developed by us, by double decomposition reaction, 99 % conversion of SrSO₄ to SrCO₃ is achieved under atmospheric pressures and at lower temperatures of about 65 °C by using an overall 60 % excess of ammonium carbonate. In this process purity of the product depends on the purity of the celestite used. With proper raw material (celestite) purification, particularly from iron, 98 % pure SrCO₃ can be obtained. To obtain higher purification than 99 %, calcination of the product followed by water solution and reprecipitation is needed. The detailed description of the process is as follows: (See Fig. 1)

The process is developed in two reactors in series. Celestite is mixed in the first reactor with recycle solutions from the centrifuges of second reactor which contain 20-25 % ammonium carbonate and 8-10 % ammonium sulfate. The recycle solution added contains 20-25 % stoichiometric excess of ammonium carbonate. To the mixture obtained thus, water is added to achieve 3.0-3.5 liquid/solid ratio. The reaction is carried out at 65 °C at atmospheric pressure for 3 hours. The conversion of SrSO₄ to SrCO₃ obtained is 70-80 %.

At the end of the reaction a suspension containing 25-30 % solids with the mean particle size of about 15 ^> is obtained. From this suspension, solids are separated of centrifugation. The filtrate containing 20-25 % ammonium sulfate and 8-10 % ammonium carbonate is sent to a filtrate tank. The filtrate is used to produce ammonia and carbondioxide and/or ammonium sulfate.

The solid obtained from centrifugation is loaded to the II. reactor. In this reactor stoichiometric excess ammonium carbonate is 60 % of the strontium sulfate, fed to the first reactor. This reaction is also carried out at 65 °C at atmospheric pressure for duration of three hours. 1,5 % excess of ammonia, needed to form ammonium carbonate consumed during reaction, is used additionally. The reason for this addition is to keep the pH between 8,5 and 9,0. As the ammonium carbonate consumed by forming ammonium sulfate, which reduced the pH otherwise. Excess ammonia is recycled in the system. After the second stage reaction also, solids are separated by a centrifuge. The solution containing 20-25 % ammonium carbonate and 8-10 % ammonium sulfate is recycled to the first reactor. The solid carries about 15 % solution, then it is washed thoroughly to remove ammonium carbonate and ammonium sulfate.

Calcination: The raw SrCO₃ obtained from the centrifuge is fed to the rotary calcinator. Where, in line with the following equation SrCO₃ is converted to SrO, at 1200-1300°C. The duration of calcination depends on the temperatures applied and is about 1 to 2 hours.

SrCO₃ ► SrO + CO₂

The CaCO₃ impurity in the raw SrCO₃, is also converted to CaO.

CaCO₃ ► CaO + CO₂ The other impurities in the raw SrCO₃, BaSO₄ is converted to BaO in a very limited amount.

It is known from the literature that at these temperatures the iron present in the raw SrCO₃ , forms SrO and CaO ferrites (SrO.6Fe₂O₃, CaO.6Fe₂O₃,) whose melting points are below the calcination temperatures applied. This is, why sticking problems were encountered during laboratory experiments, which were eliminated by magnetic separation of iron from the celestite used at the beginning. To find a solution to this problem, in the patent of Kaiser Aluminum & Corp. (US 3875298, 1975), some additives such as petrocoke is recommended which is not a real solution and is very difficult to apply.

Leaching Process: The strontium oxide product obtained after calcination containing calcium oxide and some other impurities, is dissolved in water where SrO is converted to strontium hydroxide and CaO is converted to calcium hydroxide.

SrO + H₂O ► Sr(OH)₂

CaO + H₂O ► Ca(OH)₂

In the dissolving tank the temperature is kept around the boiling temperature of water due to the fact that solubility of Sr(OH)₂ is low under 100 °C while solubility of Ca(OH)₂ decreases as the temperature increases. In addition to keep the dissolved Ca(OH)₂ at a minimum level, the Sr(OH)₂ concentration of the solution must be around 10 %. The leaching process is carried out in a mixing tank. During this process iron oxide (Fe₂O₃), silicon dioxide (SiO₂), calcium hydroxide (Ca(OH)₂), sulfates and insoluble complex strontium compounds remain undissolved and are separated from the Sr(OH)₂ solution by centrifugation during which attention must be paid to keep the temperature about 96-100 °C.

