Title of Invention

"AN IMPROVED PROCESS FOR PURIFICATION OF PHOSPHOGYPSUM"

Abstract

The present invention relates to "an improved process for purification of phosphogypsum". The improved process of the present invention will be particularly useful for the purification of phosphogypsum, a by-product of the phosphoric acid, by removal of the harmful impurities of phosphates, fluorides, organic matter and alkalis. The purified phosphogypsum will find use in the production of building materials. The improved process of the present invention consists of extracting impurities of P2Os, F, organic matter and alkalis present in phosphogypsum with aqueous citric acid solution, followed by washing of the leached impurities with a stream of water, drying the gypsum cake at 40-5 0°C to constant weight.

Full Text

The present invention relates to " an improved process for purification of phosphogypsum". The improved process of the present invention will be particularly useful for the purification of phosphogypsum , a by-product of the phosphoric acid, by removal of the harmful impurities of phosphates , fluorides , organic matter and alkalies. The purified phosphogypsum will find use in the production of building materials. Phosphogypsum is produced as a by-product of wet process phosphoric acid industry by interaction of phosphate rock with sulphuric acid.It is produced to a tune of 5.0 million tonnes per annum in India posing a serious problem of disposal and health hazards. The material has certain harmful impurities of phosphate, fluoride, organic matter etc.which interfere with the normal functioning of gypsum and gypsum products produced from it. To eliminate pollution and health hazards, a proper and effective utilization of phosphogypsum is required. Attempts have been made from time to time to find ways and means of utilization of phosphogypsum. Currently not more than 10% of the bulk of phosphogypsum produced in India is utilized in the manufacture of cement and ammonium sulphate fertilizer and soil ammendment. The consumption of gypsum may be increased many- fold if the level of harmful impurities (P2O5 /F/ organic matter etc.) present in the phosphogypsum are mitigated to the permissible limits. The purified phosphogypsum may be utilized as a retarder for regulating setting time of ordinary portland cement and portland slag cement in place of natural gypsum.Being high in purity in terms of calcium sulphate content ( more than 85% CaS04.2H2O ), the requirement of gypsum may be lower than the normally used 4-5% with the additional advantage of reduced power consumption. The purified gypsum is eminently suitable for the production of calcined gypsum/gypsum plaster as per relevant Indian Standards ( IS : 2547 (Part-1)-1976 - Gypsum building plaster : Part-1 excluding premixed lightweight plasters and IS : 8272 - 1976, The gypsum plaster for use in the manufacture of fibrous plaster boards) using fibres such as sisal, coir,glass fibers etc. The gypsum plaster can be used making building blocks, tiles and in plastering works ( finish coat, under coat ) and for many other multifarious applications in the building sector.The purified phosphogypsum can be fruitfully utilized in the production of durable gypsum binder for use in the exposed situations hitherto the use of gypsum and set gypsum plaster products is banned in moist and outside construction activities. The gypsum binder made by blending gypsum plaster with active alumino-silicious materials and chemical additives has been found suitable for use in masonry mortars, plasters, plaster boards and door shutters both for external and internal applications. Purified phosphogypsum can be used for making ceramic grade plaster as per IS : 2333-1981 and surgical plasters as per Indian Pharmacopoeia ( Vol.1, p. 399, 3rd. Edn. 1985 ) Government of India.The purified phosphogypsum has a tremendous scope in the simultaneous manufacture of sulphuric acid and production of cement. Since the impurities of ?2°5 an from dihydrate process plant in India contains larger proportions of impurities than the permissible limits of P2°5 and F. Whereas phosphogypsum has been utilized to greater extent in countries like Japan and Germany. In Japan , most of the plaster boards and building blocks are made out of phosphogypsum. The quality of phosphogypsum is better in these countries than the phosphogypsum