GB2026996A

GB2026996A - Monocalcium Phosphate and Phosphoric Acid Production

Info

Publication number: GB2026996A
Application number: GB7917161A
Authority: GB
Original assignee: Pennzoil Co
Current assignee: PennzEnergy Co
Priority date: 1978-08-02
Filing date: 1979-05-17
Publication date: 1980-02-13
Also published as: JPS5523087A; JPS5827207B2; DE2930825C2; IT1194869B; DE2930825A1; GB2026996B; FR2432477B1; ZA792199B; CA1115483A; IN150442B; ES480523A1; IL57301A; BE876325A; BR7903045A; FR2432477A1; IT7912625A0; MA18439A1

Abstract

Monocalcium phosphate, phosphoric acid and/or potassium phosphate containing fertilizers are produced in a process involving acidulation of phosphate rock with phosphoric acid in the presence of added silicon dioxide and potassium ion whereby fluorides contained in the rock are converted to K2SiF6 and monocalcium phosphate dissolved in phosphoric acid is formed during acidulation. In an important feature, the K2SiF6 is separated and hydrolyzed to regenerate the K2O from K2SiF6 recycled KH2PO4/H3PO4 solution for further reaction with fluoride from fresh phosphate rock feed. A portion of the MCP/H3PO4 solution and/or crystallized monocalcium phosphate can then be reacted with potassium sulphate, potassium bisulphate, or mixtures thereof to form KH2PO4, or KH2PO4/H3PO4, solutions, and gypsum. In a closely related embodiment, the remaining MCP/H3PO4 solution is reacted with sulphuric acid to produce phosphoric acid product and/or the recycle phosphoric acid required in the phosphate rock acidulation step.

Description

SPECIFICATION Process for the Preparation of Monocalcium Phosphate and Phosphoric Acid This invention relates to a process for the preparation of monocalcium phosphate and phosphoric acid.

More particular, this invention relates to a process for the production of monocalcium phosphate and phosphoric acid by the acidulation of phosphate rock with phosphoric acid.

Phosphoric acid plants are currently operated utilizing a basic and well known process for the acidulation of phosphate rock which comprises reaction of the rock with sulphuric acid to form phosphoric acid and subsequent reaction of the phosphoric acid, with for example ammonia, to produce monoammonium phosphate (MAP) and diammonium phosphate (DAP). The phosphoric acid formed in this process is called wet process phosphoric acid.

In this reaction, a byproduct is gypsum having the chemical formula CaSO4. 2H20. Essentially, all phosphate rock contains some fluoride, normally in the 3.0 to 4.0% range, and the acidulation reaction usually generates gaseous fluorides. Because of the fluoride content, an important problem in the operation of these wet process phosphoric acid plants has been the expensive methods for handling the large amounts of fluorine compounds which are liberated in the gaseous and aqueous effluents. It is only in recent years that studies have been made on the effects of fluorides contained in the final product and indications seem clear that they may have a deleterious effect on the long term producing ability of the soil when present in fertilizers.We have, therefore, sought to produce relatively pure phosphoric acid and relatively pure monocalcium phosphate which are essentially free of fluorides, iron, aluminium, magnesium and other impurities, in such a manner as to eliminate or greatly reduce K20 losses and concentrate insoluble fluoride compounds in recoverable form so that they can be processed for fluorine-and K20-recovery and reuse, and so that contamination of the environment and final products by the presence of fluorine compounds is minimized.

Thus, the present invention provides a process for the production of monocalcium phosphate and phosphoric acid which comprises acidulating phosphate rock with an excess of phosphoric acid in the presence of added silicon dioxide and alkali metal ion to produce a first slurry of insoluble alkali metal fluosilicate in a solution of monocalcium phosphate in phosphoric acid; separating the resulting slurry to produce a clarified solution of monocalcium phosphate in phosphoric acid and a second slurry comprising monocalcium phosphate in phosphoric acid which contains insoluble alkali metal fluosilicate; hydrolysing the second slurrv at an elevated temperature to regenerate an alkali metal dihydrogen phosphate/phosphoric acid solution and to produce calcium fluoride and silicon dioxide; recovering the calcium fluoride and silicon dioxide and recycling the alkali metal dihydrogen phosphate/phosphoric acid solution to the acidulation reaction.

