Integrated method for the commercial and industrial utilisation of calcium sulphate whilst obtaining rare earth elements from the production of phosphoric acid
View Patent ↗A process may involve treating calcium sulfate separated from phosphoric acid with acid to obtain a suspension comprising purified calcium sulfate, separating the purified calcium sulfate in solid form from a liquid phase of the suspension, treating the purified calcium sulfate with water or with a salt- and/or chelate ligand-containing aqueous solution to leach rare earths out of the calcium sulfate, separating the further-purified calcium sulfate in solid form from the liquid phase of the suspension, mixing the purified calcium sulfate that is separated off 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 and thereby forming sulfur dioxide as offgas, and feeding the sulfur dioxide as raw material to sulfuric acid production to produce the sulfuric acid.
1 . An integrated process for producing sulfuric acid and cement clinker and for obtaining rare earths, the process comprising performing the following steps in an integrated plant complex:
(a) treating calcium sulfate separated from phosphoric acid in step (f) 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,
(b1) treating the purified calcium sulfate that is separated in step (b) with water or with a salt- and/or chelate ligand-containing aqueous solution to leach rare earths out of the calcium sulfate, wherein a weight ratio of solids to liquid for the leaching of the rare earths is 1:1 to 1:10, wherein a dwell time for the leaching of the rare earths is 5 h to 68 h,
(b2) separating the further-purified calcium sulfate from step (b1) in solid form from a liquid phase of a suspension obtained in step (b1) with one or more rare earths present in the liquid phase;
(c) mixing the further-purified calcium sulfate that is separated off in step (b1) 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 offgas;
(e) feeding the sulfur dioxide as raw material to sulfuric acid production to produce the sulfuric acid; and
(f) forming the calcium sulfate and separating the calcium sulfate off as a solid by-product during production of the phosphoric acid in a reaction of raw phosphate with the sulfuric acid to form the phosphoric acid.
2 . The process of claim 1 wherein the calcium sulfate used in step (a) is calcium sulfate obtained from a separation unit in the phosphoric acid production directly or after one or more washes with liquid.
3 . The process of claim 1 wherein at least one of:
in step (a) the acid is added such that a weight ratio of solids to liquid in the suspension in step (a) is in a range from 1:10 to 1:1;
an acid resulting from the treatment in step (a) is a 1 to 12 molar acid;
the acid used in step (a) is hydrochloric acid, nitric acid, sulfurous acid, and/or sulfuric acid;
the treatment in step (a) is conducted at a temperature in a range from 15 to 100° C.; and
a duration of the treatment in step (a) is in a range from 5 to 120 minutes.
4 . The process of claim 1 wherein a D v (50) of a grain size distribution of the separated, purified calcium sulfate obtained in step (b) is in a range of 0.5-100 μm.
5 . The process of claim 1 wherein the aqueous solution used for leaching in step (b1) comprises a salt and/or chelate ligand in a concentration of 1% to 50% by weight.
6 . The process of claim 1 wherein at least one of:
the salt used for leaching in step (b1) comprises a chloride and/or a nitrate; or
the chelate ligand-containing aqueous solution comprises EDTA and/or citrate.
7 . The process of claim 1 wherein a reaction mixture in step (b1) is kept in motion during a reaction time by circulating, stirring, or blowing in gas.
8 . The process of claim 1 wherein the weight ratio of solids to liquid for the leaching in step (b1) is in a range from 1:2 to 1:5.
9 . The process of claim 1 wherein the separation of the further-purified calcium sulfate in solid form from the liquid phase comprising one or more rare earths in step (b2) comprises a centrifugation, a filtration, a sedimentation, and/or an evaporation of the liquid phase.
10 . The process of claim 1 comprising obtaining rare earths by way of solvent-solvent extraction, ion exchange methods, precipitation as hydroxides, oxalates, phosphates, and/or carbonates, from the solution comprising one or more rare earths obtained in step (b2).
11 . The process of claim 1 wherein the admixtures for production of the raw meal mixture in step (c) are raw materials or compounds comprising one or more oxides selected from oxides of Si, Al, Fe, Ca, or precursors thereof.
12 . The process of claim 11 wherein carbon, hydrocarbons, and/or elemental sulfur is mixed into the calcium sulfate as a reducing agent.
13 . The process of claim 1 wherein carbon, hydrocarbons, and/or elemental sulfur is mixed into the calcium sulfate as a reducing agent.
14 . The process of claim 1 wherein an extent of purification of the calcium sulfate in step (a) is established taking account of impurities present in the admixtures used for the raw meal mixture, wherein guide values for phosphorus and fluorine are observed for the cement clinker.
15 . The process of claim 14 wherein a liquid budget of the phosphoric acid production is not altered by the integrated process because input of liquid for purification of the calcium sulfate in the integrated process is coupled to the liquid budget of the phosphoric acid production.
16 . The process of claim 14 wherein the guide value for phosphorus is not more than 1.0% by weight of P 2 O 5 and the guide value for fluorine is not more than 0.5% by weight of F.
17 . The process of claim 14 wherein the guide value for phosphorus is not more than 0.5% by weight of P 2 O 5 and the guide value for fluorine is not more than 0.25% by weight of F.
18 . The process of claim 14 wherein the guide value for phosphorus is not more than 0.1% by weight of P 2 O 5 and the guide value for fluorine is not more than 0.1% by weight of F.