produced in India as they contain lesser amount of phosphate, fluoride and organic matter contents in former than the latter case. These impurities exist either on the surface of dihydrate gypsum crystals, built in the crystal lattice or present as undecomposed phosphate rock. Reference may be made to O.K.Kitchen and W.J.Skinner, Chemistry of by-product gypsum and plaster,Jr. Appld. Chem. and Biotech, Vol. 21, No. 2, Feb. 1971, pp 53-60 and Manjit Singh, Physico - Chemical Studies on Phosphogypsum for Use in Building Materials, Ph.D. Thesis, University of Roorkee, Roorkee, 1979, pp 16-4. The impurities [Ca(H2P04)2.H20j found on gypsum crystals are water- soluble whereas the impurities held up in the gypsum crystals (CaHPO4.2H2O) are sparingly soluble and released slowly in the aqueous media and influence the properties of cements slowly. These impurities,in fact, are the products of reaction which takes place between rock phosphate and the sulphuric acid during the production of phosphoric acid. These impurities affect the development of strength and other hydraulic properties of the calcined gypsum as well as the cements produced from them. Reference may be made to Keiichi Murakami , Utilization of chemical gypsum for portland cement, Proc. International Symposium on the Chemistry of Cement, Tokyo, Part IV, 1968, pp 457-503; Eipeltauer, Edward, Applicability of phosphoric acid gypsum sludge, Tonind-Ztg. Kerm Rindsch., Vol. 97, No. 1, 1971, pp 4-8, German & C.A.Taneja and Manjit Singh, Evaluation of phosphogypsum for different building materials, Chemical Age of India, Vol. 20, No.2, Feb. 1977, pp 108-111. Hence,a careful study ia required. The rmturo mid quantity of those impurities being very specific, any variation in these will cause unwanted and wide variations in the quality of the end products. Due to this paramount reason, it is utmost essential to remove these impurities or make them innocuous to enable utilization of the phosphogypsum in the development of different types of building materials. Attempts have been made in several countries to beneficiate phosphogypsum to obtain a useful industrial material. In Japan, Onoda Chemical Ind. Co. Ltd. investigated phosphogypsum to find its applications in wider perspective for cement and plaster industries to overcome acute problem on nonavailability of mineral gypsum in the country . Reference may be made to Fukuda,0nishi et al. Onoda Chemical Industry Co., Ltd.,; and Improving quality of phosphogypsum, Chemical Abstract,Vol.80, No.14, 1974 73777 v. Rhone Progil of France has developed a method of purification of phosphogypsum by passing it through series of hydrocyclones or through different flotation cells.Reference may be made to Calcium Sulphate, Franz Wirsching, Ulmann's Encyclopedia of Industrial chemistry, Vol. A 4, p. 571, 1985 . Two plants were set up at Les Roches de Candrien and Products and Rouen in seventies for the production of hemihydrate plasters respectively. Knauf, the West Germany Company Gebr Knauf. Westdeutsche gipswerke of Ipofen developed three different processes of purification and washing by floatation to eliminate water-soluble impurities and organic matter etc. Reference may be made to F. Wirsching, TIZ, Vol 105, 981, pp 383-389. In all these processes, phosphogypsum is first calcined to hemihydrate stage and then recrystalized with calcium hydroxide and water which converts hemihydrate into dihydrate, thus lime senstivity caused by the acid phosphate is eliminated and at the same time the particle structure changes which again help in overcoming the thixotropic behaviour of the plaster. CDF Chimie / Air Industries has developed a process for purification of phosphogypsum which is being used in one commercial plant at Douvrin, France. Reference may be made to J. Baron , B. Neven and D. Stegehrr,The new phospho plaster plant in Douvrin, near Arras/ France, Zement - Kalk - Gips , Vol. 30, No.8, 1977, pp 385-390. The process of purification chiefly consists of passing the gypsum slurry through series of hydrocyclones to remove soluble and organic impurities, vacuum filtered and the solid cake is dried in hot air device. Giulini process developed by Chemie Linz of Germany for beneficiation involves crystalization of dihydrate filter cake to autoclave plaster at 120°C after washing the phosphogypsum slurry in series of hydrocyclones. Reference