According to one aspect the present invention provides a process for the acidulation of phosphate rock and the production of phosphoric acid and mono-calcium phosphate which may subsequently be converted to potassium dihydrogen phosphate, a valuable fertilizer, as well as the recovery and isolation of the fluoride compounds initially as K2SiF6 and ultimately as calcium fluoride. A particularly preferred embodiment of the process according to the invention comprises the acidulation of phosphate rock in the presence of added silicon dioxide and recycle phosphoric acid which contains potassium ions, the reaction being conducted to produce monocalcium phosphate in phosphoric acid solution while converting the fluorides to insoluble potassium fluosilicate.The resulting slurry is then thickened to produce a clarified solution of monocalcium phosphate in phosphoric acid and a concentrated suspension comprising monocalcium phosphate in phosphoric acid which will contain the slimes and fluosilicate insolubles from the reaction. The concentrated suspension of the fluosilicates which still contains monocalcium phosphate and phosphoric acid, is hydrolyzed to regenerate K20 for recycle in the process of fluoride elimination in the acidulation of phosphate rock The clarified monocalcium phosphate/phosphoric acid solution may then be reacted with K2SO4, KHS04, or mixtures thereof to produce potassium dihydrogen phosphate and phosphoric acid.A major portion of the monocalcium phosphate/phosphoric acid solution is reacted with sulphuric acid to precipitate calcium sulphate hydrate which is removed from the system, and phosphoric acid, a portion of which may be removed as product, with the balance being recycled to the acidulation reactor as determined by material balance considerations.

As indicated above, this invention is concerned with a multi-step process for the preparation of essentially fluoride-free products, that is alkali metal phosphates and phosphoric acid, by acidulation of phosphate rock, which procedure is conducted in the substantial absence of fluorine pollution and from which the fluorides may be recovered in usable form, and wherein phosphoric acid may be regenerated for reuse in the system and/or separated as product. As is known, most of the commercially important phosphate ores mined in the United States and particularly those mined in Florida, contain 34% fluorine after benefication. The fluorine is a constituent of fluoapatite which is commonly expressed as Cå9(PO4)8 CaF2 and may also be present as calcium fluosilicate (CaSiF6).Silica is a component of phosphate rock and is usually abundant in most grades of rock that are commonly used in the production of wet process phosphoric acid. In conventional processes the fluorine compounds in the phosphate rock react with sulphuric acid during the attack on the rock so that the fluorine appears in vapor form as hydrofluoric acid (HF), silicon tetrafiuoride (SiF4), or other gas, and in the phosphoric acid solution as fluosilicic acid (H2SiFó) and/or fluosilicate salts or other forms.

Acid from a rock low in reactive silica may also contain free hydrogen fluoride. The present invention provides a significant solution to problems of fluoride pollution by means of a process which minimizes fluoride evolution while recovering substantially all of the fluorides in usable form thereby preventing the fluorides from contaminating the environment and also the desired products. The present invention also provides a series of substantially purer and useful products as well as novel procedures for obtaining these products without pollution.

According to one aspect, the process of this invention is concerned with the preparation of alkali metal phosphates and/or phosphoric acid in a particular embodiment of the invention, the alkali metal phosphate is an alkali metal dihydrogen phosphate. A preferred product is KH2PO4 and/or its admixture with phosphoric acid, which contain high plant food nutrients, and is highly valued as a fertilizer.

NaH2PO4, an alternative product, is widely used in the detergent industry and other areas. However, potassium products are preferred and the reaction is particularly described with respect to potassium reactants and products. The process of the present invention is carried out in a continuous manner in the optimum embodiment.

In the initial step of the process of this invention, phosphate rock form any origin, but usually of the type described above containing at least some fluorides, is acidulated with a solution of phosphoric acid containing potassium ion recycle values at a temperature from room temperature up to about 950C., and preferably about 700 to 900C., for a sufficient time to achieve substantially complete acidulation, usually about 1/2 to 4 hours depending on the reaction temperature and using a sufficient amount of the phosphoric acid solution completely to solubilize the calcium phosphate formed.

