METHOD FOR PRODUCING PHOSPHORIC ACID AND CALCIUM SULPHATE QUALITY SUITABLE FOR A CLINKER PROCESS FOR THE COMMERCIAL AND INDUSTRIAL UTILIZATION OF CALCIUM SULPHATE

Abstract

A process may involve digesting raw phosphate with concentrated sulfuric acid and converting the raw phosphate to calcium sulfate in the form of dihydrate and/or hemihydrate, and phosphoric acid, separating off calcium sulfate as solid from a liquid phase of a suspension that is obtained, treating the calcium sulfate that is separated off or from a stockpile with an acid to give a suspension with purified calcium sulfate and P.sub.2O.sub.5-containing acid solution, separating off the purified calcium sulfate as solid from a liquid phase of a suspension obtained, using the P.sub.2O.sub.5-containing liquid phase as a portion of the sulfuric acid required for digesting the raw phosphate or as feedstock for treating phosphogypsum from the stockpile to give a suspension of purified calcium sulfate and P.sub.2O.sub.5-containing acid solution, which is thereafter processed.

Claims

1.-28. (canceled)

29. A process for producing phosphoric acid and purified calcium sulfate by reaction of raw phosphate with sulfuric acid, the process comprising: a) digesting the raw phosphate in a first step with concentrated sulfuric acid and converting the raw phosphate to calcium sulfate in a form of dihydrate, hemihydrate, or a combination of hemihydrate and dihydrate, and phosphoric acid; b) separating off the calcium sulfate as a solid from a liquid phase of a suspension that is obtained; c) treating the calcium sulfate from step (b), separated off from the phosphoric acid, and/or calcium sulfate/phosphogypsum from a stockpile with an acid to give a suspension with purified calcium sulfate and a P.sub.2O.sub.5-containing acid solution; d) separating off the purified calcium sulfate after step (c) as a solid from a liquid phase of the suspension that is obtained, wherein the separation of the purified calcium sulfate from the suspension is started at a time in a range from t.sub.MIN+30 minutes to t.sub.MIN−30 minutes, wherein t.sub.MIN is a time at which an acid concentration during the treatment in step (c) is a minimum; and e) at least one of e1) using a P.sub.2O.sub.5-containing liquid phase obtained from step (d) as feedstock in step (a) as a portion of the sulfuric acid required for digesting the raw phosphate, or e2) using a P.sub.2O.sub.5-containing liquid phase obtained from step (d) as feedstock for treating phosphogypsum from the stockpile in step (c) to give a suspension of purified calcium sulfate and an P.sub.2O.sub.5-containing acid solution.

30. The process of claim 29 wherein the separation of the purified calcium sulfate from the suspension in step (d) is started at a time in a range from t.sub.1+20 minutes to t.sub.1−20 minutes, wherein t.sub.1 is a time at which the acid concentration during the treatment in step (c) has been reduced by at least 1.0% of an initial acid concentration.

31. The process of claim 29 wherein the separation of the purified calcium sulfate from the suspension in step (d) is started at a time in a range from t.sub.MIN+15 minutes to t.sub.MIN−15 minutes.

32. The process of claim 29 wherein the treatment of the calcium sulfate/phosphogypsum from the stockpile without the calcium sulfate from step (b) is treated in a separate step with an acid and either a suspension that is formed is supplied to step (d), or a suspension that is formed is supplied to a separate separating unit and in the separate separating unit a solid is separated from a liquid phase comprising a P.sub.2O.sub.5-containing acid solution.

33. The process of claim 29 wherein the calcium sulfate is separated off from the phosphoric acid in step (b) by filtration.

34. A process for producing sulfuric acid and cement clinker, the process comprising: a) digesting raw phosphate in a first step with concentrated sulfuric acid and converting the raw phosphate to calcium sulfate in a form of dihydrate, hemihydrate, or a combination of hemihydrate and dihydrate, and phosphoric acid; b) separating off the calcium sulfate as a solid from a liquid phase of a suspension that is obtained; c) treating the calcium sulfate from step (b), separated off from the phosphoric acid, and/or calcium sulfate/phosphogypsum from a stockpile with an acid to give a suspension with purified calcium sulfate and a P.sub.2O.sub.5-containing acid solution; d) separating off the purified calcium sulfate after step (c) as a solid from a liquid phase of the suspension that is obtained, wherein the separation of the purified calcium sulfate from the suspension is started at a time in a range from t.sub.MIN=30 minutes to t.sub.MIN−30 minutes, wherein t.sub.MIN is a time at which an acid concentration during the treatment in step (c) is a minimum; and e) mixing the purified calcium sulfate that is separated off and obtained in step (d) with admixtures and reducing agent to give a raw meal mixture for cement clinker production; f) burning the raw meal mixture to give the cement clinker, with sulfur dioxide being formed as an offgas; and g) supplying the sulfur dioxide as raw material to sulfuric acid production to produce the sulfuric acid.

35. The process of claim 34 wherein the separation of the purified calcium sulfate from the suspension in step (d) is started at a time in a range from t.sub.1+20 minutes to t.sub.1−20 minutes, wherein t.sub.1 is a time at which the acid concentration during the treatment in step (c) has been reduced by at least 1.0% of an initial acid concentration.

36. The process of claim 34 wherein the separation of the purified calcium sulfate from the suspension in step (d) is started at a time in a range from t.sub.MIN+15 minutes to t.sub.MIN−15 minutes.

