AU2008202077B2 - Treatment of metal-containing water - Google Patents

Abstract

A process for treating metal-containing water includes using a dissolved sulphide complex having the formula A(HS)(B) where A is an alkaline earth metal and B is HS~ or HCOa~, to precipitate metals present in the water as 10 metal sulphides. In this fashion, the metal concentration of the water is reduced.

Description

Australian Patents Act 1990 - Regulation 3.2 ORIGINAL COMPLETE SPECIFICATION STANDARD PATENT Invention Title Treatment of metal-containing water The following statement is a full description of this invention, including the best method of performing it known to me/us: P/00/0 II 1 THIS INVENTION relates to the treatment of metal-containing water. It relates in particular to a process for treating metal-containing water. 5 Metals are present in many industrial effluents such as acid mine water. Owing to the toxicity of such metals, metal concentrations in such effluents must be at a low level in order that such effluents can be discharged into public streams or used for drinking water production. It is an aim of the present invention to provide a process whereby metal concentrations in such effluents can be reduced. 10 SUMMARY OF THE INVENTION According to the invention, there is provided a process for treating metal containing water, which process includes reacting, in an aqueous component, an 15 alkaline earth metal sulphide, AS, optionally mixed with a Ca alkali and/or a Mg alkali, with an acid or C0 2 , thereby to form a dissolved sulphide complex having the formula A(HS)(B) where A is an alkaline earth metal and B is HS' or HCO&; admixing the aqueous component with the metal-containing water; and using the dissolved sulphide complex having the formula A(HS)(B) to precipitate metals 20 present in the water as metal sulphides, thereby to reduce the metal concentration of the water. The metal-containing water may, in principle, be any metal containing water. Thus, in particular, it may be mine water, which is usually acidic. However, 25 instead, it can be more-or-less neutral metal-containing water. While, at least in principle, A may be any of the other alkaline earth metals of Group IIA of the Periodic Table of elements, such as Ba or Sr, it preferably is Ca. 30 In this version, it is expected that the complex formed will be Ca(HS) 2 , particularly at a pH>10. This is advantageous, as the solubility of Ca(HS) 2 in water is high so that the Ca(HS) 2 will react quickly with metals in the water, 2 In one embodiment, formation of the complex Ca(HS)(B) may be effected by passing the aqueous component through a bed of particulate CaS while simultaneously treating it with CO 2 . In particular, the aqueous component may then be a slipstream of the metal-containing water separated from a main stream 5 of the metal-containing water which is thus treated by passing it through the bed of CaS while simultaneously treating it with C02, and thereafter recombining the slipstream with the main stream of metal-containing water. It is believed that this embodiment could be used in particular to treat manganese (Mn 2 ) containing water which is at a pH of about 7, which is an aqueous component found on some 10 gold mines. The manganese present in the slipstream is converted to MnS, which precipitates out; however, an excess of the complex Ca(HS)(B) forms and this results in MnS precipitation when the treated slipstream is recombined with the main stream. 15 In another embodiment, formation of the complex Ca(HS)(B) may be effected by adding the acid or CO 2 to a slurry comprising CaS mixed with water, to form the complex and to lower the pH of the slurry thereby to increase the dissolved sulphide concentration CaS, and then mixing the resultant treated aqueous component with the metal-containing water. 20 The pH of the slurry comprising CaS and water, is normally in the range of 12.0 12.4. Within this pH range, the solubility of CaS in water is only about 1.4 g/t, at ambient conditions. By reducing the pH of the slurry by means of 3 an acid, such as an inorganic acid, or CO 2 , the dissolved sulphide concentration (as S) can increase significantly, e.g. up to about 48 g/f or even higher. 5 Preferably, CO 2 is used for pH reduction of the slurry. Thus, CO 2 may be added to the slurry, thereby dissolving CaS, with dissolved Ca(HS) 2 complex forming and CaCO 3 precipitating, In accordance with reaction (1):

CO

2 + 2CaS + H 2 0 -* Ca(HS) 2 + CaCO 3 (1) 10 Care must be taken not to allow reaction (1) to proceed to the point where H 2 S is evolved according to reaction (2) or even reaction (3) with more CO 2 addition. This will occur when excess CO 2 is used, le when the pH is reduced to less than 8. 15 Co 2 + Ca(HS)2 + H 2 0 -> Ca(HC0 3