Sufficient potassium ion is present in the mixture to cause precipitation of the fluorides as a precipitate, primarily as K2SiF6 together with SiO2 and impurities. In the preferred embodiment, the potassium ion values are provided by KH2PO4 salts contained in recycle phosphoric acid solution.

In conducting this initial step, the phosphoric acid solution is utilized in sufficient excess to effect substantially complete acidulation and solubilization of the calcium in phosphate rock. The P205 content of the phosphoric acid should range from about 2055% and preferably about 25 40% by weight. In general, there should be used an excess of phosphoric acid and preferably about 35 to 90 moles of phosphoric acid for each 6 moles of phosphate in phosphate rock, or a molar ratio of P205 in the acid to P205 in the rock, of about 6:1 to 15:1, respectively.Also, about 1.0 to 10 moles of K20, preferably more than about one mole, to provide a slight excess of K20, should be present for each three moles of phosphate rock of the formula Ca(PO4)6 Ca CaF2. The K20 or potassium ion is preferably added as KH2PO4- As pointed out, the phosphoric acid is present in sufficient amounts to solubilize the calcium phosphate contained in the phosphate rock. Further, the K20 values such as the KH2PO4 salt are contained in the phosphoric acid in a sufficient amount to precipitate the fluorides present as dense crystalline solids which may be recovered readily. Thus, during the acidulation step, while the calcium phosphates are solubilized, there is precipitated a mixture of solids from which the fluorides may be recovered. This precipitate contains the fluorides essentially as K2SiF6.

It is to be appreciated that the phosphoric acid as the treating acid is to be distinguished from the stronger mineral acids such as sulphuric acid, nitric acid, hydrochloric acid, and the like. As is shown in many standard reference books, phosphoric acid has a weaker ionization constant than stronger mineral acids. By use of the term phosphoric acid, it is meant that it is an acid that is ionized at less than 90% at a strength of concentration of 0.1 Normal, and has an ionization constant of no more than 7.5x 1 0-. In conducting the initial step of the reaction, the phosphate rock and phosphoric acid are reacted in the presence of reactive silica. There is also present a recycle solution comprising a solution of potassium dihydrogen phosphate and phosphoric acid.In general, there is sufficient potassium ion and reactive silica present in this initial reactor to convert fluorides contained in phosphate rock to potassium fluosilicate.

The silica added during the reaction of this invention may be amorphous silicon dioxide in any suitable form so long as it is not deleterious to the reaction under consideration. The silicon is preferably obtained from materials combinable with the phosphate rock, such as slag, or commercially available products such as those sold under the tradename "Dicalite", sold by Grafco Corporation.

The product resulting from the initial reaction comprises a relatively low concentration of suspended solids (e.g., in the range of from 3 to 15 wt.%), in a solution of monocalcium phosphate and phosphoric acid. This mixture is preferably passed to a thickener for separation of the solids and solution to produce a clarified monocalcium phosphate solution. This clarified monocalcium phosphate may then be treated as described herein to produce phosphoric acid and/or potassium dihydrogen phosphate.

An important feature of the invention is the utilization of the calcium ion from phosphate rock to remove fluorides as 3CaF2 and/or 3CaF2/SiO2 and thereby eliminate the need of using an external source of calcium such as limestone. While the potassium ion is a critical component of this sytem, it is not consumed, but simply recycled to perform the required fluoride removal function. As a consequence, the cost of K20 in fluoride removal is no longer a significant factor since only makeup K20 will be needed as governed by losses.

It is also within the scope of the invention to utilize an external source of phosphoric acid and/or an external source of K2SiF6 in the initial acidulation reaction. However, in the preferred embodiment, recycle of these materials is especially preferred for purposes of economics.