37. The process of claim 34 wherein the treatment of the calcium sulfate/phosphogypsum from the stockpile without the calcium sulfate from step (b) is treated in a separate step with an acid and either a suspension that is formed is supplied to step (d), or a suspension that is formed is supplied to a separate separating unit and in the separate separating unit a solid is separated from a liquid phase comprising a P.sub.2O.sub.5-containing acid solution.

38. The process of claim 34 wherein the calcium sulfate is separated off from the phosphoric acid in step (b) by filtration.

39. The process of claim 34 wherein the calcium sulfate in step (c) is either: from the stockpile, or obtained from a separating unit of step (b), wherein calcium sulfate filtercake obtained after a first separation from the phosphoric acid is used directly or after one or more washes with liquid.

40. The process of claim 34 wherein at least one of: in step (c) the acid is added in an amount such that a weight ratio of solids to liquid in the suspension is in a range from 1/5 to 1/1; an acid resulting from the treatment in step (c) is a 3 to 10 molar acid; the acid in step (c) is hydrochloric acid, nitric acid, sulfurous acid, and/or sulfuric acid; the treatment in step (c) is performed at a temperature in a range from 30 to 80° C.; or a duration of the treatment in step (c) is in a range from 15 to 90 minutes.

41. The process of claim 34 wherein a D.sub.v(50) of a grain size distribution of calcium sulfate anhydrite obtained in step (d) is in a range of 0.5-100 μm.

42. The process of claim 34 wherein the purified calcium sulfate from step (d) accounts for more than 70% by weight of a total calcium sulfate in the raw meal mixture.

43. A plant for producing phosphoric acid and purified calcium sulfate by reaction of raw phosphate with sulfuric acid, the plant comprising: a) a reaction unit that includes (i) a raw phosphate feed and (ii) a concentrated sulfuric acid feed line, wherein the reaction unit is configured to form a suspension comprising phosphoric acid and calcium sulfate, with the calcium sulfate being present in the form of dihydrate, hemihydrate, or a combination of hemihydrate and dihydrate, wherein the reaction unit includes (iii) an outlet for the suspension; b) a first separating unit configured to separate the suspension as a solid from a liquid phase, the first separating unit comprising an outlet for a substantially solid-containing phase and an outlet for a substantially liquid-containing phase, wherein the first separating unit is fluidically connected to the reaction unit; c) a purifying unit that is fluidically connected to the first separating unit and includes a feed for an acid, wherein the purifying unit is configured to convert the calcium sulfate separated off in the first separating unit and/or calcium sulfate/phosphogypsum from a stockpile into a suspension with the acid supplied to the purifying unit, wherein the suspension includes at least calcium sulfate and a P.sub.2O.sub.5-containing acid solution, wherein the purifying unit includes an outlet line for the suspension; d) a second separating unit configured to treat the suspension from the purifying unit, wherein the second separating unit includes an outlet for the P.sub.2O.sub.5-containing acid solution and an outlet for solid calcium sulfate, wherein the second separating unit is disposed downstream of the purifying unit; and e) at least one of e1) a first fluidic connection configured to return the P.sub.2O.sub.5-containing acid solution from the second separating unit into the reaction unit or provided upstream of the reaction unit, or e2) a second fluidic connection, starting from the second separating unit, provided upstream of the second separating unit and downstream of the first separating unit, wherein the purification unit and the second separating unit are configured such that separation of the calcium sulfate from the suspension starts at a time in a range from t.sub.MIN+30 minutes to t.sub.MIN−30 minutes, wherein t.sub.MIN is a time at which an acid concentration during a treatment in the purifying unit is at a minimum.

44. The plant as claimed in claim 43 wherein the purifying unit and the second separating unit are configured such that separation of the calcium sulfate from the suspension is started at a time in a range from t.sub.MIN+15 minutes to t.sub.MIN−15 minutes.

45. A plant for producing sulfuric acid and cement clinker using calcium sulfate that is formed and separated off as a solid byproduct in phosphoric acid production, in a reaction of raw phosphate with sulfuric acid to form phosphoric acid, wherein the plant comprises: a) a reaction unit that includes (i) a raw phosphate feed and (ii) a concentrated sulfuric acid feed line, wherein the reaction unit is configured to form a suspension comprising phosphoric acid and calcium sulfate, wherein the calcium sulfate is present as dihydrate, hemihydrate, or a combination of hemihydrate and dihydrate, wherein the reaction unit comprises (iii) an outlet for the suspension; b) a first separating unit configured to separate the suspension as a solid from a liquid phase, wherein the first separating unit comprises an outlet for a substantially solids-containing phase and an outlet for a substantially liquid-containing phase, wherein the first separating unit is fluidically connected to the reaction unit; c) a purifying unit that is fluidically connected to the first separating unit and includes a feed for an acid, wherein the purifying unit is configured to convert the calcium sulfate that is separated off and/or calcium sulfate/phosphogypsum from a stockpile into a suspension with the acid, wherein the suspension comprises at least calcium sulfate and a P.sub.2O.sub.5-containing acid solution, wherein the purifying unit includes an outlet line for the suspension; d) a second separating unit configured to treat the suspension, wherein the second separating unit includes an outlet for the P.sub.2O.sub.5-containing acid solution and an outlet for solid calcium sulfate, wherein the second separating unit is disposed downstream of the purifying unit; e) a raw meal mixing unit configured to mix the separated purified calcium sulfate with admixtures and a reducing agent to give a raw meal mixture for cement clinker production; and f) a cement clinker process unit, including a combination of a preheater, a burner device, and a cooler that are configured for preheating, burning, and cooling the raw meal mixture to give the cement clinker, wherein the cement clinker process unit is configure to form sulfur dioxide as an offgas.

