INTEGRATED METHOD FOR PRODUCING SULPHUR DIOXIDE QUALITY SUITABLE FOR A SULPHURIC ACID PROCESS FROM CALCIUM SULPHATE/PHOSPHOGYPSUM FROM PHOSPHORIC ACID PRODUCTION

Abstract

A process for producing sulfuric acid and cement clinker may use calcium sulfate that is formed as a solid by-product and separated off in phosphoric acid production in a reaction of raw phosphate with sulfuric acid to form phosphoric acid. The process comprises treating calcium sulfate separated from the phosphoric acid with an acid to obtain a suspension comprising purified calcium sulfate, separating the purified calcium sulfate in solid form from the liquid phase of the suspension, mixing the purified calcium sulfate with admixtures and reducing agents to obtain a raw meal mixture for cement clinker production, burning the raw meal mixture to obtain the cement clinker, with formation of sulfur dioxide as offgas, and subjecting the sulfur dioxide formed to offgas purification and feeding the sulfur dioxide as raw material to sulfuric acid production to produce the sulfuric acid. The sulfuric acid produced may be used as starting material in phosphoric acid production.

Claims

1.-15. (canceled)

16. A process for producing sulfuric acid and cement clinker using calcium sulfate that is formed as a solid by-product and separated off in phosphoric acid production in a reaction of raw phosphate with sulfuric acid to form phosphoric acid, wherein the process comprises: (a) treating the calcium sulfate that is separated off from the phosphoric acid with an acid to obtain a suspension comprising purified calcium sulfate; (b) separating the purified calcium sulfate in solid form from a liquid phase of the suspension, wherein the purified calcium sulfate that is separated off contains at least 50% by weight of anhydrite based on dry calcium sulfate; (c) mixing the purified calcium sulfate that is separated off with admixtures and reducing agents to obtain a raw meal mixture for cement clinker production; (d) burning the raw meal mixture to obtain the cement clinker, forming sulfur dioxide as an offgas; (e) subjecting the sulfur dioxide to offgas purification, wherein the offgas purification comprises: (e1) dry depositioning particles having a size greater than 100 μm, (e2) wet depositioning particles having a size of 20 to 100 μm, (e3) removing particles having a size less than 20 μm, and (e4) removing nitrogen oxides; and (f) feeding the sulfur dioxide as raw material to a sulfuric acid production to produce the sulfuric acid

17. The process of claim 16 being an integrated process.

18. The process of claim 16 wherein the phosphoric acid production is executed by a process selected from a dihydrate (DH) process, a hemihydrate (HH) process, a dihydrate-hemihydrate (DHH) process, a hemihydrate-dihydrate (HDH) process, and a dihydrate attack-hemihydrate filtration (DA-HF) process.

19. The process of claim 16 wherein the calcium sulfate used in step (a) is obtained from a separation unit in the phosphoric acid production, obtained after a first separation from the phosphoric acid.

20. The process of claim 16 wherein the calcium sulfate used in step (a): is calcium sulfate filtercake; is from the phosphoric acid production directly or after one or more washes with water; and/or is stockpiled calcium sulfate from the phosphoric acid production.

21. The process of claim 16 comprising at least one of: wherein in step (a) the acid is added in such an amount that a weight ratio of solids to liquid in the suspension is in a range from 1/10 to 1/1; wherein the acid resulting from the treatment after step (a) is a 1 to 12 molar acid; wherein the acid is hydrochloric acid, nitric acid, sulfurous acid, and/or sulfuric acid; wherein the treating in step (a) is conducted at a temperature in a range from 15 to 100° C.; or wherein a duration of the treating in step (a) is in a range from 5 to 120 minutes.

22. The process of claim 16 wherein a D.sub.v(50) of a grain size distribution of the calcium sulfate obtained in step (b) is in a range of 0.5-100 μm.

23. The process of claim 16 comprising using the liquid phase obtained in step (b) as feedstock in the phosphoric acid production as a portion of sulfuric acid required to digest raw phosphate.

24. The process of claim 16 wherein at least one of: the admixtures of step (c) are raw materials or compounds comprising one or more oxides of Si, Al, Fe, Ca, or precursors thereof, carbon and/or hydrocarbons are mixed into the calcium sulfate as reducing agents; or the cement clinker produced in step (d) is used for cement production.