The underflow, when a thickener is used, is a slurry of monocalcium phosphate/phosphoric acid solution which contains the fluorides, usually as potassium fluosilicate, and any slimes. A feature of this invention is that this mixture is hydrolyzed, preferably by heating at 100-11 50C. Or up to the reflux point, to form potassium dihydrogen phosphate in phosphoric acid and convert the fluorides to calcium fluoride and silicon dioxide.

As shown, this hydrolysis reaction proceeds as illustrated by the following equation: 3Ca(H2PO4)2+ 1 oH3Po4+K2SiF6+R2o3/P2o5+2H2oo SiO2/3CaF2+R203/P20s+2KH2PO4+ 1 4H3PO4 wherein R is a metal such as Fe or Al.

As may be seen from this equation, the fluorides, in the form of K2SiF6, are converted to SiO2/3CaF2 as a solid in admixture with Al203, Fe203 and the like. This solid mixture is separated from the solution of 2KH2PO4+1 4H3PO4 and valuable fluorides may be recovered from the solids as described herein.

The resulting solution is suitable for recycle to the system to provide at least a portion of the potassium ion necessary to produce additional potassium fluosilicate and also provide a source of phosphoric acid. As a result, some of the SiO2 and K20 are not consumed in the reaction but rather are recycled in the continuous process. It is, of course, to be understood that additional amounts of potassium ion and SiO2 from external sources may be added to the acidulation reactor as may be required by the system. An external source of phosphoric acid may also be used.

In one embodiment, a portion of the resulting clarified monocalcium phosphate and phosphoric acid solution is reacted with potassium sulphate, potassium bisulphate or a mixture thereof to produce KH2PO;H3PO4 solutions from which KH2PO4 may be recovered as a fertilizer grade material.

Phosphoric acid may also be produced in this embodiment and may be recovered or recycled as makeup phosphoric acid.

The remaining monocalcium phosphate/phosphoric acid solution is reacted with sulphuric acid to produce calcium sulphate hydrate which may be recovered and the phosphoric acid regenerated as a result of this reaction may be recovered as product and/or recycled to the main reactor to effect acidulation of the phosphate rock feed.

The essential steps described above for the reaction provide a number of advantages in the process. Thus the process serves to regenerate valuable hydrogen ions as illustrated by the following equation: Recycle K2SiF6+3Ca(H2P04)2+ 1 0H3P043CaF2+ Si02+2KH2PO4+ 1 4H3PO4 Thus the phosphoric acid concentration increases from 1 Oto 14 moles or an increase of 40%. More importantly, this 14 moles of free H3PO4 can now accommodate additional unreacted phosphate rock.

In effect, approximately 3Ca0/30Ca0 or 10% of the original rock feed can be acidulated in this manner; e.g.

Ca3(PO4)2+ 1 4H3PO4=3Ca(H2PO4)2+ 1 OH3PO4.

The process of the invention also removes unreacted phosphate rock from the acidulation reaction and subjects this rock to much more vigorous acidulation conditions to provide: a) increased phosphate acid concentration as illustrated above and b) increased temperatures from 80-900C. The process accomplishes these functions using a relatively modest defluorination/hydrolyses loop which is only 10% of the main loop or system. Further, it permits recovery of the considerably more dense fluorspar component and will also separate unhydrolyzed K2SiFs with CaF2. In this instance, subsequent treatment with NH40H can be utilized to produce a chemical grade fluorspar.The process also eliminates the R203 component after removal of the dense CaF2/K2SiF6 components-preferably by the addition of clean gypsum to assist in the separation (centrifuge) step and to simulate the 0-20-0 NSP grade. The 1 101 1 50C temperatures involved in hydrolysis will help flocculate the R203 component and simplify separation. Reference is now made to the accompanying drawings in which figure 1 is a flow sheet illustrating one embodiment of the process according to the invention and figure 2 is a flow sheet illustrating another embodiment of the process. With reference to figure 1, phosphate rock from line 1 and phosphoric acid from line 2 are reacted in acidulation reactor 3.

