GB1564663A

GB1564663A - Process for thermal decomposition of phosphogypsum

Info

Publication number: GB1564663A
Application number: GB27297/77A
Authority: GB
Original assignee: Vish Khim T I
Current assignee: Vish Khim T I
Priority date: 1977-06-22
Filing date: 1977-06-29
Publication date: 1980-04-10
Also published as: DE2728149A1; FR2395956A1; FR2395956B3

Description

(54) PROCESS FOR THERMAL DECOMPOSITION OF PHSOPHOGYPSUM (71) We, VISH CHIMIKO-TECHNOLOGICHESKI INSTITIR of Sofia, Darvenitza, Bulgaria, a State research Institute organised under the laws of Bulgaria, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a method for the production of granulated phosphogypsum in a form suitable for thermally decomposing it into sulphur dioxide and calcium oxide under fluidised bed consitions.

Phosphogypsum is a waste material of the production of phosphoric acid by sulphuric acid digestion of apatite and phosphorite, the phosphoric acid generally being concentrated subsequently to form phosphoric acid or used in the production of complex fertilizers. The phosphogypsum waste material consists largely of calcium sulphate in the form of gypsum and phosphoric acid and is produced in an amount of from 1.3 to 1.6 tons for each ton of apatite and phosphorite employed.

Phosphogypsum may be used in the production of a variety of products, including sulphuric acid and calcium oxide, sulphuric acid and cement, plaster of Paris, building materials and filler compositions.

Sulphur dioxide and calcium oxide have been produced by the thermal dissociation of phosphogypsum which is carried out under reducing conditions in rotating furnaces. This type of processing takes a relatively long time, generally taking from 2 to 4 hours and suffers from a number of basic shortcomings, including poor utilisation of the volume of the furnace, low efficiency, high operational expenses and capital costs, low sulphur dioxide concentration in the furnace gas produced, the sulphur dioxide concentration generally only being from 5 to 7%, and poor quality of lime obtained owing to interaction of the calcium oxide produced with silica at the elevated temperatures employed.

Improvement in utilisation of the furnace and efficiency of the process can be obtained if fluidising conditions are employed; this leads to a multifold increase in the rate of dissociation. However, the phosphogypsum obtained in the phosphoric acid is polydisperse and possesses a high moisture content and is not suitable for use directly in fluidised bed processing. It is necessary that it be subjected to a preliminary drying and granulation to suitable particle size for fluidised bed processing.In handling particulate phosphogypsum, care must be taken to ensure that sintering thereof does not take place at the temperature at which it is subjected to decomposition, which is optimally 1200 to 12200C. A slight decrease in the processing temperature below 1200"C has been found to reduce sharply the degree and rate of dissociation making the process largely impracticable. A rise in temperature much above 1200"C. increases the degree of dissociation but also increases the danger of aggregation of the particles as a result of the sintering of the starting material.

According to the present invention, there is provided a method for the production of granulated phisphodypsum for thermal decomposition into calcium oxide and sulphur dioxide under fluidised bed conditions, which comprises heating the phosphogypsum at a temperature of 90 to 1500C over a period of from 1 to 5 hours to dry it and granulating the phosphogypsum, a binder for the phosphogypsum, being added to the phosphgypsum during the course of granulation thereof or primar to the granulation thereof.