)

2 + H2S (2) Ca(HCO 3 )(HS) + CO 2 + H 2 0 - Ca(HCOa) 2 + H 2 S (3) The CaCO 3 that is produced remains in the slurry, and is thus also contacted 20 with the metal-containing water. The CaCO 3 assists In controlling the pH of the water, once it is admixed with the slurry. If the CaCO 3 is not retained in the slurry, the slurry addition will cause the water pH to drop rapidly, owing to formation and precipitation of sulphides of the metals in the water, such as FeS. When the metals are precipitated as metal sulphides, hydrogen ions are 25 released, and need to be neutralized, which is effected by means of the CaCOs in the water. Thus, the slurry that is admixed with the metal-containing water may contain dissolved CaS, solid (undissolved) CaS, dissolved and/or solid Ca(HS) 2 and 30 dissolved and/or solid CaCO 3 , depending on the pH; thus, when the pH>8, there is probably some dissolved CaCQ 3 and Ca(HS) 2 ; however, if the pH drops to below 8, then Ca(HS)(HCO 3 ) will probably form, with redissolution of precipitated CaCO 3

.

4 However, If desired, at least some of the CaCO 3 may be removed from the aqueous component, before mixing the treated aqueous component with the metal-containing water. 5 Sufficient carbon dioxide may be added to the CaS slurry so that the dissolved Ca(HS) 2 complex that is formed is at or near its maximum concentration In the slurry, at the temperature of the slurry. Thus, C02 may be added to a point just before H 2 S starts stripping off in accordance with reaction (2). The 10 process may include thereafter heating at least some of the slurry, thereby precipitating Ca(HS) 2 as a solid since its solubility In water is less at elevated temperatures than at low temperatures; and recovering the precipitated pure solid Ca(HS) 2 as a by-product, 15 The admixing of the aqueous component with the metal-containing water may be effected in a plurality of reaction stages through which the water passes sequentially and to each of which aqueous component is introduced, with each reaction stage being at a different pH, so that different metals precipitate at successively higher pH's In successive reaction stages. 20 Thus, In the first such reaction stage, the addition of the slurry will raise the pH, resulting in precipitation of a particular metal or metals as metal sulphides; further addition of slurry to the second reaction stage will then raise the pH further, resulting in precipitation of a different metal or metals as metal 25 sulphides; and so on. In other words, multiple additions of slurry raises the pH in Increments, to achieve selective precipitation of metal sulphides. In the reaction stages, the following reactions thus take place: 30 MS0 4 + CaS - MS + CaSO 4 (4) 2MS0 4 + Ca(HS) 2 --> 2MS + CaSO 4 + H 2 S0 4 (5) CaCO 3 + H 2 S0 4 -+ CaSO 4 + CO 2

+H

2 0 (6) 5 It will, however, be appreciated that, instead of selective precipitation of metal sulphides as hereinbefore described, precipitation can be effected in a single reaction stage, in which case a mixture of different precipitated metal sulphides is obtained. 5 A gypsum crystallization inhibitor may, if desired, be added to the metal containing water upstream of at least one of the reaction stages. Addition of such an inhibitor will prevent or inhibit gypsum crystallization so that the metal sulphides can be recovered in a more-or-less pure form i.e. not contaminated 10 with gypsum. An example of a gypsum crystallization inhibitor is sodium hexa metaphosphate, available under the trade name Antiprex. It can be dosed at a rate between 1 and 60mg/t Alternatively, metals can be precipitated as MS together with gypsum and 15 leached from the sludge. C02 leaching will dissolve Mg(OH) 2 . Other metals can be leached with H 2 S04. As hereinbefore indicated, the metal-containIng water may, In particular, be an acid metal-containing water, such as acid mine water or another acid industrial 20 effluent containing dissolved metals. However, It can also be neutral metal containing water. The process may include pre-treating the acid metal-containing water, before it is admixed with the aqueous component, with CaCO 3 in a pre-treatment stage, 25 to neutralize free acid in the water and to precipitate any Fes* and Al 3 ' as Fe(OH) 3 and AI(OH) 3 respectively, and with CO 2 being generated. When the acid metal-containing water contains sulphuric acid as well as iron and aluminium as dissolved metals, and which is often the case, neutralization 30 may proceed in accordance with reaction (7), so as to neutralize sulphuric acid in the water and to precipitate iron and aluminium as Fe(OH) 3 and AI(OH) 3 respectively, and with C02 being generated: 6

