Method for producing calcium sulfate

Abstract

A method for preparing calcium sulphate comprising a production of DCP by the attack of a source of phosphate by an acid, a digestion of the isolated DCP by the sulphuric acid under conditions giving rise to the formation of a first slurry of gypsum suspended in an acidic aqueous phase having a content of free SO.sub.3 equal to or less than 1.5% and a content of free P.sub.2O.sub.5, a conversion of at least part of said first slurry by heating to a temperature greater than 80 C. and potentially by adding sulphuric acid, with solubilisation of the gypsum crystals and recrystallisation of the solubilised calcium sulphate in a second slurry of -calcium sulphate hemihydrate crystals suspended in an aqueous phase based-on phosphoric acid, wherein the content of free SO.sub.3 is less than 10% by weight, and a separation between said aqueous phase and a filter cake based on particularly pure -calcium sulphate hemihydrate.

Claims

1. A method for preparing calcium sulphate comprising the steps of: producing calcium monohydrogen phosphate by: attack in an aqueous medium of a source of phosphate by an acid with the formation of a pulp comprising an aqueous phase containing water-soluble calcium phosphate and a solid phase containing impurities, separation of the aqueous phase, neutralization of the latter by a neutralizing calcium compound with precipitation of said calcium monohydrogen phosphate in an aqueous solution, and isolation of the precipitated calcium monohydrogen phosphate from said aqueous solution, and digesting the isolated calcium monohydrogen phosphate in an aqueous medium by sulphuric acid under conditions giving rise to the formation of a first slurry of calcium sulphate dihydrate crystals suspended in an acidic aqueous phase having a content of free P.sub.2O.sub.5, wherein digestion is carried out at a temperature greater than 65 C. and less than 75 C., at a content of free P.sub.2O.sub.5 in the acidic aqueous phase of 30-38% by weight of the first slurry and at a content of free SO.sub.3 in said acidic aqueous phase greater than 0.5% by weight of the first slurry and in order to obtain, in the first slurry, a content of free SO.sub.3 equal to or less than 1.5% by weight of the first slurry, and in that the method further comprises the steps of: converting at least part of said first slurry by heating to a temperature greater than 80 C and optionally adding sulphuric acid, wherein solubilisation of the calcium sulphate dihydrate crystals and recrystallisation of the solubilised calcium sulphate give rise to a second slurry of -calcium sulphate hemihydrate crystals suspended in an aqueous phase based on phosphoric acid, wherein the content of a free SO.sub.3 is less than 10% by weight of the second slurry, and separating, in said second slurry, the aqueous phase based on phosphoric acid and a filter cake based on -calcium sulphate hemihydrate.

2. The method for preparing calcium sulphate according to claim 1, further comprising a step of drying and grinding of the separated -calcium sulphate hemihydrate.

3. The method according to claim 2, wherein the dried -calcium sulphate hemihydrate has a crystallization water content between 6.2% and 1.5% by weight.

4. The method according to claim 3, wherein the separated -calcium sulphate hemihydrate has a residual acidity, and the method further comprises a neutralisation of said residual acidity.

5. The method according to claim 2, wherein the grinding and the drying of the separated -calcium sulphate hemihydrate take place simultaneously or separately.

6. The method according to claim 1, wherein the content of free SO.sub.3 in the aqueous phase based on phosphoric acid of the conversion step is less than 8% by weight of the second slurry.

7. The method according to claim 1, wherein the source of phosphate is a phosphate ore, a phosphate rock, ashes, wastewater treatment plant sludge, bones, pig manure, or a mixture thereof.

8. The method according to claim 1, further comprising, before said conversion, the steps of: dividing the first slurry into a first fraction and a second fraction, and filtering the first fraction with separation, as a filtrate, of production phosphoric acid and, as a filter cake, of calcium sulphate dihydrate, which is mixed with said second fraction to be submitted to said conversion, and wherein said conversion is performed at a temperature from 85 to 90 C., with added sulphuric acid, in order to obtain, in the aqueous phase based on phosphoric acid of the second slurry, a content of free SO.sub.3 greater than 2% by weight of the second slurry.

9. The method according to claim 1, wherein said aqueous phase based on phosphoric acid separated from the filter cake based on -calcium sulphate hemihydrate is recycled during the aforementioned digestion step.

10. The method according to claim 1, wherein the digestion is performed at a temperature between 70 C. and 90 C., at a content of free P.sub.2O.sub.5 in the acidic aqueous phase of 38 to 50% by weight of the first slurry and at a content of free SO.sub.3 in the acidic aqueous phase of less than 0.5% by weight and greater than 0.05% by weight of the first slurry.