When carrying out the method of the invention, the starting phsophogypsum is dried at a temperature of from 90 to 1500C. for a period of from 1 to 5 hours. To the dried phosphogypsum is then added the binder substance which is preferably employed in an amount of from 0.01 to 6% by weight of the dried phosphogypsum and is preferably selected from urea, carboxymethyl cellulose, ammonium nitrate, calcium nitrate, polytehylene oxide and distillation fluid from soda production. In soda production, sodium bicarbonate is produced by reaction of sodium chloride solution with ammonia and carbon dioxide and subsequently filtered off and converted to soda. After addition of slaked lime to the filtrate, distilling off of ammonia is effected to yield the aforesaid distillation fluid containing, inter alia CaC12 and NaC1.Free carbon-containing mineral substances may be incorporated in the granulate in an amount preferably of from 1 to 15% by weight of the dry phosphogypsum, the free carbon-containing mineral substances preferably being coke, anthracite or coal. A free sulphur-containing mineral may also include in the granulate, preferably in an amount of from 2 to 25% by weight of the phosphogypsum. In addition to using elemental sulphur, it is possible to use pyrites, pyrites generally not being considered to be a true compound, that is sulphide, but rather a substance intermediate a compound and an elemental mixture. Hence the term "free sulphur" as used herein includes pyrites.

Granulation of the mixture thus produced may be effected in a drum, conveyor, plate or Blounget granulator, the mixture preferably having a moisture content of 15 to 30% by weight. The granules obtained are then submitted to thermal treatment usually at from 100 to 7000C., usually for from 10 to 90 minutes, to achieve hardening thereof. In order to obtain granules having a particle size of from 1 to 4 mm which are most suitable for use in the fluidised bed production of line and sulphur dioxide, the granules are subsequently sieved.

The use of the free carbon-containing and free sulphur-containing mineral starting materials in the production of the phosphogypsum granules has been found to favour the processing of phosphogypsum into calcium oxide and sulphur dioxide by thermal dissociation.

The free carbon-containing mineral starting material, the carbon generally being in solid form, serves to accelerate and increase the degree of thermal dissociation of phosphogypsum by shifting the decomposition reaction to the right as a result of freed oxygen being combined with the carbon to form carbon monoxide as follows: Ca S04 + C e CaO + SO2 + CO The free carbon-containing mineral material in the granules enables the required slightly reducing atmosphere in the reaction space to be obtained as a result of the interaction of the free carbon with carbon dioxide from the combustion of fuel for the heating up of the reaction space, in accordance with Boudoirs reaction which, at the optimum reaction temperature, that is at about 1200"C., is almost completely shifted to the right.

The addition of the free carbon-containing mineral material as reduing agent enables full use to be made of the fuel heat as it undergoes complete combusion, while the availability of reducing components in the gaseous phase is ensured by the solid carbon-containing reducing agent.

The thermal dissociation of phosphogypsum is an endothermic process and the purpose of the sulphur is to compensate for the heat absorbed in the thermal dissociation as the sulphur itself undergoes combustion. The presence of sulpur leads to the production of a gaseous phase having a high concentration of sulphur dioxide which renders it particularly favourable for processing into sulphuric acid.

The following Examples illustrate this invention.

Example 1 1 kg of phosphogypsum which had been dried for 3 hours at 1050C. was introduced into a drum granulator. As the drum was rotated at a speed of 41 r.p.m., 240 ml of water and 40 g of urea were added to the contents of the drum. Granulation was continued for 8 minutes, whereupon the granules obtained were heat treated for 65 minutes at 1200C., then sieved and the fraction sized 1 to 3 mm was collected. This useful fraction constituted 85.9% of the granulation product. The granulate possessed a compressive strength of 3.0 kg/cm2 and airesistance to rubbing of 62.85%.

The granules were then subjected to thermal dissociation at 12500C. in a pilot installation for a fluidised bed process. The installation was 60 mm in diameter and 1000 mm in height.

The rate of flow of gas through the reactor was 0.32 m/sec and the rate of supply of granules thereto was 750 g per hour. A reducing atmosphere was provided in the reactor by supply of propane-butane gas. The calcium sulphate underwent 98.7% dissociation into sulphur dioxide and calcium oxide and the dissociation of the entire amount of phosphogypsum granules was completed within the space of several minutes.