can be made to The Giulini method for processing synthetic gypsum, Company Brochure, Gebr., Giulini GmbH, Ludwigshafen, West Germany, March 1967; Getting rid of phosphogypsum, I-IV, Phosphorous and Potassium, vol. 87, p. 37, 1977; Vo. 89, 1977, p 36; Vol. 94, 1978, p 24, and Vol. 96, 1978 p 30 . The plaster is eminently suitable for production of building blocks, partition panels, jointing and moulding plasters. Reference can be made to Building blocks from by-product gypsum by the Giulini Process , Phosphrous and Potassium, No. 37, September/ October, 1968, pp 26-28 .Processing of phosphogypsum by washing and wet sieving has also been developed . Reference can be made to R.K.Colling, Evaluation of phosphogypsum for gypsum products, Canadian Institute of Mining and Metallurgy Bulletin, Vol. 65, Sept. 1972, pp 41-51 . Simple water washing helps in extraction and removal of water-soluble phosphates and fluorides present in phosphogypsum. In wet sieving of the material through different sieves ( 65 and 100 mesh ), coarse fraction retained over the sieves, is rejected being rich in impurities whereas the fraction ( more than 90% ) passing through the selves are further washed and dried at 45^C, calcined and ground to a fineness of 3000-5000 cm2/g ( Elaine ) . The stucco/plaster produced is fit for use as raw material for making plaster boards an building blocks. A pilot plant of capacity one tonne/shift had been installed at the Central Building Research Institute, Roorkee for the purification of phosphogypsum. Reference may be made to Manjit Singh, Mridul Garg, C.L.Verma, S.K.Handa and Rakesh Kumar, Purification and utilization of down stream effluent from phosphoric acid production, National seminar on Emerging Challanges in Process Industries, University of Roorkee, Roorkee, pp V-8-v-II, 28-29 Sept. 1995. The plant is based on the wet aeiving of phosphogypsum through 300 micron sieve. Successful processing of phosphogypsum has been carried out in the plant. Several thermal treatments have been suggested for the purification of phosphogypsum. These techniques varied from calcination of phosphogypsum to hemihydrate or soluble anhydrite stage and subsequent washing with water to remove the released P205 & F.Reference can be made to Manjit Singh,S.S.Rehsi and C.A.Taneja Beneficiating phosphogypsum for the manufacture of gypsum plaster and plaster products, Indian Ceramics, Vol. 26, No. 1, April 1983, pp 3-8. In another treatment, phosphogypsum is calcined to hemihydrate stage and then it is neutralized with CaO, Ca(OH)2 and MgO, Mg(OH)2 solution. Reference may be made to Yamaguchi Taro, Take Takao and Sasano Tetsue, Improving quality of phosphogypsum, Chemical Abstract,Vol. 80, No. 1, 18824 h, 1974 ; Ehara Manabu and Matsuki takato, Gypsum from phosphogypsum, Chemical Abstract, Vol. 80, 136847 r, 1974 ; K. Miyajawa, Report on Meeting for Study on " Utilization of Chemical Gypsum for Portland Cement" , 1966 and Manjit Singh, Processing of phosphogypsum for the manufacture of gypsum plaster, Research and Industry, Vol. 27, June 1982, pp 167-169 .By treating the phosphogypsum plaster with Ca(OH)2/ the impurities of P20s and F are converted into insoluble & inert Ca3(PO4)2 & CaF2 compounds. The processing of phosphogypsum by heating it at 800 - 850°C is report whereby, the P2°s held up in gypsum lattice as CaHP04.2H2O is transformed into insoluble and inert Ca2P2O7 and the fluorides get voltalized. Reference can be made to L.A.Gudorich and B.I. Gurevich, Removal of fluorine from H3P04 acid CaS04.2H2O during preparation of insoluble anhydrite, Chemical Abstract, Vol.75, no. 14, 24756 a, 1971 and Manjit Singh, S.S.Rehsi and C.A.Taneja, Development of phosphogypsum anhydrite plasters, Zement-Kalk-Gips, Vol. 3, Nov. 1981, pp 597-598 . The chemical processing of phosphogypsum by extracting impurities with hot aqueous ammonium sulphate solution ( Manjit Singh, A chemical process for purifying phosphogypsum, Environmental Health, Vol. 25, No. 4 1983, pp 300-306 ) diluted ammonium hydroxide ( Manjit Singh, M.Garg, and S.S.Rehsi, Purifying phosphogypsum for cement menufacture, Construction and Building Materials (U.K.), Vol. 7, No. 1, 1993, pp 3-7) and with H2SO4 - SiO2 mixture ( Manjit Singh, S.S.Rehsi and C.A. Taneja, Rendering phosphogypsum suitable for plaster manufacture, Indian Journal of Technology, vol. 22, June 1984, pp 28-32 ) have