11. The method according to claim 10, wherein the conversion is performed for all of said first slurry at a temperature equal to or greater than 90 C., so as to obtain, in the aqueous phase based on phosphoric acid of the second slurry, a content of free P.sub.2O.sub.5 of 35 to 45% by weight and a content of free SO.sub.3 concentration of less than 2% by weight in relation to the weight of the second slurry, the aqueous phase based on phosphoric acid being, after said separation, a production phosphoric acid.

12. The method according to claim 11, wherein the conversion step does not comprise the addition of sulphuric acid.

13. The method according to claim 1, further comprising maturing the separated -calcium sulphate hemihydrate in order to provide a -calcium sulphate dihydrate.

14. The method according to claim 1, wherein the digestion and the conversion take place in the separated reactors.

15. The method according to claim 1, wherein the content of free SO.sub.3 in the aqueous phase based on phosphoric acid of the conversion step is less than 5% by weight of the second slurry.

Description

(1) The method according to the invention will now be described in a more detailed manner, with reference to the accompanying figures.

(2) In the different figures, identical or similar elements keep the same reference numerals.

(3) FIG. 1 shows, in the form of a flow diagram, an installation example implementing one embodiment of the method according to the invention.

(4) FIG. 2 shows, in the same manner, an installation example implementing another embodiment of the method according to the invention.

(5) The installation shown in FIG. 1 comprises an attack reactor 1 into which is introduced in 2 the crushed phosphate rock and in 3 a strong acid, for example hydrochloric acid. The following conditions are applied in this reactor:

(6) Temperature: 50-70 C.

(7) Residence time: <1 hour

(8) The pulp 4 obtained in the reactor 1 is formed from an aqueous phase, in which calcium dihydrogen phosphate (MCP) and calcium chloride are dissolved, and a solid phase containing metallic impurities and radioactive elements present in the rock. These two phases are separated in a filtration device 5. The impurities are discarded in 6, whereas the aqueous phase 7 is neutralised in a tank 8 by the addition therein of a basic agent 9, for example calcium carbonate. This addition is controlled so as to increase the pH to reach a value preferably between 2.5 and 3.0. The calcium monohydrogen phosphate (DCP) is therefore precipitated, whereas other salts, such as calcium chloride, remain in the dissolved state. The suspension obtained is then transferred to a filtration device 10, which is used to isolate, in solid form, the calcium monohydrogen phosphate 11 from an aqueous solution of calcium chloride 32, which is discarded to be used in other treatments or for recycling.

(9) The calcium monohydrogen phosphate is then added inside a digestion reactor 12, where it is subjected to the action of the sulphuric acid added in 13, for example sulphuric acid at a concentration of 98-99% by weight. This reaction is exothermic and does not require the calorie intake, but merely a thermic adjustment of the reactor by known means.

(10) The following operating conditions are applied in this reactor:

(11) Temperature: >65-<75 C., preferably 70 C.

(12) % free P.sub.2O.sub.5: 30-38% by weight

(13) % free SO.sub.3: >0.5%-1.5% by weight, preferably 0.8-1.2%, in particular 1.0% by weight.

(14) Residence time: 2-4 hr, preferably 3 hr.

(15) Under these conditions, a gypsum slurry is formed in an acidic aqueous phase having a significant content of free P.sub.2O.sub.5. A first fraction 14 of this slurry is transferred to a filtration device 15. The filtrate 16 obtained is a high-quality production phosphoric acid and the filter cake 17 formed from gypsum is transported, as the second fraction 18 of the slurry from the digestion, to a conversion tank 19. The temperature is increased in this tank in a known manner, for example by adding water vapour in 20, and a small additional quantity of sulphuric acid is added to the reaction medium in 21.

(16) The following operating conditions are applied in the conversion tank:

(17) Temperature: >80 C., preferably 85-90 C.

(18) % free P.sub.2O.sub.5: 24-32% by weight

(19) % free SO.sub.3: 2-10% by weight, preferably 5-8.0% by weight

(20) Residence time: 45 to 90 min., preferably 60 min.

(21) Under these conditions, the calcium sulphate dihydrate is solubilised, releases the P.sub.2O.sub.5 values that it contains and is recrystallised in the form of a particularly pure and easily filterable -calcium sulphate hemihydrate which has only an extremely low content of P.sub.2O.sub.5.

(22) The second slurry thus formed 22 is then transferred to a filtration device 23. The filtrate 24, formed from a mixture of phosphoric acid and sulphuric acid, can be recycled in the digestion reactor 12. The filter cake formed from -calcium sulphate hemihydrate that is still impregnated with water can be immediately transferred in 25 to a drying device 26, where it is dried so that it advantageously contains no more than 6.2% of crystallisation water, which prevents it from becoming rehydrated. The dried hemihydrate is then conveyed in 27 to a grinding device 28 (for example a cutter mill), from which the ground product to the desired grain size can be transferred in 29 to applications where it can be directly implemented, eventually with different suitable additives, to manufacture products with particularly developed mechanical properties.