Example 2 1 kg of phosphogypsum, dried for 3 hours at 1050C was placed in a drum granulator. As the drum underwent rotation at a speed of 38 r.p.m., 240 ml of water and 10 g of carboxymethyl cellulose were added to the contents of the granulator. Granulation was continued for 4 minutes whereupon the granules obtained were heat treated for 45 minutes at 1200C., sieved and the fraction having a particle size of from 1 to 3 mm was collected for further use. The useful fraction amounted to 73.3% by weight of the granulation product, this fraction having a compressive strength of 15 kg/cm3, a dynamic strength of 81% and a resistance to rubbing of 99.7%. The granules were then subjected to thermal dissociation at 1220"C. in a pilot installation for carrying out a fluidised bed process.The rate of gas flow (propane-butane gas used as reducing agent) was 0.36 m/sec and the rate of supply of granules was 600 g er hour. The degree of dissociation of calcium sulphate obtained in the process which took several minutes to complete was 96.8%.

Example 3 1 kg of phosphogypsum was dried for 3 hours at 105"C. and placed in a drum granulator.

While continuously rotating the drum at a rate of 40 r.p.m., 240 ml of water and 30 g of ammonium nitrate were added to the contents thereof. The granulation process was allowed to proceed for 7 minutes, whereupon the granules obtained were subjected to a heat treatment at 1200C for 65 minutes. The granules were then sieved and the fraction 1 to 3 mm in size was retained for further use. This fraction amounted to 75% by weight of the granulation product and the granules possessed a compressive strength of 6.51 kg/cm2 and a resistance to rubbing of 70%. The granules were then subjected to thermal dissoiation under fluidised bed conditions at temperatures ranging from 1200 to 13500C.

Example 4 1 kg of phosphogypsum was dried for 2.5 hours at 1100C. and then placed in a drum granulator. Continuous rotation of the drum at a rate of 45 r.p.m. was then allowed to take place and during the rotation of the drum, 255 ml of water and 48 g calcium nitrate were added to the contents thereof. The granulation process took 10 minutes and the granules obtained were then heated at 350"C. for 40 minutes. The granulation product was then sieved and the fraction of size 1 to 3 mm was retained for further use. This fraction corresponded to 75% by weight of the granulation product. The granules possessed a compressive strength of 19.5 kg/cm2 and a resistance to rubbing of 84%. The granules thus obtained were subjected to thermal dissociation under fluidised bed conditions at temperatures ranging from 1200 to 13500C.

Example 5 1 kg of phosphogypsum was dried for 2 hours at 1100C. and was then introduced into a drum granulator. 1.5 g of polyethylene oxide inthe form of a fraction, 0.1 mm in size were added. The contents of the granulator were uniformly mixed and were then mixed with 225 ml of water as the drum underwent continuous rotation at 48 r.p.m. Granulation took 11 minutes and the granules were heat treated at 6500C. for 15 minutes, then seived and a fraction 1 to 3 mm in size was retained for further use. The higher temperature of the heat treatment resulted in an increase in the strength of the granules. The granules were then submitted to thermal dissociation under fluidised bed conditions at temperatures ranging from 1200 to 13500C.

Example 6 0.5 kg of phosphogypsum were dried for 3 hours at 1058C., mixed with 40 g of fine coke particles and then introduced into a drum granulator. As the granulator underwent rotation at 40 r.p.m. 120 ml of water together with 15 g of urea were added to the contents thereof.

The granulation took 9 minutes to complete, The granules were then submitted to a treatment at 1200C. over the course of 60 minutes and the fraction of size 1 to 3 mm thus obtained were etained for further use. This fraction represents 6.5% of the granulate and possessed a static strength of 2.5 kg/cm2 and a resistance to rubbing of 62.7%. The granules obtained were then subjected to thermal decomposition in a fluidised bed at a temperature of 1200 to 1250 C.