25. The process of claim 16 wherein the dry depositioning of particles from the sulfur dioxide in steps (e1) and/or (e3) is undertaken by means of a cyclone, settling separator, rotational flow separator, and/or electrofilter.

26. The process of claim 25 wherein step (e2) is undertaken by means of a Venturi scrubber, rotary scrubber, and/or jet scrubber, wherein residual moisture in the sulfur dioxide is removed by condensation and/or a wet electrofilter.

27. The process of claim 16 comprising separating nitrogen oxides from the sulfur dioxide in step (e4) by way of a breakdown of the nitrogen oxides to N.sub.2 and H.sub.2O.

28. The process of claim 16 wherein the purified calcium sulfate obtained in step (b), before being fed to step (c), is subjected to an additional purification step for removal of rare earths in which the purified calcium sulfate obtained in step (b) is treated with a liquid and the further-purified calcium sulfate is separated in solid form from the liquid phase in the suspension that is obtained, wherein the further-purified calcium sulfate is sent to step (c), with one or more rare earths being present in the liquid phase.

29. The process of claim 16 wherein an extent of purification of the calcium sulfate in step (a) is established taking into account impurities present in the admixtures used for the raw meal, wherein guide values for phosphorus and fluorine are observed for the cement clinker, which are not more than 1.0% by weight of P.sub.2O.sub.5 and/or not more than 0.5% by weight F, wherein a liquid budget of the phosphoric acid production is altered only insignificantly, if at all, by an integrated process because an input of liquid for purification of the calcium sulfate in the integrated process is coupled to the liquid budget of the phosphoric acid production.

30. A plant for production of sulfuric acid and cement clinker using calcium sulfate by the process of claim 16, wherein the calcium sulfate is formed and separated off as a solid by-product in phosphoric acid production in a reaction of raw phosphate with sulfuric acid to form phosphoric acid, wherein the plant comprises: a calcium sulfate reaction unit for treating the calcium sulfate separated from the phosphoric acid therein with an acid to obtain a suspension comprising purified calcium sulfate; a calcium sulfate separation unit for separating the purified calcium sulfate in solid form from a liquid phase of the suspension; a raw meal mixing unit suitable for mixing the purified calcium sulfate separated off with admixtures and reducing agents therein to obtain a raw meal mixture for cement clinker production; a cement clinker production unit comprising a preheater, at least one burner device, and a cooler that are suitable for preheating, burning, and cooling the raw meal mixture to obtain the cement clinker, forming sulfur dioxide as offgas; a sulfur dioxide offgas purification plant that is suitable for removing impurities from the sulfur dioxide offgas; and a sulfuric acid production plant configured to be supplied with the sulfur dioxide offgas that has been formed in the cement clinker production unit to produce sulfuric acid.

Description

[0161] The invention is described hereinafter by working examples, which are elucidated in detail by the figures. The working examples are not intended to restrict the scope of the invention claimed in any way.

[0162] FIG. 1 shows a schematic flow diagram for utilization of calcium sulfate from phosphoric acid production with the aid of an integrated complex for production of cement clinker and sulfuric acid.

[0163] FIG. 2 shows a schematic flow diagram for utilization of calcium sulfate from phosphoric acid production by means of an integrated complex for production of cement clinker and sulfuric acid as in FIG. 1, additionally showing alternative or additional process steps.

[0164] FIG. 3 shows a schematic flow diagram for purification of the SO.sub.2 gas which is obtained in the production of cement clinker.