H

2 S0 4 +CaCO 3 -- + CaSO 4 + CO 2 + H 2 0 (7) Thus, in the pre-treatment stage, a sludge comprising CaSO 4 , Fe(OH) 3 and

AI(OH)

3 is produced. At least part of the CO 2 generated in the pre-treatment 5 stage in accordance with reaction (7) may be used to reduce the pH of the aqueous component. The process may include the preliminary step of producing the slurry by reacting, in a CaS production stage, gypsum (CaSO 4 ) with a reductant at 10 elevated temperature, to produce CaS and CO 2 in accordance with reaction (8); and admixing the CaS with water, to obtain the slurry: CaSO4 + 2C -+ CaS + 2CO 2 (8) 15 The reductant may, in particular, be coal. However, it is to be appreciated that, instead, any other suitable reductant can be used. The process may include, in a polishing stage which takes place after the treatment of the water with the slurry, contacting the water with Fe(OH) 3 to 20 remove residual sulphides therefrom. The Fe(OH)a used in the polishing stage may be obtained by feeding at least a portion of the sludge from the pre-treatment stage to the polishing stage. 25 The residual sulphide removal in the polishing stage thus proceeds in accordance with the following reaction: 3HS"+2Fe(OH)3 -+ Fe 2

S

3 + 3H 2 0 (9) 30 The Fe 2

S

3 precipitates out, The Fe(OH) 3 addition may counteract the gypsum formation inhibition obtained by having added the inhibitor, to allow eventual removal of gypsum from the 7 treated water. When it is desired to remove gypsum from the water, lime is added to the polishing stage to stimulate gypsum crystallization, in accordance with the reaction (10): 5 Ca 2 * + S0 4 2 + 2H 2 0 - CaSO4-2H20 (10) However, when A is Ba rather than Ca, then BaS rather than CaS is used. Then, in a BaS production stage, barium sulphate can be reacted with a reductant such as carbon to produce BaS which is then admixed with water to obtain the aqueous 10 component in the form of a slurry, in accordance with reactions (11) and (12): BaSO 4 + 2C BaS + 2CO2 (11) BaS + C02 + H20 -- BaCO 3 + H 2 S (12) 15 Also described is a process that includes forming the complex A(HS)B in situ in the metal-containing water. In particular, the process may then include simultaneously adding CaCO 3 and H 2 S to the metal-containing water, in a primary reaction stage, to form Ca(HS) 2 and/or Ca(HS)(HCO 3 ) in situ, in accordance with reactions (13) and (14): 20 CaCO 3 + H 2 S Ca(HS)(HCO 3 ) (13) CaCO3 + 2H 2 S Ca(HS) 2 + CO 2 + H 2 0 (14) The process may then include withdrawing treated water from the reaction stage, 25 passing it into a secondary reaction stage, and, in the secondary reaction stage, adding calcium hydroxide, Ca(OH) 2 , to the water, thereby to precipitate magnesium hydroxide, gypsum and ettringite from the water. The process may also include passing treated water from the secondary reaction stage to a tertiary reaction stage, and introducing barium carbonate, BaCO 3 , into the water in the 30 tertiary reaction stage, thereby to precipitate barium sulphate, BaSO 4 , and calcium carbonate, CaCO 3 , from the water, thus removing sulphates from the water. The process may further include adding carbon dioxide, C02, to the water simultaneously with, or ahead of, the BaCO 3 . The solids that are formed in the M-Vv 1-WU T QWiMq VOg VI.UV%- /UW VI1 8 tertiary stage may be separated from residual water, and one or more products recovered from the solids. The processes hereinbefore described use an aqueous component which is not 5 biologically derived. However, it can, instead, be biologically derived. Further described is a process that includes adding an aqueous component which has been derived from a biological sulphate removal process and which is rich in Ca(HS)(HCO 3 ), to the metal-containing water, to obtain metal precipitation. 10 Embodiments of the invention will now be described in more detail, by way of example only, with reference to the accompanying drawings. In the drawings, 15 FIGURE 1 shows a simplified flow diagram of a process according to one embodiment of the invention, for treating metal-containing water; FIGURE 2 shows a simplified flow diagram of a process according to another embodiment of the invention, for treating metal-containing water; and FIGURE 3 shows a simplified flow diagram of a process according to yet 20 another embodiment of the invention, for treating metal-containing water. Referring to Figure 1, reference numeral 10 generally indicates a process according to one embodiment of the invention for treating metal-containing water. 25 The process 10 includes a pre-treatment stage 12, with an acid metal-containing water feed line 14 leading into stage 12, as does a CaCO 3 addition line 16. A