The reaction is conducted at a temperature in the range of about 40-950C. and the materials are reacted utilizing an excess of the phosphoric acid. The phosphoric acid contains potassium, usually added as KH2PO4, in sufficient amounts to react with fluoride contained in the phosphate rock and produce potassium fluosilicate. In addition, reactive silica is added by line 4 to provide sufficient reaction with potassium to form the potassium fluosilicate. In the acidulation reactor 3, monocalcium phosphate is formed as a solution in phosphoric acid with an insoluble precipitate comprising slimes and a portion of the potassium fluosilicate. Sufficient phosphoric acid is present to dissolve the monocalcium phosphate.

The reaction mixture is then passed by line 5 directly to a defluorination reactor or thickener 6 for removal of the fluorides.

In the defluorination thickener 6, a product or underflow is removed which is a slurry of potassium fluosilicate, SiO2, slimes and other solids in a solution of monocalcium phosphate in phosphoric acid. In accordance with particularly preferred embodiment of the invention, the potassium fluosilicate in the slurry is withdrawn by line 7 to a hydrolyzer 8. The hydrolysis reaction in the hydrolyzer 8 is conducted by heating at a temperature in the range of 100--1 50C. or at the reflux point of the system preferably by introduction of steam at 9, to convert the potassium fluosilicate to silicon dioxide, calcium fluoride, and potassium dihydrogen phosphate and/or phosphoric acid using monocalcium phosphate.The resulting mixture is passed by line 10 to a separator 11 where calcium fluoride and some silicon dioxide are recovered at line 1 2. In a preferred embodiment, the mixture from separator 11 is passed to separator 1 5 by line 13 after addition of a suitable amount of gypsum by line 13. Thereafter, there is recovered from separator 1 5 an 0-20-0 fertilizer by line 1 6 which contains most of the R203 components. or slimes.The gypsum is added primarily as substrate to provide a filterable solid 0-20-0 (N-P-K) product, and to facilitate the separation of slimes from the solution the separator 15. KH2PO4/H3PO4 solution, which may contain some SiO2, is then recycled by line 17. While the bulk of the R203 is removed here, it can also be expected that portions will be removed with other products.

In the meantime, the overflow or solution from the defluorinator or thickener 6 is recovered in line 1 8 as a solution of monocalcium phosphate in phosphoric acid. This product may be processed by any of several methods to recover valuable products, including monocalcium phosphate, phosphoric acid including recycle H3PO4, and gypsum, all of which are substantially free of fluoride contamination.

As a result of this process, there is recovered from the defluorinator 6 by line 1 8 the product from the reaction of this invention. This reaction product comprises a solution of monocalcium phosphate in phosphoric acid, which is a valuable reaction product of high quality substantially free of fluoride contamination. This product solution may be treated by various alternative processing techniques to recover monocalcium phosphate and/or phosphoric acid, which products may also be converted to other valuable products including KH2PO4 and recycle phosphoric acid. Preferred further processing techniques are shown in Figure 2.

In the embodiment illustrated in Figure 2, the monocalcium phosphate/phosphoric acid solution product from line 1 8 is passed to intermediate storage 1 9 where the stream may be divided into two portions for further processing. The division of the MCP/H3PO4 stream at this point may be in a desired ratio, e.g., about 40 to 60 wt. % of the stream may be removed, and processed to recover KH2PO4/H3PO4. in this aspect, a portion of the stream is withdrawn by line 20 and passed to a reactor 21. In the reactor 21, the stream is reacted with a potassium sulphate reactant such as potassium sulphate, potassium hydrogen sulphate or a mixture thereof, added by line 22.The potassium sulphate reactant may be added as a solid or aqueous solution and is added in sufficient stoichiometric amounts to react with all the monocalcium phosphate present. As necessary, for solution purposes, water may be added by line 23. This reaction is conducted at a temperature of about 50 C to 1 0O0C. with agitation.

In the reactor 21, the monocalcium phosphate and potassium sulphate react to produce potassium dihydrogen phosphate as product together with gypsum and phosphoric acid as illustrated by the following equation when the reactant is potassium sulphate: 4H20 Ca(H2PO4)2+K2sO4 > 2KH2PO4+CaS04 2H2O (YH3PO4) where Y is the amount of phosphoric acid in the system.

