Title of Invention

PROCESS FOR THE MANUFACTURE OF IMPROVED QUALITY PHOSPHORIC ACID

Abstract

ABSTRACT OF THE INVENTION This invention refers to a process for manufacture of substantially pure phosphoric acid low in F and other impurities from rock phosphates, wherein, the impurities present in rock phosphate, such as CaF<SUB>2</SUB>, days, micas, silica, silicates, organics, compounds of elements like As, Pb, Cd, Hg, are separated from the tricalcium phosphate component as first step by digesting the rock phosphate in a phosphoric acid medium under such operating conditions that the tricalcium phosphate and CaC0<SUB>3</SUB> components of rock phosphate are converted and dissolved as mono-calcium phosphate and the impurities are left unreacted, filtering the digested slurry to recover the mono-calcium phosphate - phosphoric acid solution, reacting the filtrate with 98.4 wt % sulphuric acid, stoichiometric to the Ca of the MCP formed to liberate phosphoric acid by precipitating Ca as gypsum dihydrate, filtering the gypsum and from the filtered phosphoric acid recirculating the quantity of digestion acid to perform next digestion cycle and transferring the balance to storage as production of phosphoric acid from the process of this invention.

Full Text

This invention refers to a process for manufacture of substantially pure phosphoric acid and gypsum from the mineral rock phosphate. The mineral rock phosphate, the principal source of raw material for commercial manufacture of phosphoric acid, after mining and size reduction, is beneficiated by a combination of physical methods to improve the concentration of tricalcium phosphate component of the rock. However due to limitations of physical methods of beneficiation, to minimise the loss of phosphate value in the tailings, the beneficiated rock generally contain 70 to 78 wt % of tricalcium phosphate, the balance being impurities such as: 1. Combination impurities: F as CaFz is the main impurity under this category. It is present in combination with tricalcium phosphate as calcium fluorapatite represented as Cai0(PO4)6F2 or 3Ca3(P04)z" CaF2« CaF2 component, therefore cannot be separated by physical beneficiation. 2. Accessory mineral impurities: Depending on the geographical location of the ore bodies, various types of clays, micas, silicates, silica, phosphates (other than calcium fluorapatite), carbonates, oxides, organics and compounds of elemetns like As, Pb, Hg, Cd, Zn, Ti are intimately associated with the calcium fluorapatite of the ore bodies and cannot be totally removed by physical beneficiation. 3. Substitution impurities: This category of impurities is due to substitution of Ca, P, F and 0 ions of fluorapatite with other ions of elements and radicals having ionic radii close to that of Ca, P, F and 0. Therefore such impurities cannot be separated by physical beneficiation. 4. Entrained impurities: Chemicals used during beneficiation of rock phosphate are to a certain extent adsorbed on the fluorapatite partides and also can be entrained in the beneficiated rock. In the known art process routes for manufacture of phosphoric acid from rock phosphate, based on xwef process principle, the rock phosphate is directly digested with H2S04 for a period of 3 to 4 hours using 3 to 3.5 wt % excess sulphuric acid at temperatures in the range of 70 to 100°C to liberate phosphoric acid from the tricalcium phosphate of rock by precipitating Ca as CaSO4V2l-l20 or as CaSCv 0.5H2O. However, in the presence of strong oxidizing sulphuric acid in excess and at operating temperatures of 70 to 100°C, the CaF2 as well as all other impurities of the rock phosphate undergo oxidation and complex chemical reactions. CaF2 is oxidised to HF and further converted to SiF4 or H2SiF6' Extensive purification steps are required to prevent escape of HF and SiF4 to atmosphere. The final reaction products of impurities are manifested as scales and sludges in the process equipment, as dissolved and suspended impurities in the phosphoric acid produced and as entrained impurities in the CaS04 by-product. The process streams are highly corrosive due to presence of free sulphuric add and flourine compounds. In the product phosphoric acid, the dissolved and suspended impurities together may be in the range of 15 to 30 wt % of P205 concentration of the acid produced. More than 85% F and all compounds of toxic elements such as As, Cd, Pb, Hg that may be present in the rock phosphate feed are transferred to the phosphoric acid produced. The acid on conversion to DAP, the toxic compounds are retained in the DAP and thereby contributing to the ecological problems with continuous use of such DAP for agricultural activity. The