46. The plant of claim 45 comprising (g) a sulfuric acid production plant configured to be supplied with the sulfur dioxide offgas as raw material to produce sulfuric acid.

47. The plant of claim 45 comprising at least one of: e1) a first fluidic connection that is upstream of the reaction unit and is configured to return the P.sub.2O.sub.5-containing acid solution from the second separating unit; or e2) a second fluidic connection starting from the second separating unit, wherein the second fluidic connection is positioned upstream of the second separating unit and downstream of the first separating unit.

48. The plant as claimed in claim 45 wherein the purifying unit and the second separating unit are configured such that separation of the calcium sulfate from the suspension is started at a time in a range from t.sub.1+20 minutes to t.sub.1−20 minutes, wherein t.sub.1 is a time at which an acid concentration during a treatment in the purifying unit has been reduced by at least 1.0% of an initial acid concentration.

Description

[0218] The invention is described hereinafter by working examples, which are elucidated in more detail by the figures. The working examples are not intended to restrict the scope of the claimed invention in any way.

[0219] FIG. 1 shows a schematic flow diagram for the workup of calcium sulfate from phosphoric acid production after a substep of the process of the invention.

[0220] FIG. 2 shows a schematic flow diagram for the utilization of calcium sulfate from phosphoric acid production by means of an integrated complex for producing cement clinker and sulfuric acid.

[0221] FIG. 3 shows a schematic flow diagram for the utilization of calcium sulfate from phosphoric acid production by means of an integrated complex for producing cement clinker and sulfuric acid as in FIG. 2, with alternative or additional process steps being additionally shown.

[0222] FIG. 4 shows a schematic flow diagram of a new integrative overall complex for producing phosphoric acid, calcium sulfate suitable for cement clinker, cement clinker and sulfuric acid by the process of the invention, with alternative or additional process steps being additionally shown.

[0223] FIG. 5 shows a schematic flow diagram of a new integrative overall complex for producing phosphoric acid, calcium sulfate suitable for cement clinker, cement clinker and sulfuric acid by the process of the invention, with preferred embodiments of the concept of the invention being additionally shown, which may be present individually or cumulatively.

[0224] FIG. 6 is a graphic representation of the acid concentration against the dwell time of the gypsum of the gypsum PG B in sulfuric acid from example 9.

[0225] FIG. 7 is a graphic representation of the acid concentration (left-hand axis), the anhydrite content (right-hand axis) and the leaching efficiency with respect to P2O5 content (right-hand axis) against the dwell time of the gypsum PG B in sulfuric acid from example 9.

[0226] FIG. 8 is a graphic representation of the acid concentration against the dwell time of the gypsum PG A in sulfuric acid from example 10.

[0227] FIG. 9 is a graphic representation of the composition of the mineralogy (left-hand axis) and of the leaching efficiency with respect to P2O5 content (right-hand axis) against the dwell time of the gypsum PG A in sulfuric acid from example 10.

[0228] FIG. 1 shows a flow diagram for the workup of calcium sulfate from phosphoric acid production by the process of the invention. Calcium sulfate sludge 14 from the raw phosphate reaction unit of the phosphoric acid plant is passed into the first separating unit, preferably filtration unit, 3 of the phosphoric acid plant, where the calcium sulfate generated in the raw phosphate reaction unit is separated off from the phosphoric acid. The calcium sulfate separated off is conducted into the purification unit 5, where the calcium sulfate is treated with acid. In this process the impurities in the calcium sulfate which adversely affect a downstream clinker process and the cement quality are reduced to the level required by the clinker process. This is an integrated process, in which the process parameters, such as dwell time, acid, temperature and S/L ratio, can be adapted in tune with the starting material qualities and with the desired properties for the product obtained, in terms of further processing. In a second separating unit (calcium sulfate separating unit) 6, which is preferably a filtration unit, the liquid and the resulting solid in the suspension obtained in step c) are separated from one another. The liquid 15, more particularly as filtrate, can be used in the existing phosphoric acid-sulfuric acid complex. The treated calcium sulfate can be processed further in a clinker process.