The resulting reaction slurry is then transferred by line 24 to a separator or filter 25 and a solution of KH2PO4 in phosphoric acid is removed by line 26 and the gypsum is removed by line 27. The solid filter cake is washed by water from line 28 and the wash water may be recycled by line 29 to the reactor 21.

The product recovered at line 26 contains potassium dihydrogen phosphate and has a fertilizer value of 0-24-6. The KH2PO4 may be recovered from this solution by evaporation and precipitation with a water miscible solvent such as methanol or extraction with a water immiscible solvent such as butanol.

In the meantime, the other portion of the clarified monocalcium phosphate/phosphoric acid solution from intermediate storage 1 9 is passed by line 30 to a crystallizer 31 and reacted with at least a stoichiometric amount of sulphuric acid from line 32. The sulphuric acid reacts with the MCP/H3PO4 solution to produce phosphoric acid and calcium sulphate hydrate and this slurry is passed by line 33 to thickener 34 wherein concentration of the slurry is achieved and the underflow slurry is then passed by line 35 to filter 36. The solid calcium sulphate hydrate is substantially pure form is recovered by line 37.

After removal of the calcium sulphate hydrate, the phosphoric acid solution/filtrate is transferred by line 39 to evaporator 40 where water is removed from the system at 41 as required. The remaining phosphoric acid may then be recovered as product by line 42 or may be combined with line 38 overflow from thickener 34 via dotted line 43 to meet the recycle phosphoric acid needs of line 2 in the phosphate rock acidulation carried out in the reactor 3.

In another embodiment of the present invention (not shown), the monocalcium phosphate/phosphoric acid solution may be processed to recover solid monocalcium phosphate from the phosphoric acid and each product may then be recovered or further processed. In one aspect, the monocalcium phosphate/phosphoric acid clarified solution from defluorinator 6 is passed to a crystallizer.

Up to this point, the monocalcium phosphate/phosphoric acid solution has been maintained at a temperature in the range of 8S95 C. to maintain the solution. However, in the crystallizer, the solution is cooled via evaporation to about 25--55"C., preferably about 40 C., to cause crystallization of solid monocalcium phosphate from the phosphoric acid solution. Therefore, it is preferred that the mixture be cooled by a temperature difference of about 35-550C. The resulting slurry is then passed from the crystallizer to a separator where a separation is effected between solid monocalcium phosphate and the mother liquor MCP/H3PO4.The solid monocalcium phosphate from the separator is then passed, for example to reactor 21, wherein reaction is carried out with a potassium sulphate reactant such as potassium sulphate, potassium hydrogen sulphate, or a mixture thereof as described above for the MCP/H3PO4 solution. In the reactor 21, the monocalcium phosphate and K2SO4 and/or KHSO4 reactant produce potassium dihydrogen phosphate and/or phosphoric acid as a product together with gypsum. The resulting mixture is then filtered and the gypsum removed by line 27. The product recovered at line 26 is an aqueous solution of potassium dihydrogen phosphate and/or phosphoric acid. This solution mav be further processed into desired products.

In this reaction, the monocarcium phosphate reacts with the potassium sulphate or potassium hydrogen sulphate as illustrated by the following equations: a) 8Ca(H2PO4)2+8K2SO4o1 6KH2PO4+8CaSO4 2H2O b) 8Ca(H2PO4)2+8KHSO4e8KH2PO4+8H3PO4+8CaSO4 2H2O In reaction (a) with K2SO4, the KH2PO4 product is a liquid (0--1 5--10 fertilizer which may be further concentrated, and in reaction (b) with KSSO4, the KH2PO,/H2P04 product is a liquid 0--244-8 fertilizer.

In the meantime, the MCP/phosphoric acid from the separator is passed to the calcium sulphate hydrate crystallizer and reacted with sulphuric acid to produce phosphoric acid product and/or recycle mother liquor and calcium sulphate hydrate as described above for the process of Figure 2. This reaction for recycle is illustrated by the following equation: 1 9Ca(H2P04)2+90H3P04+ 1 9H2SO4+38H2)e1 28H3P04+ 1 9CaSO4 2H2O The "1 28H3PO4" portion represents the phosphoric acid available for recycle.