CO

2 withdrawal line 18 leads from the stage 12, as do a water transfer line 20 and a sludge withdrawal line 22.

9 The water transfer line 20 leads to a first reaction stage 24, with a gypsum crystallization Inhibitor addition line 26 leading Into line 20 ahead of the reaction stage 24. A water transfer line 28 leads from the reaction stage 24 to a second reaction stage 30, with a water transfer line 32 leading from the 5 second reaction stage 30 to a third reaction stage 34. It will be appreciated that, in other examples, a greater or lesser number of reaction stages can be provided, depending on the number of different metals present In the effluent that need to be recovered. 10 Precipitated metal sulphide withdrawal lines 27, 31 and 35 lead from the reaction stages 24, 30 and 34 respectively. A water transfer line 36 leads from the reaction stage 34. 15 The process 10 also includes a CaS production stage 40, with a gypsum addition line 42 and a coal addition line 44 leading respectively into the stage 40. A CaS withdrawal line 66 leads from the stage 40 to a treatment stage 50. A CaS/Ca(HS)2/CaCO 3 withdrawal line 52 leads from the stage 50. A line 54 leads from the line 52 into the reaction stage 24, a line 56 leads from the line 20 52 into the reaction stage 30, and the line 52 leads finally to the reaction stage 34. A water addition line 60 leads into the treatment stage 50, as does a CO 2 addition line 62. 25 The process 10 also includes a polishing stage 70. The water transfer line 36 leads into the polishing stage 70, as does a sludge addition line 68. The line 68 leads from the sludge withdrawal line 22 exiting the pre-treatment stage 12. A lime addition line 72 leads into the stage 70 while a product water withdrawal 30 line 76 leads from it. The water line 60 leading into the treatment stage 50, leads from the water withdrawal line 76. A gypsum withdrawal line 74 leads from the stage 70.