[0229] FIG. 2 shows a flow diagram of phosphoric acid production (existing complex) and an integrated process for producing cement clinker and sulfuric acid from calcium sulfate which comes from the phosphoric acid production (integrated complex). In a processing unit 1 the phosphate rock is processed to give the raw phosphate. In the raw phosphate reaction unit of the phosphoric acid plant 2, the raw phosphate is reacted with sulfuric acid which comes from the sulfuric acid production plant, to form phosphoric acid and solid calcium sulfate as a byproduct. The calcium sulfate generated in phosphoric acid production is separated from the phosphoric acid in the first separating unit 3, which is preferably a filtration unit, of the phosphoric acid plant, and is supplied to the purification unit 5. There the calcium sulfate is admixed with acid, to give for example a 1-12 molar acid, more particularly a 1-12 molar sulfuric acid, after the treatment. For example a 1-12 molar sulfuric acid may be added for the treatment. Following addition of the acid, the treatment may be carried out for example at a temperature of 15-100° C. for 5 to 120 min, during which the resulting suspension is preferably agitated, by stirring, for example. In this procedure, the impurities in the calcium sulfate that adversely affect the downstream cement clinker process and the cement quality are reduced to the level required by the cement clinker process. In a second separating unit 6, which is preferably a filtration unit, the liquid and the resulting solid are separated from one another. The liquid, more particularly as filtrate, can be used in the existing phosphoric acid-sulfuric acid complex. The treated calcium sulfate is supplied to the raw meal mixing unit 7 positioned upstream for the cement clinker process. In this unit the calcium sulfate is mixed with the required admixtures for the required cement clinker quality in the correct ratio. The cement clinker process unit 8 is charged with the prepared cement clinker raw meal, with the raw meal preferably being preheated in a heat exchanger (not shown) before being supplied to the process unit 8. In the cement clinker process unit 8, sulfur dioxide is separated from the calcium sulfate and supplied as offgas from the cement clinker process unit to the sulfur dioxide offgas treatment facility 9. The treated sulfur dioxide gas may optionally be supplied to the existing sulfuric acid production plant 4. Alternatively the treated sulfur dioxide gas may optionally be supplied to a new sulfuric acid production plant (cf. 13 in FIG. 3). The calcium which remains in the cement clinker process unit is reacted with the admixtures to give cement clinker. The burning temperature for cement clinker production may be effected, for example, in the range from 1200° C. to 1600° C. and a burning time of 5 minutes to 60 minutes. The cement clinker produced in this way can be processed further to give cement.

[0230] FIG. 3 shows a schematic flow diagram for the utilization of calcium sulfate from phosphoric acid production by means of an integrated complex for producing cement clinker and sulfuric acid according to FIG. 2, with alternative or additional process steps being additionally shown. The text below addresses the alternative or additional process steps; otherwise, reference is made to the explanations given for FIG. 2. FIG. 3 shows an alternative source of the calcium sulfate used in step c). Instead of the calcium sulfate from the filtration unit of the phosphoric acid plant 3, the calcium sulfate used in step c) may be a calcium sulfate from a stockpile 10, which is deposited calcium sulfate from phosphoric acid production. FIG. 3 also shows the optional processing step for the removal of rare earths, comprising a reaction unit for recovering rare earth metals from the calcium sulfate 11, and the calcium sulfate separating unit 12 for separating the liquid phase from the purified calcium sulfate. Also shown in FIG. 3 is that the sulfur dioxide obtained from the SO.sub.2 treatment 9 can be used for the recovery of sulfuric acid in the existing sulfuric acid production plant 4 and/or in a new sulfuric acid production plant 13.

[0231] FIG. 4 shows a schematic flow diagram of a total plant for phosphoric acid production, and a process for producing cement clinker and sulfuric acid from calcium sulfate which comes from phosphoric acid production. In this variant of the flow diagram, in contrast to the approaches from FIG. 2 and FIG. 3, a new overall complex is described, rather than integration into existing complexes. In a phosphate rock processing unit 1a, the phosphate rock is processed to give the raw phosphate. In the raw phosphate reaction unit of the phosphoric acid plant 2a, the raw phosphate is reacted with sulfuric acid coming from the sulfuric acid production plant 13, to form phosphoric acid and solid calcium sulfate as a byproduct, where the calcium sulfate byproduct according to processes of the invention may take the form, without preference, of dihydrate, hemihydrate or a combination of hemihydrate and dihydrate. The calcium sulfate generated in phosphoric acid production is separated off from the phosphoric acid in the first separating unit 3a, which is preferably a filtration unit, of the phosphoric acid plant, and is supplied to the purification unit 5. There the calcium sulfate, as already described in FIG. 2, is treated with acid and the resulting suspension is separated into liquid and solid in a second separating unit 6. The liquid, more particularly as filtrate, can be used in the existing phosphoric acid-sulfuric acid complex and/or optionally in a second calcium sulfate reaction unit 16 for the treatment of calcium sulfate from the stockpile 10, in which case the reaction conditions of the second calcium sulfate reaction unit 16 are within the parameter ranges of the purification unit 5. The suspension treated in the second calcium sulfate reaction unit 16, with calcium sulfate from the stockpile, is subsequently supplied to the second separating unit 6, and the subsequent procedure is as described in FIG. 2. As described in FIG. 3, optionally, a further purification step may take place for the recovery of rare earth metals 11 and subsequent separation of the calcium sulfate by way of a calcium sulfate separating unit 12. The treated calcium sulfate is then treated further as described in FIG. 2 in a raw meal mixing unit 7, supplied subsequently to the cement clinker process unit 8, and the sulfur dioxide obtained from the SO.sub.2 treatment 9 is used for the recovery of sulfuric acid in a new sulfuric acid production plant 13. The suspension in the optional calcium sulfate reaction unit 16 may also be transferred for separation not into the second separating unit 6 but instead into a separate calcium sulfate separating unit (not shown), and the calcium sulfate separated off may then be supplied to the raw meal mixing unit 7 and/or to the optional unit for the recovery of rare earths from calcium sulfate 11.