[0165] FIG. 1 shows a flow diagram of phosphoric acid production (existing complex) and an integrated process for production of cement clinker and sulfuric acid from calcium sulfate that originates from phosphoric acid production (integrated complex). The phosphate rock is processed in a processing unit 1 in order to obtain the raw phosphate. The raw phosphate is reacted in the reaction unit of the phosphoric acid plant 2 with sulfuric acid coming from the sulfuric acid production plant, in order to form phosphoric acid and solid calcium sulfate as by-product. The calcium sulfate generated in phosphoric acid production is separated from the phosphoric acid in filtration unit 3 of the phosphoric acid plant and sent to the calcium sulfate reaction unit 5. The calcium sulfate is treated there with acid, such that, after the treatment, for example, a 1-12 molar acid, especially a 1-12 molar sulfuric acid, is obtained. For example, it is possible to add a 1-12 molar sulfuric acid for treatment. The treatment can be conducted after addition of the acid, for example, at a temperature of 15-100° C. for 5 to 120 min, preferably with movement of the suspension obtained, for example by stirring. This reduces the level of impurities in the calcium sulfate that adversely affect the downstream cement clinker process and cement quality to the content required by the cement clinker process. In a calcium sulfate separation unit 6, which is preferably a filtration unit, the liquid and the resulting solids are separated from one another. The liquid, especially the filtrate, can be used in the existing phosphoric acid-sulfuric acid complex. The treated calcium sulfate is sent to the raw meal preparation unit 7 upstream of the cement clinker process. The calcium sulfate is mixed therein with the necessary admixtures for the required cement clinker quality in the correct ratio. The cement clinker raw meal prepared is used to charge the cement clinker process unit 8, preferably with preheating of the raw meal in a heat exchanger prior to supply to the process unit 8 (not shown). In the cement clinker process unit 8, sulfur dioxide is separated from the calcium sulfate and fed as offgas from the cement clinker process unit to the sulfur dioxide offgas treatment 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. 2). The calcium remaining in the cement clinker process unit is reacted with the admixtures to give cement clinker. The combustion temperature for cement clinker production may, for example, be at a temperature in the range from 1200° C. to 1600° C. and a combustion time of 5 minutes to 60 minutes. The cement clinker thus produced is cooled and can be processed further to cement.

[0166] FIG. 2 shows a schematic flow diagram for utilization of calcium sulfate from phosphoric acid production by means of an integrated complex for production of cement clinker and sulfuric acid according to FIG. 1, additionally showing alternative or additional process steps. There follows a discussion of the alternative or additional process steps; otherwise, reference is made to the elucidations relating to FIG. 1. FIG. 2 shows an alternative source for the calcium sulfate used in step a). Rather than the calcium sulfate from the filtration unit of the phosphoric acid plant 3, it is possible to use a calcium sulfate from a stockpile 10 for the calcium sulfate used in step a), this being deposited calcium sulfate from phosphoric acid production. In addition, FIG. 2 shows the optional processing step for removal of rare earths, which comprises a reaction unit for recovering rare earth metals from the calcium sulfate 11 and the calcium sulfate separation unit 12 for separation of the liquid phase from the purified calcium sulfate. FIG. 2 also shows 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 a new sulfuric acid production plant 13.

[0167] FIG. 3 shows a schematic flow diagram for purification of the SO.sub.2 gas in the sulfur dioxide offgas treatment plant 9. For this purpose, the SO.sub.2 offgas from the clinker plant is first sent to a dry deposition 14, preferably in order to separate out particles having a size of greater than 100 μm. In addition, the gas processing operation may comprise a downstream optional wet separation 15, preferably in order to separate out particles having a size of 20 to 100 μm, and an optional separation of particles 16 preferably having a size of less than 20 μm. Downstream of the particle deposition plants 14, 15, 16 is preferably connected a plant to separate off nitrogen oxides 17.

LIST OF REFERENCE NUMERALS

[0168] 1 Processing unit for phosphate rock or phosphate ore [0169] 2 Reaction unit of the phosphoric acid plant [0170] 3 Filtration unit of the phosphoric acid plant [0171] 4 Sulfuric acid production plant (existing) [0172] 5 Calcium sulfate reaction unit [0173] 6 Calcium sulfate separation unit [0174] 7 Raw meal preparation unit [0175] 8 Cement clinker process unit [0176] 9 Sulfur dioxide offgas treatment [0177] 10 Calcium sulfate from stockpile (from phosphoric acid production) [0178] 11 Recovery of rare earths from calcium sulfate [0179] 12 Calcium sulfate separation unit [0180] 13 Sulfuric acid production plant (new) [0181] 14 Dry deposition [0182] 15 Optional wet deposition [0183] 16 Optional final purification stage [0184] 17 Optional removal of nitrogen oxides