been claimed.In case of hot aqueous (NH4)2SO4 and NH4OH treatments, the impurities of P205 and fluorides are converted into water removable mono and diammonium phosphates, ammonium fluoride, ammonium fluoroferrate and aluminate. Whereas on treating phosphogypsum with H2S04 - SiO2 mixture at 70°C, impurities of CaHP04.2H20 and Ca3(PO4)2 are converted into water soluble H3P04 and Ca(H2P04).2H20 compounds and the fluorides get changed into H2SiF6 on reaction with Si02. These processess are not economical because of involvement of costly corrosion resistant equipment and machineries. In the purification of phosphogypsum by wet sieving process, several steps such as mixing, screening, filtering, centrifuging and drying have to be followed. The process involves huge expenditure on plant and machinery as well as washing which makes it uneconomical vis - a vis to natural gypsum. Whereas thermal treatment of phosphogypsum transforms gypsum into hemihydrate or soluble anhydrite which are washed or neutralized with CaO/Ca(OH)2 leading to formation of Ca( P04 )2 and CaF2 compounds.These compounds form coatings on the surface of unhydrated hemihydrate plaster and thus suppress further conversion of hemihydrate into dihydrate gypsum leading to fall in strength of the plaster.On treating phosphogypsum with aqueous ammonium sulphate and ammonium hydroxide solutions , corrosion of plant and machinery may take place which will ultimately affect the economy of the process. Moreover, large volume of ammonium hydroxide solution of strength 10 - 20 % are required to treat phosphogypsum which will further add to the cost. Similarly during treatment of phosphogypsum with H2SO4 - Si02 , lot of free acidity is produced due to availability of free H2SO4 and HF acids which may result into immense rusting of the plant and machinery leading to high expenditure on the maintenance of the process as well as drop in production. The above drawbacks have been overcome in the improved process of the present invention. The main object of the present invention is to provide" an improved process for the purification of phosphogypsum" which obviates the use of costly equipment and is quite simple and economical as compared to the existing chemical processes. The improved process of the present invention consists of extracting impurities of P205,F, organic matter and alkalies present in phosphogypsum with aqueous citric acid solution,followed by washing of the leached impurities with a stream of water, drying the gypsum cake at 40-50°C to constant weight.The treatment of gypsum converts the phosphatic and fluoride impurities into water removable citrates, aluminates and ferrates. The purification of phosphogypsum with aqueous citric acid solution is feasible both technically as well as economically as compared to other chemical processes as this process does not require costly equipment and machinery as in the prior art process described above. The citric acid is easily available both as analytical compound as well as a by-product. Moreover, the concentration of citric acid recommended is not more than 5.0 percent.The treated phosphogypsum has been found to comply with the requirement of the properties of cements and plaster produced using the purified phosphogypsum. Accordingly, the present invention provides an improved process for the purification of phosphogypsum which comprises mixing thoroughly unprocessed phosphogypsum with 2.0 to 5.0 percent aqueous solution of citric acid in a mechanical shaker for a period of 15 to 25 hours to get a slurry, filtering by known method the slurry to obtain gypsum cake, washing the gypsum cake so obtained with 0.5 to 1.0 percent aqueous citric acid solution followed by thorough washing with water the for at least three times for a period of 15 to 20 minutes using ordinary tap water. Drying the resultant washed gypsum cake at a temperature in the range of 40 to 50°C to obtain purified phosphogypsum, the said process characterized in the steps of treating unprocessed phosphogypsum with 2 to 5% aqueous solution of citric acid. In the embodiment of the present invention the unprocessed phosphogypsum used may contain impurities such as PiOs in the range of 0.47 to 0.92 wt.%, F in the range of 0.44 to 0.86 wt.%, organic matter in the range of 0.11 to 0.65 wt.