[0232] FIG. 5 shows an extended representation of FIGS. 2 to 4. The explanations made so far for FIGS. 2 to 4 are valid here correspondingly. Additionally represented are the feed lines for the acid, preferably sulfuric acid, and in accordance with the concept of the invention a circuit is made by the feed of sulfuric acid from a sulfuric acid production plant 4 and/or 13 to a reaction unit in a phosphoric acid plant 2 and/or 2a. It is possible for the sulfuric acid plants 4 and 13 to coexist. It is likewise possible for the phosphoric acid plants 2 and 2a to coexist. An acid, preferably a sulfuric acid, may additionally be introduced from an external source into the process. The sulfuric acid produced in the sulfuric acid plant 4 and/or 13 may be introduced into the second calcium sulfate reaction unit 16. Here as well, alternatively, an acid, preferably a sulfuric acid, from an external source can be utilized for supplying the second calcium sulfate reaction unit 16 with an acid. The sulfuric acid from 4 and/or 13 may likewise be introduced into the purification unit or calcium sulfate reaction unit 5. For the sake of clarity, this fluidic connection is not represented by a direct arrow in FIG. 5, but the asterisk (*) is intended to make it clear. For treating the calcium sulfate/phosphogypsum from a stockpile, preferably a phosphoric acid plant stockpile, the suspension obtained, which originates from the uniting of the acid from, for example, 4 and/or 13 with the calcium sulfate/phosphogypsum of the stockpile 10 within the second calcium sulfate reaction unit 16, can be transferred into an additional second separating unit or calcium sulfate separating unit 6′. Here the suspension is separated into calcium sulfate and a P.sub.2O.sub.5-containing acid solution. The calcium sulfate from 6′ may therefore optionally be supplied to the raw meal preparation facility 7. The suspension of the second calcium sulfate reaction unit 16 may optionally also be transferred into the second separating unit or calcium sulfate separating unit 6. The P.sub.2O.sub.5-containing acid solution as is obtained in the second separating unit or calcium sulfate separating unit 6 and/or in the additional second separating unit or calcium sulfate separating unit 6′ may be returned to the purification unit or calcium sulfate reaction unit 5 and/or the reaction unit of the phosphoric acid plant 2 and/or 2a, and/or optionally to the second calcium sulfate reaction unit 16. The dashed lines/arrows, accordingly, represent material streams which are present optionally, which may be present individually or else simultaneously.

EXAMPLES

[0233] Set out below are a number of examples relating to the purification of various phosphogypsums. The phosphogypsums were each dried before and after treatment by described methods for at least 24 h at 50° C. to remove free water. Before and after treatment, the chemical composition of the gypsums was determined using x-ray fluorescence analysis (XFA) on an Axios Advanced spectrometer from PANalytical with the software package SuperQ 5.3B. For the purpose of analysis the gypsum was digested using lithium tetraborate. The loss on ignition of the gypsums was ascertained at 1050° C. The fluoride content was determined following digestion of the gypsum with sodium peroxide and hydrochloric acid, by means of an ion-selective electrode. All of the values reported below for the results from the XFA pertain to the gypsum samples free from loss on ignition. A number of gypsums were additionally studied for their mineralogical composition, before and after treatment, by powder diffractometry, on a D4 Endeavor diffractometer from Bruker. Evaluation using the Rietveld method took place using the software package Topas 4.2 from Bruker. The D.sub.v(50) value of the grain size distribution was ascertained on a Mastersizer 3000 from Malvern, using ethanol as dispersing medium. The scattering model employed was the Fraunhofer model.

Example 1

[0234] 50 g of a phosphogypsum designated as “PG A” with P.sub.2O.sub.5 contents of 1.29 wt % and F contents of 1.25 wt % was stirred using a KPG stirrer with 200 ml (S/L=0.25) of 8-molar sulfuric acid for 30 minutes at 60° C. After this time had elapsed, the suspension was rapidly filtered and washed twice with 57.5 ml of water at room temperature. After treatment, the P.sub.2O.sub.5 and F contents are 0.02 and 0.01 wt %, respectively (corresponding to a leaching efficiency of 98% and 99%, respectively). The mineralogical composition before treatment was found to be 2.8 wt % quartz, 91.5 wt % dihydrate (CaSO.sub.4*2H.sub.2O), 3.7 wt % hemihydrate (CaSO.sub.4*0.5 H.sub.2O) and 2.9 wt % anhydrite (CaSO.sub.4). Following treatment, the composition found was as follows: 3.3 wt % quartz, 1.2 wt % dihydrate (CaSO.sub.4*2H.sub.2O), 0.1 wt % hemihydrate (CaSO.sub.4*0.5 H.sub.2O) and 95.5 wt % anhydrite (CaSO.sub.4).

Example 2

[0235] 75 g of the same gypsum PG A as in example 1 was stirred in a further experiment using a KPG stirrer with 150 ml (S/L=0.5) of 6-molar sulfuric acid for 30 minutes at 60° C. After the time had elapsed, the suspension was rapidly filtered and washed twice with 86.3 ml of water at room temperature. Following treatment, the P.sub.2O.sub.5 and F contents are 0.29 and 0.03 wt %, respectively (corresponding to a leaching efficiency of 78% and 98%, respectively). The mineralogical composition found after treatment was as follows: 3.2 wt % quartz, 73.2 wt % dihydrate (CaSO.sub.4*2H.sub.2O), 2.4 wt % hemihydrate (CaSO.sub.4*0.5 H.sub.2O) and 21.2 wt % anhydrite (CaSO.sub.4). The D.sub.v(50) after treatment is 59.1 μm.