V "rnq T. OP 0443 Ua 1.uA~k;..1b'Pnr 4 10 In use, gypsum is introduced into the stage 40 along the line 42 while coal as reductant is introduced into the stage 40 along the line 44. In the stage 40, CaS is produced in accordance with reaction (9). The CaS thus produced is transferred to the treatment stage 50 along the line 66. 5 In the treatment stage 50, the CaS is scurried with water entering the stage along the line 60. The pH of the slurry is reduced by means of C02 entering along the line 18, supplemented, if necessary, by external CO 2 introduced along the line 62 or CO 2 produced in the stage 40. As the slurry pH is reduced -in the stage 50, 10 Ca(HS) 2 is produced in accordance with reaction (1). A slurry comprising dissolved CaS, dissolved Ca(HS) 2 and precipitated CaCO 3 is thus produced in the treatment stage 50 and is withdrawn along the line 52 to be added to reaction stage 34, and, by means of the lines 54 and 56, to the reaction stages 24 and 30 respectively. 15 In another version of this embodiment of the invention (not shown), sufficient C02 can be added to the CaS slurry in the stage 50, so that the dissolved Ca(HS) 2 that is formed is at or near its maximum concentration in the slurry, at the temperature of the slurry. A portion of the slurry can then be heated, so that Ca(HS) 2 , which is 20 less soluble in water at elevated temperatures than at lower temperatures, then precipitates out, and can be recovered as a solid by-product. 3+ In the pre-treatment stage 12, acid mine water which includes Fe and A1 3 *, that enters the stage along the line 14, is contacted with CaCO 3 which enters the stage 25 along the line 16. Free acid in the water is thereby neutralized in accordance with reaction (7). The C02 that is thus produced is transferred, by means of line 18, to the treatment stage 50. A sludge comprising gypsum, Fe(OH) 3 and AI(OH) 3 is withdrawn along the line 22. 30 Metal-containing water is withdrawn along line 20 from the stage 12, and a gypsum crystallization inhibitor is added thereto along the line 26. The water thereafter sequentially passes through the reaction stages 24, 30 and 34. In the reaction stages 24, 30 and 34, the pH increases sequentially, with the addition of 11 the slurry entering along the lines 54, 56 and 52 respectively, Thus, selective metal precipitation is achieved in the reaction stages 24, 30 and 34 in accordance with reactions (4), (5) and (6). The precipitated metals are withdrawn from the reaction stages 24, 30 and 34 along the lines 27, 31 and 35 respectively. 5 Typically, the metals that are removed in reaction stage 24, 30 and 34 include Fe 2 +, Mn 2 +, Nj*, Zn 2 4 and Co2+ In view of the concentration of dissolved CaS in the slurry that enters the reaction stages 24, 30 and 34, a relatively small volume of slurry, relative to the water 10 passing through the reaction stages 24, 30 and 34, is required. It will be appreciated that each of the reaction stages 24, 30 and 34 will include some form of separation means e.g. a clarifier, a filter or a cyclone, to recover the precipitated metal sulphides. 15 The addition of slurry to the reaction stages 24, 30 and 34 will increase the gypsum concentration in the water therein and may result in gypsum crystallization. Such crystallization is, however, optionally inhibited or prevented through the addition, along the line 26, of a gypsum crystallization inhibitor such as 20 that available under the trademark ANTIPREX A. Instead, or additionally, the inhibitor can be added directly to the stage 12, to prevent formation of gypsum crystals or nuclei. The activity of such an inhibitor is then easily destroyed, in the polishing stage 70 by means of the addition of lime, to allow gypsum precipitation and separation. 25 Water from the final reaction stage 34 passes into the polishing stage 70 where it is admixed with sludge entering the stage 70 along the line 68. Sludge preferably only enters the stage 70 when the gypsum therein is pure. Lime is added along the line 72 to stimulate gypsum crystallization. In the polishing stage 70, residual 30 sulphide is removed in accordance with reaction (9) while gypsum is crystallized in accordance with reaction (10). Gypsum 12 and Fe 2

S

3 are removed along the line 74 and preferably recycled (along line 78) to the stage 40 after being dewatered and dried, while treated product water Is withdrawn along the line 76. Some of the treated water is recycled to the stage 50 along the line 60. 5 In the process 10, metals are thus recovered from stages 24, 30 and 34 selectively as relatively pure compounds which can then be processed individually. Additionally, pure gypsum is produced which is suitable as a raw material for the production of sulphur and calcium carbonate, or can be 10 recycled for CaS production to recover metals, Still further, the process 10 is simple and safe to operate. It will be appreciated that, in another embodiment of the invention, Instead of all the metal-containIng water being subjected to treatment with the CaS slurry, 15 only a side stream thereof need be treated. In such case, the sulphide rich side stream will then be recombined with the main metal-rich stream so that the sulphide reacts with the metals to form the insoluble metal sulphides. The benefit is that a large portion of the metal rich stream is contacted with soluble sulphate which reacts faster than CaS, where the latter first need to dissolve 20 before 82- can react with the metals. In the side stream excess CaS In contact with metal containing water will result in complete removal of metals (e.g. Mn) and free sulphide will be present in the effluent. The concentration of sulphide will be of the order of 270 mg/I (as S) which is much higher than the solubility of CaS. 25 CaS/Ca(HS)2 can be dosed Into the reaction stages 24, 30 and 34 as a solution or slurry to the pH required for the metal that needs to be removed in that stage. 30 It is believed that the process 10 offers the following advantages over other processes for removing metals from acid mine water: Cheap and abundant CaCO 3 is used for neutralization of the bulk of the acid as well as for the removal of Fe 3 * and A 3