% and alkalis in the range of 0.27 to 0.46 wt%. In another embodiment of the present invention the citric acid used may be of commercial grade. In yet another embodiment of the present invention the thorough mixing may be effected for a period of 15 to 25 hours in a mechanical shaker. In still another embodiment of the present invention the filtering of the slurry may be done using a buckner funnel under vacuum. In another embodiment of the present invention the thorough washing with water of the gypsum cake may be effected at least three times for a period of 15 to 20 minutes using ordinary tap water. In yet another embodiment of the present invention the drying of the gypsum cake may be effected using drying means such as a drier chamber, incubator. The detailed process steps of the present invention are given below: First of all,the phosphogypsum sample whether produced by dihydrate or hemihydrate-dihydrate processes, is chemically analysed for various ingredients and subjected to Differential thermal analysis ( DTA ) to know the level of impurities in the phosphogypsum. The phosphogypsum sample is then mixed with 2.0 to 5.0 percent aqueous solution of citric acid in a mechanical shaker for a period of 15-25 hours whereby the impurities of P205 an The removal of impurities ( P2O5 an Reaction with Phosphates : 1. Monocalcium phosphate monohydrate, Ca(H2P04).H20 C6H807 + 3Ca(H2PO4).H20 Ca3(C6H507)2 + 2H3P04 Calcium citrate Phosphoric acid 2. Dicalcium phosphate dihydrate, CaHPO4.2H20 C6H807 + 3CaHP04.2H20 Ca3(CgH5O7)2 + H3PO4 + H2O Calcium citrate 3. Tricalcium phosphate, Ca3(PO4)2 C6H807 + Ca3(P04)2 Ca3(C6H507 )+ 2H3P04 Calcium citrate The phosphoric acid formed in the above reactions is water-soluble and could be easily washed with water.The other compound formed is calcium citrate which is less soluble in hot than cold water i.e. 0064g/100 ml in cold water which is too small and is therefore harmless even if it remains in the purified phosphogypsum, it will remain uneffected on calcination as it does not decompose at the temperature of calcination. Reaction with Fluoride 1. Sodium fluoride, NaF 2CgH807 + 3NaF Na3(C6H507)2 + HF Sodium citrate Hydrofluoric acid 2. Sodium silico fluoride, Na2SiFg 2C6H807 + 3Na2SiF6 Na3(C6H507)2 + 3H2SiF6 3. Sodium fluoro aluminate, Na3AlFg 2C6H807 + 3Na3AlF6 Na3(C6H507)2 + H2AlF6 4. Sodium fluoroferrate, Na3FeFg 2C6H807 + 3Na3FeFg Na3(C6H507)2 + 3H2FeF6 Sodium citrate 5. Calcium fluoride, CaF2 2C6H807 + 3CaF2 Ca3(C6H507)2 + 3HF Calcium citrate The sodium citrate formed in the above reaction is soluble in water and could be removed by washing it with water. The hydofluoric acid and hydroflualuminate, ferrate compounds formed are water soluble and could be washed with water easily. The extent of removal of impurities of phosphates and fluorides through the formation of above compounds has been confirmed by determining their solubility in aqueous citric acid solutions corresponding to these impurities (Tables 1 and 2). Table 1 - Solubility of Phosphatic Compounds in Aqueous Citric Acid Solution at 20°C (TableRemoved) Table 2 - Solubility of Fluoride Compounds in Aqueous Citric Acid Solution at 20°C (TableRemoved) Data confirm that with the increase in citric acid concentration, the solubility of phosphatic compounds also increases. Among phosphatic compounds, the solubility of Ca (H2PO4) 2 .H20 was found maximum, whereas in case of fluoride compounds, the solubility increased upto 4%. Maximum solubility of NaF was recorded among fluoride compounds. It has been observed that calcium citrate formed during the interaction of phosphogypsum with aqueous citric acid is not harmful as compared to CaHP04.2H20, Na2S04/ NaOH etc. formed during other chemical treatments ((NH4)2SO4/ NH4OH, H2SO4-Si02) which are deleterious and may adversely affect the hydraulic properties of calcined phosphogypsum. The following example is given by way of illustration of the present invention and should not be construed to limit the scope of the present invention. Example 1 Two samples of phosphogypsum designated as dihydrate and hemihydrate-dihydrate were procured from M/S Coromondal Fertilizers Ltd., Vishakhapatnam (Dihydrate process) and M/S Rashtriya Chemicals and Fertilizers Ltd., Mumbai (Hemihydrate-dihydrate process) respectively. These samples were chemically analysed for different constituents (Table 3) . Sample hemihydrate-dihydrate possesses Table 3 - Chemical Analysis of Phosphogypsum