Example 3

[0236] 75 g of a further gypsum (designated as PG B) having P.sub.2O.sub.5 contents of 1.70 wt % and F contents of 2.13 wt % was stirred in a further experiment using a KPG stirrer with 150 ml (S/L=0.5) of 6-molar sulfuric acid for 30 minutes at 75° C. After the time had elapsed, the suspension was rapidly filtered and washed twice with 86.3 ml of water at room temperature. Following treatment, the P.sub.2O.sub.5 and F contents are 0.07 and 0.12 wt %, respectively (corresponding to a leaching efficiency of 96% and 94%, respectively). The mineralogical composition before treatment was found to be 3 wt % quartz and 97 wt % dihydrate (CaSO.sub.4*2H.sub.2O). After treatment, the composition found was as follows: 2.5 wt % quartz, 0.4 wt % dihydrate (CaSO.sub.4*2H.sub.2O) and 97.2 wt % anhydrite (CaSO.sub.4). The D.sub.v(50) after treatment is 9.77 μm.

Example 4

[0237] 75 g of the same gypsum (PG B) was stirred in a further experiment using a KPG stirrer with 150 ml (S/L=0.5) of 6-molar sulfuric acid for 45 minutes at 75° C. After the time had elapsed, the suspension was rapidly filtered and washed twice with 86.3 ml of water at 75° C. temperature. Following treatment, the P.sub.2O.sub.5 and F contents are 0.03 and 0.11 wt %, respectively (corresponding to a leaching efficiency of 98/and 95%, respectively). The mineralogical composition found after treatment was as follows: 2.9 wt % quartz, 0.2 wt % dihydrate (CaSO.sub.4*2H.sub.2O) and 97.0 wt % anhydrite (CaSO.sub.4). The D.sub.v(50) after treatment is 9.34 μm.

Example 5

[0238] 75 g of the same gypsum (PG B) was stirred in a further experiment using a KPG stirrer with 150 ml (S/L=0.5) of 7-molar sulfuric acid for 30 minutes at 75° C. After the time had elapsed, the suspension was rapidly filtered and washed twice with 86.3 ml of water at room temperature. Following treatment, the P.sub.2O.sub.5 and F contents are 0.03 and 0.06 wt %, respectively (corresponding to a leaching efficiency of 98% and 97%, respectively). The D.sub.v(50) after treatment is 6.99 μm.

Example 6

[0239] 75 g of the same gypsum (PG B) was stirred in a further experiment using a KPG stirrer with 150 ml (S/L=0.5) of 4-molar sulfuric acid for 20 minutes at 30° C. After the time had elapsed, the suspension was rapidly filtered and washed twice with 86.3 ml of water at room temperature. Following treatment, the P.sub.2O.sub.5 and F contents are 0.43 and 0.17 wt %, respectively (corresponding to a leaching efficiency of 75/o and 92%, respectively). The mineralogical composition found after treatment was as follows: 1.7 wt % quartz, 96.3 wt % dihydrate (CaSO.sub.4*2H.sub.2O), 2.0 wt % hemihydrate (CaSO.sub.4*0.5 H.sub.2O) and 0 wt % anhydrite (CaSO.sub.4). The D.sub.v(50) after treatment is 16.1 μm.

Example 7

[0240] In order to adjudge whether the process described can also be used with merely separated and hence unwashed phosphogypsum from the phosphoric acid plant, 75 g of the same gypsum PG B as in example 3 were admixed with 25 g of 25% strength P.sub.2O.sub.5 solution (in the form of phosphoric acid) and stirred using a KP stirrer with 150 ml (S/L=0.5; 25 g of P.sub.2O.sub.5 solution disregarded) of 7-molar sulfuric acid for 30 minutes at 75° C. After the time had elapsed, the suspension was rapidly filtered and washed twice with 86.3 ml of water at room temperature. After treatment, the P.sub.2O.sub.5 and F contents are 0.07 and 0.08 wt %, respectively (corresponding to a leaching efficiency of 96% and 96%, respectively). Following treatment, the mineralogical composition found was as follows: 2.5 wt % quartz, 0.2 wt % dihydrate (CaSO.sub.4*2H.sub.2O) and 97.3 wt % anhydrite (CaSO.sub.4). The D.sub.v(50) after treatment is 6.62 μm.

Example 8

[0241] In order to evaluate the filterability of the suspensions in dependence on the influencing variables such as, for example, temperature, dwell time and acid concentration, the filtration times of the suspensions from examples 3, 4 and 5 on a suction filter with a filter area of 56 cm.sup.2 were determined at a reduced pressure of 500 mbar. In all cases the height of the filtercake was between 13 and 14.5 mm. In the case of the suspension from example 5, the resulting filtration times were 30 s and 52 s and also 39 s in washes 1 and 2, respectively. In the case of the suspension from example 3, the resulting filtration times were 11 s and 22 s and also 19 s in washes 1 and 2, respectively. In the case of the suspension from example 4, the resulting filtration times were 11 s and 20 s and also 11 s in washes 1 and 2, respectively. It is apparent that the filterability can be optimized by adaptation of the influencing variables, with retention of the quality of purification.