(23) Advantageously, the drying step 26 and the grinding step 28 can simultaneously be realised in a dryer mill; in this case, no conveyor device 27 is required.

(24) It must be understood that this method, which is represented as done in one process, may be subject to intermediate proceeding. For example, the isolated DCP can be stored before being used in the digestion.

(25) FIG. 2 shows a method according to the invention which, after isolation of the DCP, takes place in an alternative manner. The part of the method preceding the digestion reactor 12 can therefore be taken from FIG. 1.

(26) In this example embodiment, the operating conditions in the digestion reactor 12 are the following:

(27) Temperature: 70-90 C., preferably 70-80 C.

(28) % free P.sub.2O.sub.5: 38-50% by weight

(29) % free SO.sub.3: 0.05%-<0.5% by weight

(30) Residence time: 2-4 hr.

(31) The gypsum slurry surprisingly formed under these conditions is not filtered and is transferred in its entirety in 30 to the conversion tank 19.

(32) It is heated, for example by water vapour, in 20 and a small quantity of sulphuric acid is potentially added, however not absolutely necessary, in 21. The operating conditions in the conversion tank 19 are following:

(33) Temperature: >90 C., preferably 90-105 C.

(34) % free P.sub.2O.sub.5: 35%-45% by weight

(35) % free SO.sub.3: <2% by weight, preferably 0.1-<1.0% by weight

(36) Residence time: 0.5-1.5 hr.

(37) A second slurry 22 of -calcium sulphate hemihydrate is thus obtained, which is then transferred into a filtration device 23. The filtrate 31 is a very high quality phosphoric acid, containing a high content of P.sub.2O.sub.5, which is the production acid. The filter cake 25 impregnated with water is, as in the first example embodiment, dried then ground to produce the -calcium sulphate hemihydrate ready for use.

EXAMPLE

(38) -calcium sulphate hemihydrate has been prepared under the operating conditions of the method shown in FIG. 1.

(39) A chemical analysis of the two dry samples has been performed, and is summarised in Table 1 below:

(40) TABLE-US-00001 TABLE 1 Analyses at 250 C. Sample 1 Sample 2 P.sub.2O.sub.5, % 0.18 0.19 CaO, % 41.48 41.16 Al.sub.2O.sub.3, % 0.0364 0.0763 F, % 0.1188 0.0626 SiO.sub.2, % 0.1550 0.1575 Fe.sub.2O.sub.3, % 0.0189 0.0187 MgO, % <0.0050 <0.0050 Na.sub.2O, % 0.0406 0.0468 K.sub.2O, % 0.0563 0.1006 Th, ppm <6.1500 <6.2500 Sr, % 0.0332 0.0525 Ti, ppm 22.5092 41.0467 U.sub.3O.sub.8, ppm 12.6691 <1.250 Cd, ppm <0.615 <0.625 As, ppm 3.5670 0.3044 Pb, ppm 148.4894 160.0000 Hg, ppm <0.123 <0.125 Total C, ppm 84.8708 85.0000

(41) This table shows that the -calcium sulphate hemihydrate produced is particularly pure, in particular suitable for use by plasterers.

(42) As shown, this calcium sulphate hemihydrate contains an extremely low content of P.sub.2O.sub.5, which is a key requirement for plasterers. Indeed, the P.sub.2O.sub.5 is an element that interferes with the setting time of cements and plasters.

(43) On the other hand, a residual content of Na.sub.2O is shown to be greatly below 1,500 ppm, even below 500 ppm, which is particularly advantageous for a use of calcium sulphate in plasterboard. Indeed, the Na.sub.2O has the drawback of causing a efflorescence of calcium sulphate after installation.

(44) Flexural strength tests have been then conducted according to the following protocol:

(45) The samples of the hemihydrate analysed above have been rehydrated in view of the plaster preparation (water/calcium sulphate ratio 0.34). The plaster thus formed was poured into rectangular parallelepipedal moulds (3.93.916.2 cm). Once solidified, the plaster test pieces were removed from the moulds and subjected to a flexural strength test. Each bar-shaped test piece was placed on three bearing points and stress was applied to the central point. The stress was gradually increased until the test piece fractured.

(46) These flexural strength tests have been conducted after different periods of time after solidification and demoulding.

(47) TABLE-US-00002 TABLE 2 Time after demoulding (days) 1 7 21 Flexural strength (MPa) 4.81 6.08 6.37

(48) These results therefore show a calcium sulphate hemihydrate having a flexural strength of more than 2.5 MPa, advantageously more than 4 MPa.

(49) In compressive strength tests, the samples also showed a strength of more than 5 MPa, advantageously of more than 10 MPa.

(50) It must be understood that this invention is in no way limited to the example embodiment disclosed hereinabove and that numerous modifications can be made without leaving the scope of the appended claims.