*

13 - Metals can be recovered separately and selectively. m Pure gypsum Is produced or can be recycled . The process is suitable for small-scale operation - Use of this process permits establishment of a CaS production facility 5 which can then be used economically to support metals removal in accordance with the process of the Invention. Referring to Figure 2, reference numeral 100 generally indicates a process according to another embodiment of the invention, for treating metal-containing 10 water. The process 100 includes a first reaction stage 102 with an acid mine drainage line 104 leading into the stage 102. The stage 102 is fitted with a mixer 106. An air Inlet line 108 leads Into the stage 102, as do a CaCO 3 addition line 110 15 and an H 2 S addition line 112. An arrow 114 indicates C02 evolution from the stage 102. A transfer line 116 leads from the stage 102 to a second reaction stage 118. The stage 118 is also fitted with a stirrer 120, and a Ca(OH) 2 addition line 122 20 leads into the stage 118. A transfer line 124 leads from the stage 118 to a settler 126, with a solids withdrawal line 128 leading from the settler 126. 25 A liquid transfer line 130 leads from the settler 126 to a third reaction stage 132, also fitted with a stirrer 134. A CO 2 addition line 136 leads into the stage 132, as does a BaCO 3 addition line 138. Another BaC0 3 addition line 140 also leads into the stage 132. 30 A transfer line 142 leads from the stage 132 to a settler 144, from which leads a water withdrawal line 146. A solids withdrawal line 148 leads from the bottom of the settler 144 to a kiln 150. A coal or CHa gas or H 2 or CO addition -wV1Wvon vw GUUTOwI4 0.1Du v.1 0//oe 2 12 14 line 152 leads into the kiln 150, with a solids withdrawal line 154 leading from the kiln. A CO 2 withdrawal line 156 also leads from the kiln 150. The solids line 154 leads to a reaction stage 157. A water addition line 158 also 5 leads into the stage 157. A BaS withdrawal line 160 leads from the stage 157 to a reaction stage 162. The Ca(OH) 2 line 122 leads from the stage 157. 10 A CO 2 addition line 164 leads from the CO 2 line 156 to the stage 162, with a H2S withdrawal line 166 also leading from the stage 162. The BaCO 3 line 140 also leads from the stage 162. 15 The H 2 S line 166 leads to a reaction stage 168. An air/0 2 addition line 170 leads into the stage 168. An H 2

SO

4 withdrawal line 172 leads from the stage 168. 20 The CO 2 line 156 which leads from the kiln 150, leads into a reaction stage 174, with the line 128 from the settler 126 also leading into the stage 174. An H 2 S line 176 leads from the stage 174 to the H 2 S line 112 (leading into the first reaction stage 102). 25 An air addition line 178 leads into the stage 174 while a Mg(HCO3) 2 transfer line 180 leads from the stage 174 to a reaction stage 182. A heat addition line 184 leads into the stage 182, while a MgCO 3 withdrawal line 186 leads from the stage 182. 30 A solids withdrawal line 188 leads from the stage 174 to a reaction stage 190. The

H

2

SO

4 line 172 from the stage 168 also leads into the stage 190. A metal sulphate withdrawal line 192 leads from the stage 190. An air addition line 194 leads into the stage 190, while a gypsum withdrawal line 196 leads from it.