Example 9

[0242] In order to look at the feasibility of integrating process analysis via determination of the acid concentration, 150 g of the gypsum PG B was stirred in a further experiment using a KPG stirrer with 300 ml (S/L=0.5) of 5-molar sulfuric acid at 75° C. After defined time intervals each of 10 minutes and directly after the start (t=1 minute), a sample (around 12 ml of suspension) was taken in each case, with filtration, and was washed twice with around 6 ml of water each time. The filtrate of the first filtration step was collected and used for further analyses. The reaction was terminated after 100 minutes, meaning that a total of 11 samples were taken. To determine the acid concentration of the individual filtrate samples, 0.5 ml of each filtrate was diluted with around 20-40 ml of ultrapure water and titrated using 1 M sodium hydroxide solution. Additionally the concentration of the acid used was also studied. The equivalence point was determined potentiometrically using a commercial automatic titrator from Metrohm. The filtered and washed phosphogypsum samples were dried at 50° C. for at least 24 h and then analyzed for mineralogy and P.sub.2O.sub.5 content.

TABLE-US-00001 TABLE 1 Resulting acid concentration after corresponding dwell time of gypsum PG B in sulfuric acid; with the reaction conditions: c = 5 mol/l; T = 75° C.; S/L = 0.5. The concentration of the acid used was likewise studied. Acid Relative concentration, decrease in Sampling sample mol/l acid concentration, Sample time min. Mean sample % 5M H.sub.2SO.sub.4 4.97 — .sup.i) (used) E0 1 5.01   0% .sup.ii) E1 10.33 4.92 1.9% E2 20 4.82 3.9% E3 30 4.77 5.0% E4 40 4.67 6.9% E5 50 4.57 8.9% E6 60 4.60 8.4% E7 70 4.61 8.0% E8 80 4.64 7.6% E9 90 4.67 7.0% E10 100 4.68 6.7% .sup.i) acid used; control measurement .sup.ii) acid concentration measured after one minute; defined as starting concentration after direct addition of the acid

[0243] FIG. 6 is a graphical representation of the acid concentration against the dwell time of the gypsum of the gypsum PG B in sulfuric acid.

TABLE-US-00002 TABLE 2 Mineralogy and leaching efficiency with respect to P.sub.2O.sub.5 content after corresponding dwell time of the gypsum PG B in sulfuric acid; with the reaction conditions: c = 5 mol/l; T = 75° C.; S/L = 0.5. P.sub.2O.sub.5 Sam- Anhy- Dihy- Hemihy- (Lol-free) P.sub.2O.sub.5 pling drite drate drate wt % leaching Sample min. wt % wt % wt % 1.70 % E0 1 18.7 74.9 4.4 0.46 72.8 E1 10.33 34.0 59.4 3.4 0.42 75.2 E2 20 35.4 58.6 3.4 0.33 80.6 E3 30 40.1 54.6 2.6 0.27 84.4 E4 40 79.4 17.3 0.7 0.18 89.7 E5 50 96.9 0.2 0.0 0.11 93.7 E6 60 97.2 0.0 0.0 0.06 96.2 E7 70 97.3 0.0 0.0 0.05 96.8 E8 80 95.1 0.7 0.0 0.04 97.5 E9 90 96.4 0.3 0.0 0.05 96.8 E10 100 97.3 0.0 0.0 0.03 98.1

[0244] FIG. 7 is a graphical representation of the acid concentration (left-hand axis), of the anhydrite content (right-hand axis) and of the leaching efficiency with respect to P.sub.2O.sub.5 content (right-hand axis) against the dwell time of the gypsum PG B in sulfuric acid.

Example 10

[0245] For the same purpose as in example 9, 150 g of the gypsum PG A was stirred in a further experiment using a KPG stirrer with 300 ml (S/L=0.5) of 6-molar sulfuric acid at 75° C. After defined time intervals each of around 6 minutes and directly after the start (t=0.5 minutes), a sample (around 12 ml of suspension) was taken in each case, with filtration, and was washed twice with around 6 ml of water each time. The filtrate of the first filtration step was collected and used for further analyses. The reaction was terminated after 55 minutes, meaning that a total of 9 samples were taken. To determine the acid concentration of the individual filtrate samples, 0.5 ml of each filtrate was diluted with around 20-40 ml of ultrapure water and titrated using 1 M sodium hydroxide solution. Additionally the concentration of the acid used was also studied. The equivalence point was determined potentiometrically using a commercial automatic titrator from Metrohm. The filtered and washed phosphogypsum samples were dried at 50° C. for at least 24 h and then analyzed for mineralogy and P.sub.2O.sub.5 content.

TABLE-US-00003 TABLE 3 Resulting acid concentration after corresponding dwell time of gypsum PG A in sulfuric acid; with the reaction conditions: c = 6 mol/l; T = 75° C.; S/L = 0.5. The concentration of the acid used was likewise studied. Acid concentration, Relative decrease in Sampling sample mol/l acid concentration, Sample min. Mean sample % 6M H.sub.2SO.sub.4 6.076 — .sup.i) (used) E0 0.5 6.102   0% .sup.ii) E1 6 5.946 2.6% E2 12 5.662 7.2% E3 17.5 5.599 8.2% E4 23 5.622 7.9% E5 28.5 5.630 7.7% E6 34.5 5.665 7.2% E7 45 5.656 7.3% E8 55 5.699 6.6% .sup.i) acid used; control measurement .sup.ii) acid concentration measured after half a minute; defined as starting concentration after direct addition of the acid

[0246] FIG. 8 is a graphical representation of the acid concentration against the dwell time of the gypsum PG A in sulfuric acid.