Carbonation (SrCO₃ Reformation) : Purified Sr(OH)₂ solution is then treated with CO₂ in a suitable CO₂ absorption tower or mixing tank to produce pure SrCO₃. Sr(OH)₂ + CO₂ ► SrCO₃ + H₂O

The CO₂ needed for the above reaction can be obtained from the calcination by cleaning the effluent gases via a cyclone and a dust collecter.

The strontium carbonate obtained from CO₂ treatment is separated by centrifugation. Since the particle size of the SrCO₃ precipitated is quite small, a decanter centrifuge must be used here. The filtrate, which is quite clean, is recycled for dissolving SrO.

Drying : 20-30 % water containing strontium carbonate obtained from the centrifuge is dried at 150 - 160 °C .

Recovery of Ammonia and Carbon dioxide: At the raw SrCO₃ production step, the filtrate obtained by centrifugation of the reactor mixture from the first reactor contains about 20- 25 % ammonium sulfate and 8-10 % ammonium carbonate. When the filtrate, which is at about 65 °C, is heated to 80-100°C the ammonium carbonate decomposes to ammonia and carbondioxide, which are distilled of with some amount of water. The ammonium sulfate in the solution is stable at this temperature and remains in the solution. Depending on what is desired either (NH₄) SO₄ is crystallized from this solution or inline with the below given equation, by Ca(OH)₂ addition (NH₄)₂SO in solution is decomposed to obtain ammonia and CaSO₄.

(NH₄)₂CO_{3 ►} 2NH₃ + CO₂ + H₂O

(NH₄)₂SO₄ + Ca(OH)_{2 ►} CaSO₄ + 2NH₃ + 2H₂O

The NH₃ and CO₂ obtained by decomposition of (NH₄)₂CO₃ is recycled to produce (NH )₂CO₃ needed for double decomposition.

Examples: Commercial celestite of Barit Maden Turk A.§. is grinded to below 50 μ, cleaned from its iron impurity by magnetic separation and used as raw material SrSO₄ to produce raw SrCO₃ .

When the raw material SrSO₄ is reacted with (NH₄)₂CO₃ as described above. The composition of the raw SrCO₃ product obtained is shown for two different studies. I (%) II (%)

SrCO₃ 96,26 95.94

CaCO₃ 1,83 1.83

BaSO₄ 1,25 0.64

BaCO₃ - 0.60

Fe O₃ 0,21 0.26

PbS 0,12 0.11

Others 0,25 0.62

An example composition of the SrO product obtained after calcination of raw SrCO₃ at 1200-1300 °C for duration of 1-2 hours is given below %

SrO 95.38

CaO 1.46

BaSO₄ 0.80

BaO 0.40 PbO 0.11

Others 0.81

As mentioned above SrO obtained from the furnace is dissolved in water at 95 °C. The dissolution time is 2 hours. The hot solution is then centrifuged at the same temperature to separate solid impurities and the finely distributed Ca(OH)₂ particles. The filtrate is then treated with CO₂ to covert Sr(OH)₂ to SrCO₃. The reactant is centrifuged again this time to separate the refined, solid product, SrCO₃, from the liquid phase. The wet cake obtained from the centrifuge is dried at 150-160 °C to obtain the final product.

On Table 1, the firs row indicates the solids separated as waste from the solution as percent of the original raw SrCO₃ used as feed for calcination and as percent of some other parameters. This waste contains SrCO₃, Sr(OH)₂, SrSO₄, Fe O₃, BaSO₄, BaCO₃, SiO₂ and other insolubles. On second row of Table 1 shows the waste of SrCO3 ratio to raw SrCO₃ produced in the reactors. The last row of Table 1 indicates total waste of strontium loss in the overall process. Table 1. Amount of Solid Waste

On the Table 2 composition of the solid wastes separated from strontium hydroxide solution are given.