water solubility of DAP is also reduced due to fixing of the phosphate values as insoluble iron and aluminium phosphates. Accordingly the object of the present invention is to manufacture phosphoric acid with very low levels of F and other toxic elements for conversion to better purity fertilizer DAP as well as to manufacture high analaysis fertilizers after further concentration and purification of the acid. Additionally, the known art wet process routes restrict the rock phosphate feed to good quality with minimum 72% TCP. Availability of good quality rock phosphate deposits are limited and are subjected to gradual depletion. The present invention does not depend only on good quality feed. Even medium and low quality rock phosphate feeds, that are more abundantly available in nature can also be processed. Accordingly, another object of the present invention is to process rock phosphate feeds not only from good quality but medium and inferior quality rock phosphate deposits also. In the known art wet processes, sulphuric acid in excess is used for digesting the rock phosphate. Generally, the excess sulphuric acid used measures 3.0 to 3.5 wt/wt% in product phosphoric acid. Also Ca from CaF2 component of rock reacts with sulphuric acid. In the present invention neither excess sulphuric acid is required to be used nor CaF2 component reacts with the sulphuric acid. Accordingly, it is yet another object of this invention to reduce requirement of sulphuric acid for production of phosphoric acid. Gypsum by-product from the known art processes is not pure and difficult to dispose. In the present invention, gypsum is obtained with improved purity and also the quantity of gypsum generated is lower as all the impurities of the rock phosphate along with CaF2 component are separated before sulphuric acid is used in the process. Accordingly another object of this invention is to minimize the gypsum production as well as to improve the purity. The process of this invention is based on chemical properties of phosphoric acid as outlined hereunder: Phosphoric acid is tribasic due to the presence of three replaceable hydrogen atoms in its molecule. The acid forms mono, di and tribasic calcium phosphates with calcium hydroxide. Phosphoric acid converts tricalcium phosphate (TCP) and dicalcium phosphate (DCP) into monocalcium phosphate (MCP) as per chemical reactions MCP is sparingly soluble in water, the maximum solubility being 1.8% wt / vol at 25°C, However, MCP has significantly higher solubility in phosphoric acid solution and the saturation solubility is related to available P205 % in the acid. Phosphoric acid is a weak oxidising agent due to low ionisation constants for the three hydrogen atoms upto 60°C. The acidic property of phosphoric acid is manifested only in forming salts. Otherwise the acid is chemically inactive. It does not have oxidising property like sulphuric acid. It is nonreactive to rock phosphate impurities such as calcium fluoride, clays, micas, silicates, silica, organics and other compounds containing elements such as Pb, U, V, Sr, Cr, As, Cd. Carbonates in rock phosphate react with phosphoric acid with the liberation of carbondioxide. Calcium carbonate forms monocalcium phosphate as per the following reaction. Calcium fluoride and calcium sulphate are sparingly soluble in phosphoric acid, the saturation solubility is proportional to the free P205 concentration of the acid. When H2S04 is mixed with MCP-P2Os-H20 solution, precipitation of Ca of MCP as gypsum is rapid. Gypsum is precipitated in dihydrate form when the reaction temperature is less than 60°C. When MCP-P2O5-H2O solution also contain dissolved CaF2, the possibility of HF formation can be avoided by controlling H2S04 addition to a lower value than stoichiometric requirement by Ca of MCP. Accordingly, basing on aforesaid properties of phosphoric acid, the process of this invention is best accomplished in the following manner. Pulverized rock phosphate is first digested in phosphoric acid solution having P205 concentration between 20 to 40% wt / wt, preferably between 25 to 35% wt/wt and containing weight ratios of P205 in the digestion acid to the P2O5 in the rock phosphate at 7 to 25 : 1, preferably between 10 to 18:1. The digestion temperature is maintained between 30 to 60°C, preferably between 35 to 50°C. The monocalcium phosphate formed as per reactions 1 & 2 is instantaneously solubilized by the free P2Os in the digestion acid. Calcium fluoride component of fluorapatite remains in solid phase suspension along with other impurities of rock phosphate. The liberated carbon dioxide is recovered by suitable method. The digestion time is a function of particle size of