TABLE-US-00004 TABLE 4 Mineralogy and leaching efficiency with respect to P.sub.2O.sub.5 content after corresponding dwell time of the gypsum PG A in sulfuric acid; with the reaction conditions: c = 6 mol/l; T = 75° C.; S/L = 0.5. P.sub.2O.sub.5 Sam- Anhy- Dihy- Hemihy- (Lol-free) P.sub.2O.sub.5 pling drite drate drate wt % leaching Sample min. wt % wt % wt % 0.99 % E0 0.5 17.9 79.9 0.0 0.55 44.9 E1 6.0 89.4 7.9 0.0 0.32 67.8 E2 12.0 95.1 2.1 0.0 0.11 88.9 E3 17.5 97.6 0.0 0.0 0.05 94.5 E4 23.0 97.7 0.0 0.0 0.06 94.4 E5 28.5 97.5 0.0 0.0 0.04 95.6 E6 34.5 97.6 0.0 0.0 0.04 95.6 E7 45.0 97.4 0.0 0.0 0.04 95.6 E8 55.0 97.7 0.0 0.0 0.04 95.6

[0247] FIG. 9 is a graphical representation of the mineralogical composition (left-hand axis) and of the leaching efficiency with respect to P.sub.2O.sub.5 content (right-hand axis) against the dwell time of the gypsum PG A in sulfuric acid.

Example 11

[0248] A further example showing the optimization of filterability by monitoring of the reaction course is given below. For this purpose, 150 g of the gypsum PG A to were stirred by means of a KPG stirrer with 150 ml (S/L=0.5) of 5-molar sulfuric acid for in one case 100 minutes at 75° C. and in another case the same amount of gypsum with the same acid concentration, same S/L ratio and same temperature but only for 40 minutes (optimum ascertained in preliminary experiment for minimum acid concentration; similarly to example 9). After the time had elapsed, the suspensions were rapidly filtered and each washed twice with 172.5 ml of water at room temperature. Following treatment, the P.sub.2O.sub.5 and F contents for 100 minutes' reaction time are 0.02 and 0.03 wt %, respectively (corresponding to a leaching efficiency of 98% and 98%, respectively) and for 40 minutes are 0.06 and <0.01 wt %, respectively (corresponding to a leaching efficiency of 95% and virtually 100%, respectively). Following treatment, the mineralogical composition ascertained was as follows: (for 100 minutes' reaction time) 5.7 wt % quartz, 0.1 wt % dihydrate (CaSO.sub.4*2H.sub.2O), 0.0 wt % hemihydrate (CaSO.sub.4*0.5 H.sub.2O) and 94.2 wt % anhydrite (CaSO.sub.4). (for 40 minutes' reaction time) 2.3 wt % quartz, 18.3 wt % dihydrate (CaSO.sub.4*2H.sub.2O), 0.2 wt % hemihydrate (CaSO.sub.4*0.5 H.sub.2O) and 79.3 wt % anhydrite (CaSO.sub.4). The D.sub.v(50) after 100 minutes' reaction time is 12.8 μm and after 40 minutes' reaction time is 18.0 μm. In terms of the filtration time there is a significant improvement if the reaction is terminated after just 40 minutes and the suspension is filtered. Using a suction filter and under reduced pressure as described in example 8, the two suspensions were filtered and the filtercakes were washed twice. A distinct difference was already evident from the filtercake heights. After 100 minutes' reaction time, the height was 24 mm, and after 40 minutes' reaction time it was 29 mm. In the case of the suspension after 100 minutes, the filtration time was 27 s and 55 s and also 55 s in washes 1 and 2. In the case of the suspension after 40 minutes, the filtration time was 21 s and 35 s and also 38 s in washes 1 and 2. Adding up the filtration times and washes, the resulting improvement in filterability is around 31% merely by optimizing the end of reaction (40 minutes rather than 100 minutes' reaction time) for virtually the same leaching efficiency.

LIST OF REFERENCE SYMBOLS

[0249] 1 Processing unit for phosphate rock/phosphate ore [0250] 1a Processing unit for phosphate rock/phosphate ore (new) [0251] 2 Reaction unit of phosphoric acid plant [0252] 2a Reaction unit of phosphoric acid plant (new) [0253] 3 First separating unit of phosphoric acid plant, preferably filtration unit [0254] 3a First separating unit of phosphoric acid plant (new), preferably filtration unit [0255] 4 Sulfuric acid production plant (existing) [0256] 5 Purification unit or calcium sulfate reaction unit [0257] 6 Second separating unit or calcium sulfate separating unit [0258] 7 Raw meal mixing unit [0259] 8 Cement clinker process unit [0260] 9 Sulfur dioxide offgas treatment [0261] 10 Calcium sulfate from stockpile (preferably from phosphoric acid production) [0262] 11 Recovery of rare earths from calcium sulfate [0263] 12 Calcium sulfate separating unit [0264] 13 Sulfuric acid production plant (new) [0265] 14 Calcium sulfate sludge from reaction unit of phosphoric acid plant [0266] 15 Liquid separated off for the existing sulfuric acid/phosphoric acid complex [0267] 16 Second calcium sulfate reaction unit