Example 7 0.5 kg of phosphogypsum were dried for 3 hours at 1058C and placed in a drum granulator. As the granulator underwent rotation at 40 r.p.m., 120 ml (133 g) of distillation fluid (see hereinbefore) from the production of soda, were added to the contents of the granulator. The granulation took 8 minutes to complete, and the granules obtained were then subjected to heat treatment at 1200C over the course of 15 minutes and, after sieving, the fraction having a particle size of from 1 to 3 mm was retained for further use. This fraction represented 88% of the granulate and possessed a compressure strength of 4 kg/cm2 and a resistance to rubbing of 62.7%. The granules obtained were then subjected to thermal decomposition in a fluidised bed at a temperature of 1200 to 12700C to yield calcium oxide and sulphur dioxide.

Example 8 0.5 kg of phosphogypsum were dried for 3 hours at 1050C. and then mixed with 80 g of sulphur and placed in a drum granulator. As the drum underwent continuous rotation at 45 r.p.m., 120 ml of water containing 20 g of urea were added. The granulation process lasted 8 minutes. The granules obtained were then sieved and the fraction of particle size 1 to 3 mm was retained for further use. The 1 to 3 mm particles were used in the thermal decomposition of the phosphogypsum thereof under fluidised bed conditions at a temperature of 1200 to 1250C.

Reference is next made to the following Table which sets out the thermal stability characteristics of phosphogypsum granules and admixtures thereof with fuel substances as produced in the foregoing Examples, and powdered phosphogypsum produced by a prior art product. Heating at different temperatures and for different lengths of time was effected.

TABLE Heating Heating time Powdered Granulated phosphogypsum produced in Example Temperature (min.) at Phosphogypsum ( C) stated produced by prior 1 2 3 4 5 6 7 8 temperature* art procedure 1100 30 - 180 a a a a a a a a 1150 30 - 180 a a a a a a a a 1200 30 b a a a a a a a a 60-80 c a a a a a a b a 1250 30 c a b a a a a b a 60 c a b a a a a b a 90 - 180 c a c a a b a c b 1300 15 c a c a a b a c a 30 c a c a a c a c a 60 c a c a b c b c b 90 c b c b b c b c b 90 - 180 c c c b c c c c c * Samples were examined every 10 minutes (unles otherwise indicates) to establish occurrence of sintering a = no sintering b = slight sintering c = sintering In summary, the method of this invention enables granules to be obtained which are suitable for use in the fluidised bed processing of their phosphogypsum content to produce calcium oxide and sulphur dioxide.As can be seen from the Table, the method allows phosphogypsum to be processed into calcium oxide and sulphur dioxide without the ooccurrence of any sintering of the working mass at temperatures as high as 1300"C. The possibilities of working at temperatures above 1200"C enables the process to be carried out with greater throughput under shorter reaction times so that there is obtained a high quality lime as well as a gaseous product having a higher sulphur dioxide concentration than hitherto.

In the foregoing Examples, reference is made to the compressive strength and resistance to rubbing of the granules obtained. The compressive strength is determined by the following formula:

kg/cm2 wherein Pi is the force necessary for the distortion of one granule in kg: and dma is the mean arithmetic diamter of the tested granules in cm.

The resistance to rubbing is measured in accordance with the equation: % Pr = G2 100 Gl wherein G1 is the weight of test granules employed, the test granules being subjected to rotation in a cylinder at a rotation speed of 24 r.p.m. for 15 minutes; and G2 is the weight of understroyed granules at the end of the rotation period. To determine the weight of understroyed granules, the test sample is subjected to sieving through a sieve having openings with a diameter of 1 mm after the rotation and the weight of granules which do not pass through this sieve is determined to 0.01 g.

WHAT WE CLAIM IS: 1. A method for the production of granulated phosphogypsum for thermal decomposition into calcium oxide and sulphur dioxide under fluidised bed conditions, which comprises heating the phosphogypsum at a temperature of 90 to 1500C over a period of from 1 to 5 hours to dry it and granulating the phosphogypsum beig added to the phosphogypsum during the course of granulation, a binder for the phosphogypsum therefore prior to the granulating thereof.

2. A method as claimed in claim 1, wherein the binder substance is employed in an amount of from 0.01 to 6% by weight of the phosphogypsum.