Samples (TableRemoved) higher CaS04.2H20 content than the gypsum sample dihydrate. After treatment with aqueous citric acid solution as per method described above, the level of residual impurities were determined and the results obtained are reported in Table 4. Data show sharp Table 4 Impurity Content after Citric Acid Treatment (TableRemoved) reduction in the impurities of P2°5' F/ or9an;'-c matter and alkali contents. The optimum concentration of aqueous citric acid solution has been found as 4.0 and 3.0 percent for the phosphogypsurn samples dihydrate and hemihydrate-dihydrate respectively. The level of impurities are within the maximum specified value given in 13:12679-1989. The same treatment can be applied to phosphogypsum obtained from other phosphatic fertilizer plants also. This novel process of purification is simple and economical as compared to other chemical treatments of the phosphogypsum involving treatment with hot aqueous (NH4)2SO4,NH4OH or H2SO4-SiO2 treatments.The inventive step resides in the treatment of unprocessed phosphogypsum, a by-product, with aqueous solution of citric acid. Example 2 Two more samples of phosphogypsum designated as dihydrate and hemihydrate-dihydrate were collected from M/S Albright Pandit Ltd., Ambernath and M/S Southern Petrochemical Industries Ltd.,Tuticorin respectively. These samples were chemically analysed for different constituents as shown in Table 5. It can be seen that phosphogypsum sample obtained from hemihydrate-dihydtate process contains higher CaS04.2H2O content and lower level of impurities than the phosphogypsum sample of dihydrate origin. After treatment with aqueous citric acid solution as per method explained above, the level of residual impurities were determined and the results are listed in Table 6. Results show reduction in impurities of Table 5 - Chemical Analysis of Phosphogypsum Samples (TableRemoved) Table 6 - Impurity Content after Citric Acid Treatment (TableRemoved) of P2O5' F/ organic matter and alkali contents.The optimum concentration of aqueous citric acid solution was 4.0 percent for both variety of phosphogypsum. The residual impurities ,however, complied with the maximum specified value of impurities laid down in IS : 12679 - 1989. The main advantages of the present invention are : 1. The process is suitable for the purification of phosphogypsum which helps in the removal of impurities of phosphates, fluorides, organic matter, alkalies etc. cotained in phosphogypsum. 2. The process is economical and cheaper as compared to the known processes of purification of phosphogypsum. 3. The process doea not require costly equipment and machinery which are essential for other chemical processes of the purification hitherto known. We Claim: 1. An improved process for the purification of phosphogypsum which comprises; mixing thoroughly unprocessed phosphogypsum with 2.0 to 5.0 percent aqueous solution of citric acid in a mechanical shaker for a period of 15 to 25 hours to get a slurry, filtering by known method the slurry to obtain gypsum cake, washing the gypsum cake so obtained with 0.5 to 1.0 percent aqueous citric acid solution followed by thorough washing with water the for at least three times for a period of 15 to 20 minutes using ordinary tap water, drying the resultant washed gypsum cake at a temperature in the range of 40 to 50°C to obtain purified phosphogypsum, the said process characterized in the steps of treating unprocessed phosphogypsum with 2 to 5% aqueous solution of citric acid. 2. An improved process as claimed in claim 1, wherein the unprocessed phosphogypsum used contains impurities such as P2O5 in the range of 0.42 to 0.92 wt.%, F in the range of 0.44 to 0.86 wt.%, organic matter in the range of 0.11 to 0.65 wt.% and alkalis - Na2O and K2O in the range of 0.27 to 0.46 wt.%. 3. An improved process as claimed in claims 1 and 2, wherein the citric acid used is of commercial grade. 4. An improved process as claimed in claims 1-3, wherein filtering of the slurry is done using a Buckner funnel under vacuum. 5. An improved process as claimed in claims 1-4, wherein the drying of the gypsum cake is effected using drying means such as a drier chamber, incubator. 6. An improved process for the purification of phosphogypsum substantially as herein described with reference to the examples.

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