rock phosphate and varies from 10 to 45 minutes. When rock phosphate is purlverized to pass atleast 80% through 100 mesh BSS screen, the preferred digestion time is 15 to 25 minutes for extracting and solubilizing more than 95% of TCP values along with calcium carbonate component of rock phosphate. The optimum selection of weight ratios of P205 in the digestion acid to the P205 in the rock phosphate is to provide enough P205 values for the formation of MCP as per reactions 1 & 2 and to solubilize the MCP in such a manner, the liquid phase MCP - P2Os - H2Q at the end of digestion is saturated with MCP atleast upto 70% preferably upto 90% of MCP saturation solubility value of the acid system. The slurry after digestion is filtered to separate MCP - P2Os - HjO liquid from the solid impurities. A traveling vacuum belt filter provided with cake washing facility is suitable for filtering the impurities. The washed cake with less than 1.0% entrained P205 and containing CaF2 and other impurities, generally in their native state, is either dumped or further processed in a separate facility to recover the CaF2 component or other valuable compounds of elements like strontium, titanium, uranium etc. depending on their concentration in the original rock phosphate and on the economies of recovery process. The MCP - P205 - H20 filtrate along with the cake washings is then acidulated with calculated quantity of 98.4% wtywt sulphuric acid to precipitate calcium from MCP as dihydrate gypsum as per reaction. The gypsum precipitation is performed between 30 to 60°C preferably between 35 to 50°C to precipitate the calcium sulphate as dihydrate gypsum. The precipation of gypsum from calcium of liquid phase MCP is rapid and require mixing for less than one minutesfollowed by holding the mixture for 5 to 20 minutes preferably for 10 to 15 minutes to complete precipitation of gypsum. The phosphoric acid - gypsum slurry is then filtered, preferably by a travelling vacuum belt filter and the gypsum cake is washed with calculated quantity of de-mineralized water to reduce the entrained P2O5 in the gypsum cake to 0.3 wt % on dry basis. The wash water is then combined with the main filtrate of phosphoric acid. From the combined acid, one part equivalent to digestion acid is recirculated to perform next digestion cycle and the other part containing P2O5 extracted from rock phosphate during digestion corrected with P2O5 losses in the washed impurities cake and gypsum cake and therefore represents production of phosphoric acid by the process of this invention. The phosphoric acid is significantly pure with F less than 0.05%, Fe less than 0.015%, Al less than 0.01%, total toxic compounds less than 0.001% and organics less than 0.005%. In the process of this invention heat is generated during rock phosphate digestion and H2S04 reaction with MCP whereas heat is released during filtration of impurities and gypsum with travelling vacuum belt filters. Additional cooiing of process streams is unlikely to be required. The main embodiment of the process for the present invention is explained in greater detail with reference to numerals in the schematic diagram in sheet number ONE accompanying the specification. Rock phosphate, pulverized to 80% passing BSS screen from storage bin (1) and digestion acid of P205 between 20 to 40% wt/wt, preferably between 25 to 35% wt/wt from storage tank (12) are simultaneously admitted into digester vessel (2) and agitated. The weight ratios of P205 values in the digestion acid to rock phosphate is maintained between 7 to 25:1, preferably between 10 to 18:1. The digestion temperature Is maintained between to 30 to 60°C, preferably between 35 to 50°C. During digestion, the liberated C02 gas is recovered via line (13) by a suitable arrangement. The digestion is performed with good mechanical agitation for 10 to 45 minutes, preferably for 15 to 25 minutes. The digested slurry is gravitated to filter(3), preferably travelling vacuum belt filter and the filtered impurities cake is washed with calculated quantity of de-mineralized water (4) with multi washing system to reduce entrained P205 in the washed cake to less than 1.0% P205 (on dry basis). The washed cake is discharged via (5). The wash water is combined with the MCP- P2O5-H2O filtrate and the combined solution is admitted to gypsum precipitator (7). Calculated quantity of concentrated (98.4% wt) H2S04 from storage tank (6) 15 dosed and gently mixed with the MCP- P2O5-H2O solution in tine gypsum precipitator for 1 to 3 minutes to precipitate calcium of MCP as gypsum dihydrate. The temperature during gypsum precipitation is maintained between 35 to 60°C, preferably between 40 to 50°C. The