It will therefore be understood that this approach also leads to valuable fertilizer products and recycle phosphoric acid.

The following Examples are presented further to illustrate the invention. In these Examples and throughout the specification, parts are by weight unless otherwise indicated.

Example I 1,278 grams (=9 moles) P205 in phosphate rock are reacted with 10,224 grams (=72 moles) P2Os as 35% recycle phosphoric acid for a P205 (acid)/P205 (rock) weight ratio of 8/1. This reaction mixture provides enough excess phosphoric acid to dissolve essentially all of the calcium in the phosphate rock as monocalcium phosphate wherein the P205 / CaO weight ratio should approach 6.75/1. The acidulation reaction is conducted at 80-900C. and contains a minimum of 1 mole of K2O and sufficient external reactive silica (SiO2) to remove substantially all of the fluoride as insoluble potassium fluosilicate. Sand, some R203 slimes and unreacted phosphate rock also remain insoluble.

Small amounts (up to 3 4 ppm) of a flocculating agent such as Nalcalite 670 are helpful in the settling the solids from this system.

This thin reaction slurry, still at 900C., is then separated via a decanter/thickener (separatory funnel may be used in the laboratory) wherein approximately 10% of the MCP/H3PO4 solution remains with the underflow insolubles. The now thickened slurry, is directed into the hydrolysis sector wherein the temperature is raised to 110-11 50C., e.g., by use of low pressure steam. Under these conditions, the hydrolysis reaction is essentially completed in 1 to 2 hours. The slurry now contains dense crystalline fluorspar (Ca F2) which is readily separated from the unreactive but somewhat flocculated R203/P2Os components by, for example, a hydraclone or by suitable gravity separation means.

Sufficient clean gypsum is then added to the remaining finely dispersed R203/P2Os to achieve a O- 20-0 grade fertilizer which simulates NSP. This requires approximately 3.64 grams of CaSO4 per gram of P205 slimes to be recovered. The R203/P205 component has already been flocculated/coalesced to a considerable degree during the 110-11 50C. hydrolysis step. However, the utilization of clean gypsum provides additional substrate so that separation of this material presents no undue difficulties. The product is readily separated via suitable means, e.g., a centrifuge or a precoat filter.

After separation of the solids, the remaining solution of 2KH2PO4+1 4H3PO4, which also contains a small amount of silicon dioxide, is recycled to the acidulation reactor as regenerated phosphoric acid containing potassium ion.

Example II The clarified monocalcium phosphate/phosphoric acid overflow from the K2SiF6 thickener is thus passed to a crystallizer wherein the temperature is lowered to 400 C. to crystallize monocalcium phosphate. The solid monocalcium phosphate and the remaining MCP/H3PO4 solutions are then separated via a filter, centrifuge or other separator. The solid monocalcium phosphate is removed and reacted with a stoichiometric amount of potassium hydrogen sulphate in an aqueous medium at a temperature of 900 C. In this reaction, the monocalcium phosphate is converted to KH2PO4+H3PO4 and gypsum. The gypsum is removed and the KH2PO4H3PO4 liquor separated and recovered as a 0--244-8 fertilizer solution.

The phosphoric acid solution still containing monocalcium phosphate from the separator is reacted with sulphuric acid in stoichiometric amounts at 850C. to produce calcium sulphate hydrate which crystalizes from solution. This solid is then filtered and removed from the system. The resulting phosphoric acid is then recycled to the acidulation reactor.

Example Ill In an alternative reaction, the solid monocalcium phosphate is reacted with potassium sulphate to yield primarily KH2PO4 with little or no H3PO4 coproduct. Conversely, if a portion of the (uncrystallized) MCP/H3PO4 liquor is reacted with potassium sulphate the resulting KH2PO4/H3PO4 solution will have a plant food value of 0--244-6. A portion of any of the K2O products may be recycled back to the acidulation vessel to provide makeup for the K2O lost in the hydrolysis sector.