3. A method as claimed in claim 1 or 2, wherein the binder employed is an aqueous solution of urea carboxymethyl cellulose, ammonium nitrate, calcium nitrate or polyethylene oxide or is the distillation fluid obtained.

4. A method as claimed in any one of the preceding claims, wherein a free carbon-containing mineral substance is added to he phosphogypsum prior to the granulation thereof.

5. A method as claimed in claim 4, wherein the free carbon-containing mineral substance is coke, anthracite or coal.

6. A method as claimed in claim 4 or 5, wherein the free carbon-containing mineral substance is included in the granules in an amount of frdm 1 to 15% by weight of the phosphogypsum content thereof.

7. A method as claimed in any one of the preceding claims, wherein a free sulphur-containing mineral substance as defined herein is added to the phosphogypsum prior to the granulation thereof.

8. A method as claimed in claim 7, wherein the free sulphur-containing mineral substance is employed in an amount of from 2 to 25% by weight of the phosphogypsum content of the granules.

9. A method as claimed in any one of claims 4 to 8, wherein the free carbon- and/or free sulphur-containing mineral substances are admixed with the phosphogypsum separately from the binding agent.

10. A method as claimed in any one of the preceding claims, wherein a mixture comprising phosphogypsum and a said binder and having a moisture content of from 15 to 30% by weight is granulated.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. In summary, the method of this invention enables granules to be obtained which are suitable for use in the fluidised bed processing of their phosphogypsum content to produce calcium oxide and sulphur dioxide. As can be seen from the Table, the method allows phosphogypsum to be processed into calcium oxide and sulphur dioxide without the ooccurrence of any sintering of the working mass at temperatures as high as 1300"C. The possibilities of working at temperatures above 1200"C enables the process to be carried out with greater throughput under shorter reaction times so that there is obtained a high quality lime as well as a gaseous product having a higher sulphur dioxide concentration than hitherto. In the foregoing Examples, reference is made to the compressive strength and resistance to rubbing of the granules obtained. The compressive strength is determined by the following formula: kg/cm2 wherein Pi is the force necessary for the distortion of one granule in kg: and dma is the mean arithmetic diamter of the tested granules in cm. The resistance to rubbing is measured in accordance with the equation: % Pr = G2 100 Gl wherein G1 is the weight of test granules employed, the test granules being subjected to rotation in a cylinder at a rotation speed of 24 r.p.m. for 15 minutes; and G2 is the weight of understroyed granules at the end of the rotation period. To determine the weight of understroyed granules, the test sample is subjected to sieving through a sieve having openings with a diameter of 1 mm after the rotation and the weight of granules which do not pass through this sieve is determined to 0.01 g. WHAT WE CLAIM IS:

1. A method for the production of granulated phosphogypsum for thermal decomposition into calcium oxide and sulphur dioxide under fluidised bed conditions, which comprises heating the phosphogypsum at a temperature of 90 to 1500C over a period of from 1 to 5 hours to dry it and granulating the phosphogypsum beig added to the phosphogypsum during the course of granulation, a binder for the phosphogypsum therefore prior to the granulating thereof.

11. A method as claimed in any one of the preceding claims, wherein the granules are

thermally after-treated at from 100 to 7000C.

12. A method as claimed in claim 11, wherein the thermal after-treatment is effected for from 10 to 100 minutes.

13. A method for the production of granulated phosphogypsum for fluidised bed processing to produce sulphur dioxide and calcium oxide, substantially as described in any one of the foregoing Examples.

14. A process for the production of calcium oxide and sulphur dioxide, which comprises thermally decomposing under fluidised bed conditions granulated phosphogypsum which has been produced by the process claimed in any one of the preceding claims.

15. A process as claimed in claim 14, wherein the thermal decomposition is effected at a temperature of 1200 to 13500C.

16. A process for the production of calcium oxide and sulphur dioxide, substantially as described in any one of the foregoing Examples.