gypsum slurry is given a holding time of 5 to 20 minutes, preferably between 10 to 15 minutes for completion of gypsum precipitation and then filtered using travelling vacuum belt filter (8). The filtered gypsum cake is washed with calculated quantity of de-mineralized water (9) in a multi wash system to reduce the entrained P2Os in the washed gypsum to a maximum of 0.3% wt. (dry basis) and discharged via (10). The quantity of H2S04 to be added to MCP- P205-H20 solution is calculated to maintain residual calcium of MCP in the range of 2 to 8 mg per ml in the filtered phosphoric acid, preferably between 3 to 6 mg per ml, to avoid any possibility of HF formation from the dissolved CaF2 in the MCP- P205-H20 acid solution as well as to ensure absence of un-reacted H2S04 in the filtrate. The gypsum wash water is combined with the main filtrate. The combined acid contains sum total of P205 in the digestion acid together with P2Os extracted from rock phosphate digested corrected with entrained P205 in the washed impurities and gypsum cakes. From this combined acid, one portion equivalent to acid used for digestion is transferred to storage tank (12) for recycling to the digestor (2) and balance portion representing phosphoric acid produced from rock phosphate by the process of this invention is transferred to product storage tank (11). The residual MCP calcium from the product acid is removed as explained with reference to numerals in schematic diagram in sheet number TWO accompanying the specification. The product acid from (11) is transferred to another gypsum precipitator (14) into which a quantity of 98.4 wt % H2S04 stoichiometric to the MCP calcium to be removed is mixed. After a holding period of 10 to 30 minutes preferably 20 to 25 minutes, precipitated gypsum is separated by filter (15) and the final acid with gypsum washings with de-mineralized water is transferred to (16) as final phosphoric acid manufactured by the process of this invention. Example: The initial digestion acid for the process of this invention is prepared from 85% commercial pure phosphoric acid by dilution of the acid with de-mineralized water in the ratio of 40 parts by volume of acid with 60 parrs by volume of water. After first cycle of operations, the system acid gets saturated or equilibrated with sparingly soluble components calcium fluoride and calcium sulphate at different stages of the process. Also in the digestion acid at the completion of first cycle, MCP calcium at 3 mg per ml is maintained. Accordingly the given example refers to second cycle of operation for the process and the analysis of digestion acid is as given under Table-II (A). 1000ml of digestion acid is taken in a vessel fitted with stirrer and 100 grams of rock phosphate of composition as given under Table-I is added in a slow stream to the digestion acid. Rock phosphate is digested for 12 minutes. The initial and final temperature of acid measured 35 and 40°C respectively. After digestion, the slurry is filtered under vacuum. The residue cake is washed with 25ml de-mineralized water and the washings are added to the main filtrate. The combined filtrate measured 1036ml and the analysis is as given under Table-II(B). 44ml of 98.4% wt H2S04 is added slowly to the combined filtrate with slow stirring. The slurry, after 10 minutes standing, is filtered under vacuum. The acid temperature before and after gypsum precipitation measured 36 and 44°C respectively. The filtered phosphoric acid measured 970ml. The gypsum cake is then washed under vacuum with 100ml de-mineralized water and wash water is mixed with the filtered phosphoric acid. The combined volume measured 1075 ml having analysis as given under Table-II(C). From above 1075ml, 75ml of acid is removed as production of phosphoric acid and the balance 1000ml is available for performing next cycle of digestion. The washed impurities cake and gypsum cake after drying weighed 22 gms and 145 gms respectively. I claim, 1) A process for manufacture of phosphoric acid from rock phosphate, of quality superior to that obtained by conventional processes, wherein the tricalcium phosphate and calcium carbonate components of rock phosphate are solubilized and extracted as monocalcium phosphate (MCP) by digesting the rock phosphate in phosphoric acid medium, filtering the insoluble rock phosphate impurities from the slurry to recover the MCP-phosphoric acid extract, acidulating the extract with sulfuric acid to liberate phosphoric acid from MCP by precipitating calcium of MCP as gypsum; filtering the gypsum slurry to recover the phosphoric acid as filtrate, consisting of acid used for digestion together with phosphoric acid produced from the digested rock phosphate and recirculating from the filtrate the portion of digestion acid to continue further rock phosphate digestion and transferring the balance phosphoric acid to storage as production of phosphoric acid from rock phosphate by the processes of this invention. 