Claims

1. A process for the production of monocalcium phosphate and phosphoric acid which comprises acidulating phosphate rock with an excess of phosphoric acid in the presence of added silicon dioxide and alkali metal ion to produce a first slurry of insoluble alkali metal fluosilicate in a solution of monocalcium phosphate in phosphoric acid; separating the resulting slurry to produce a clarified solution of monocalcium phosphate in phosphoric acid and a second slurry comprising monocalcium phosphate in phosphoric acid which contains insoluble alkali metal fluosilicate; hydrolysing the second slurry at an elevated temperature to regenerate an alkali metal dihydrogen phosphate/phosphoric acid solution and to produce calcium fluoride and silicon dioxide; recovering the calcium fluoride and silicon dioxide and recycling the alkali metal dihydrogen phosphate/phosphoric acid solution to the acidulation reaction.

2. A process as claimed in Claim 1, wherein the alkali metal ion is potassium.

3. A process according to Claim 1 or 2, wherein the acidulation of the phosphate rock is carried out at a temperature in the range of from 25 to 950C.

4. A process according to Claims 2 or 3, wherein the mixture is recovered from the acidulation reaction is separated in a decanter/thickener to produce an overflow comprising the clarified solution of monocalcium phosphate in phosphoric acid and an underflow slurry of K2SiF6 in a solution of monocalcium phosphate and phosphoric acid.

5. A process according to Claim 4, wherein the underflow is subjected to hydrolysis by heating at a temperature in the range of about 950C. to the reflux temperature of the system to convert the K2SiF6 to calcium fluoride and silicon dioxide.

6. A process according to any of Claims 1 to 5, wherein the calcium fluoride is initially separated from the hydrolysis reaction product, gypsum is added to the remaining reaction product, a solid 0 20-0 fertilizer is removed, and wherein the remaining solution is recycled to the acidulation reaction.

7. A process according to any of Claims 2 to 6, wherein from 1.0 to 10 moles of potassium ion are present in the acidulation reactor for each three moles of phosphate rock.

8. A process according to Claim 7, wherein the potassium ion is added as KH2PO4.

9. A process according to any of Claims 1 to 8, wherein the monocalcium phosphate and phosphoric acid clarified solution is cooled to precipitate at least a portion of the monocalcium phosphate as a solid product, and wherein the remaining solution of monocalcium phosphate and phosphoric acid is separated.

10. A process according to Claim 9, wherein the solid monocalcium phosphate is reacted with potassium sulphate, potassium hydrogen sulphate, and/or a mixture thereof to produce KH2PO4, KH2POJH2PO4 mixtures and gypsum.

11. A process according to Claim 9, wherein the solid monocalcium phosphate is reacted with K2SO4 in aqueous medium to produce KH2PO4 and gypsum, or the solid monocalcium phosphate is reacted with KHSO4 in an aqueous medium to produce a solution of KH2PO4 in H3PO4, and solid gypsum.

12. A process according to Claim 9, wherein the monocalcium phosphate/phosphoric acid solution is reacted with sulphuric acid to produce gypsum solids and phosphoric acid, the gypsum solids are filtered off and wherein the phosphoric acid is recycled to the acidulation reactor.

13. A process according to any of Claims 1 to 12, wherein the clarified solution of monocalcium phosphate in phosphoric acid is divided into two portions for separate processing.

14. A process according to Claim 13, wherein one portion of the clarified solution is reacted with potassium sulphate, potassium hydrogen sulphate and/or a mixture thereof at a temperature in the range of 60-800C. to produce a solution of KH2PO4 in phosphoric acid and insoluble gypsum.

1 5. A process according to Claims 13 or 14, wherein a portion of the clarified solution is reacted with sulphuric acid to produce gypsum solids and phosphoric acid, the gypsum solids are filtered off, and the phosphoric acid is recycled to the acidulation reactor.

1 6. A process for the production of monocalcium phosphate and phosphoric acid substantially as herein described with reference to the accompanying drawings and/or the specific Examples.

1 7. Monocalcium phosphate when produced by a process according to any of Claims 1 to 1 6.

1 8. Phosphoric acid when produced by a process according to any of Claims 1 to 1 6.