Table 2. Chemical Composition of the Solid Waste.

As seen from Table 1 on the basis of the amount of raw SrCO₃ fed to the calcinator, from the refmation process 5-10 % solid wastes are obtained. The amount of the some solid waste on the basis of the raw SrCO₃ is 3-7 %. The product obtained is pure, reagent grade SrCO₃ products. The overall yields are above 90%. The chemical composition of this pure SrCO₃ is shown on Table 3. As seen from this table the product obtained is compared with the commercial products according to the content of the critical impurities of Ba, Ca, SO₄ ⁼, Fe. The product obtained with our novel process seems better than other commercial products. In addition the product does not contain other objectionable impurities such as Na⁺ and Cl^" because of the process. Table 3. Chemical Composition of the Refined Strontium Carbonate Product

REFERENCES

1- XIONG, S., "Preparation of Strontium Carbonate", Chinese Patent, CN 1043482A, 1990

2- JTNG, Z., "Method for Preparation of SrCO₃ Using NH CI", Chinese Patent, CN 88105880, 1988

3- GAPRINDASHNILI, N.Ν., DJAOSHNILI, O.A:, NADIRADZE, D.G., "The Production Process of Strontium Carbonate", Russian Patent, 565877, 1977

4- MARTINEZ, M., MIGUEL, A., "Manufacturing of Strontium Compounds and Fertilizer From Strontium Sulfate", Spain Patent, ES-2006 338, 1989

5- PIEDRAFITA PALACIOS, A., "Continuous Production of Fertilizing Salts With Derivatives of Strontium and Gypsum", WO 92/01630, 1992

6- OKADA, H., "Strontium Carbonate Production", Japan Patent, Japan Kokai, SHO 4851895, 1973

7- COATNEY, R.L., HOUSH, L.M., NAN DREWER, M ., "Calcination of Strontium Carbonate", USA Patent, US 3875298, 1975

8- TREW, L.J., "Purification of Strontium Carbonate", US 3743691, 1973

Claims

- A novel process consisting of grinding enriched celestite mineral to below 50 μ, cleaning of the grinded celestite from iron via magnetic separation, production of raw strontium carbonate via double decomposition reaction of grinded and magnetic separation cleaned celestite with ammonium carbonate solution, calcination of the raw strontium carbonate to strontium oxide, leaching of strontium oxide in water followed by filtration of insolubles to obtain clean strontium hydroxide, precipitation of pure strontium carbonate from the clean strontium hydroxide solution by treatment with carbon dioxide, filtration of the precipitated strontium carbonate followed by drying of it. - Carrying out the double decomposition reaction mentioned in Claim 1 at 60 -70 °C and atmospheric pressure. - Carrying out the double decomposition reaction mentioned in Claim 1 and Claim 2 in two steps in 50-60 % overall excess of ammonium carbonate. - Recycling of the excess ammonium carbonate mentioned in Claim 3 as ammonia and carbon dioxide by distillation with some water vapour prior to ammonium sulfate production. - Prevention of sticking of raw strontium carbonate to the surface of the calculator during calcination mentioned in Claim 1 by means of removing iron from the raw celestite via magnetic separation. - Carrying out the calcination mentioned in Claim 1 at 1200-1300 °C. - Carrying out leaching of SrO to obtain Sr(OH)₂ solution, separation of solid impurities from it, precipitation of SrCO₃ from the Sr(OH)₂ solution by CO₂ and separation of the precipitated SrCO₃, which are all mentioned in claim 1, at high temperatures of over 90 °C always. - Drying of the precipitated strontium carbonate from strontium hydroxide solution mentioned in Claim 1 without washing after centrifugation.