2) A process according to claim 1, wherein, the phosphoric acid used for digestion contains P205 at concentration between 20 to 40% wt/wt, preferably 25 to 35% wt/wt and of quantity having weight ratio of P2O5 in the digestion acid to P205 in rock phosphate at 7 to 25:1 preferably 10 to 18:1 so as to provide at least 10% excess P205 than the P205 demand for formation and solubilization of MCP from the rock phosphate being digested. 3) A process according to claim 1, wherein, the digestion of rock phosphate with phosphoric acid is performed at temperatures between 30 to 60°C, preferably between 35 to 50°C to maintain the non-oxidising (inert) property of phosphoric acid and avoid chemical reactions between impurity components of rock phosphate and phosphoric acid. 4) A process according to claim 1, wherein, the slurry after completion of digestion is filtered to recover MCP-phosphoric acid extract as filtrate, washing the impurities cake with calculated quantity of water to recover the entrained extract acid from the cake and adding the wash water to the main filtrate to obtain total filtered extract. 5) A process according to claim 1, wherein, the MCP-phosphoric acid filtrate together with washings is acidulated with sulphuric acid of minimum 50% wt/wt concentration, preferably 98.4% wt/wt concentration, to precipitate calcium of MCP as dihydrate gypusm CaS04.2H20; using 98.4% wt/wt sulphuric add avoids entry of additional water into the system. 6) A process according to claim 1, wherein, sulphuric acid addition during the first cycle of operation is restricted to a residual MCP calcium of 3 to 12 mg/ml preferably 5 to 8 mg/ml of liquid phase and stoichiometric to calcium of MCP formed in subsequent cycles of operation in order the residual MCP calcium serves to improve the kinetics of gypsum precipitation and also avoids presence of free sulphuric acid in recycled phosphoric acid for digestion. 7) A process according to claim 1, wherein, the temperature during sulphuric acid acidulation is maintained less than 60°C, preferably between 40 to 50°C, in order the temperature during digestion stage with recycled phosphoric acid is within the preferred range. 8) A process according to claim 5, wherein, the sulphuric acid acidulation is performed with moderate agitation for a period of 3 minutes and the gypsum slurry is given a retention time of 5 to 20 minutes in the precipitator, preferably 10 to 15 minutes to complete the precipitation of gypsum as the kinetics of gypsum precipitation is reduced at low concentration of calcium and sulfate ions in the system. 9) A process according to claim 1, wherein, the gypsum-phosphoric acid slurry is filtered to recover the phosphoric acid, gypsum cake is washed with calculated quantity of water to recover entrained phosphoric acid in the gypsum cake and the wash water is combined with main phosphoric acid filtrate, the total quantity of acid thus obtained representing phosphoric acid used for digestion together with phosphoric acid obtained from digestion of rock phosphate. 10) A process according to claim 1, wherein, from the total filtered phosphoric acid, the portion of acid used for digestion is recirculated to the digestion step and the balance acid is transferred to storage as production from rock phosphate. 11) A process according to claim 10, wherein the residual MCP calcium from the portion of phosphoric acid produced from rock phosphate, is precipitated as gypsum dihydrate by treating with 98.4% wt/wt sulphuric acid of quantity stoichiometric to the MCP calcium. 12) A process according to claim 11, wherein, the gypsum slurry after a retention period of 30 minutes is filtered, the gypsum cake washed with calculated quantity of water, equivalent to water removed from the system as dihyrate gypsum, the wash water added to the phosphoric acid filtrate. 13) A process according to claim 1% wherein, the phosphoric acid filtrate together with washings is the product phosphoric acid, substantially pure as described in Example-I and illustrated in the flow diagrams No.l and No.2 accompanying the specification.

Documents:

1555-mas-98 abstract.pdf

1555-mas-98 claims duplicate.pdf

1555-mas-98 claims.pdf

1555-mas-98 correspondence others.pdf

1555-mas-98 correspondence po.pdf

1555-mas-98 description (complete) duplicate.pdf

1555-mas-98 description (complete).pdf

1555-mas-98 drawings.pdf

1555-mas-98 form-1.pdf

1555-mas-98 form-19.pdf