15 The gypsum line 196 leads into a reaction stage 200, with a coal addition line 202 also leading Into the stage 200. A CaSlash withdrawal line 204 leads from the stage 200 to a reaction stage 206. A CO 2 line 208 also leads from the 5 stage 200 to the stage 206, An H 2 0 addition line 210 leads into the stage 206, with a H 2 S withdrawal line 212 leading from the stage 210. The H 2 S line 212 leads into a Pipco reaction stage 214, with a sulphur withdrawal line 216 leading from the stage 214. A H2S line 218 leads from the 10 line 212 to the H 2 S line 112. In use, the process 100 functions in accordance with the reactions as set out hereunder, that take place in the various reaction stages. Central to the process 100 is, however, that in the reaction stage 102, metal precipitation is 15 effected by virtue of the Ca(HS)HC0 3 complex that forms by virtue of reaction of CaCO3 with H 2 S in accordance with reaction (13). REACTIONS IN THE PROCESS 100 Reaction Stage 102: 20 CaC03 & H 2 S dosing CaCOs + H 2 S - Ca(HS)(HCO 3 ) (13) H2S0 4 + CaCO -+ CaSO4 + CO2 + H 2 0 2Al1 3 + 3CaCO 3 + 3H 2 0 2Al(OH)a + 3CO 2 + 3Ca 2 + 2Fe 3 * + 3CaCO 3 + 3H 2 0 - 2FeAI(OH) + 3C0 2 + 3Ca2+ 25 F0 2 * + H 2 - FeS + 2H* Mn2++ H 2 S - MnS + 2H* Ni 2 + + H 2 S -NIS + 2H* Co 2 + + H 2 S - CoS + 2H* 30 Reaction Stage 118: Ca(OH) 2 dosing Mg2+ + Ca(OH)2 -- Mg(OH) 2 + Ca2+ Ca 2 ++ SO 4 2 '+ 2H 2 0 - CaSO 4 .2H 2 0 2A(OH)a + 3Ca(OH) 2 + 3CaSo4 -- Ettringite 35 Reaction Stage 132: BaCOs dosing CaSO 4 + BaCO 3 - BaSO 4 (s) + CaCO 3 (s) 16 Reaction Stage 174:

CO

2 dosing to dissolve Mg(OH) 2 Mg(OH) 2 + 2CO2 + H 2 0 Mg(HCO 3

)

2 (pH 8.5) 5 Reaction Stage 190:

H

2

SO

4 dosing to leach metals out MS + H 2

SO

4

MSO

4 + H2S; M Fe, Mn, Ni, Co 2M(OH) 3 + 3H 2

SO

4 2M(SO 4

)

3 + 3H 2 0; M = Fe(Ill), Al(Ill) 10 Reaction Stage 168:

H

2

SO

4 production

H

2 S + 202

H

2 SO4 15 Reaction Stage 182: Heating to form MgCo 3 Mg(HCO 3

)

2 MgCO3 + C02 + H 2 0 (70*C) 20 Kiln 150: BaSO 4 + 2C BaS + CO 2 CaCO 3 CaO + CO 2 Reaction Stage 157: 25 Leaching of BaS BaS + H 2 0 Ba(SH)(OH) Reaction Stage 162: BaCO 3 precipitation and H2S-stripping 30 BaS + C02 + H 2 0 BaCO 3 + H 2 S In summary, metal hydroxides and metal sulphides that are formed in the stage 102, are transferred to the stage 118 along the line 116. CO2 that is produced in 35 the stage 102 is withdrawn as indicated by arrow 114. In the stage 118, Ca(OH) 2 dosing takes place, with the products of the stage 118 passing to the settler 126. 40 Solids in the form of Ettringite, Mg(OH) 2 , gypsum (CaSO 4 .2H 2 0) metal sulphides and metal hydroxides are withdrawn from the settler 126 along the line 128 and enter the stage 174.

17 Liquid from the stage 126 passes along the line 130 into the reaction stage 132 where BaCO 3 dosing takes place. 5 The products from the reaction stage 132 pass to the settler 144, with water being withdrawn as a product along the line 146. BaSO 4 and CaCO 3 pass from the settler 144, along the line 148, into the kiln 150 where they are subjected to heat treatment at 900"C through the combustion of 10 coal entering along the line 152. The reaction in the remaining stages are as set out above, with the eventual products of the process 100 thus being water (from the settler 144), sulphur (from the Pipco stage 214), metal sulphates along the line 192 (from the reaction stage 15 190) and magnesium carbonate (withdrawn along the line 186 from the stage 182). Advantages of the process 100 include the following: - The main raw material is cheap coal 20 - CaCO 3 and CaO are recirculated as is CO 2 which is produced in the kilns - All sludges are treated so that the only sludge disposal costs that need to be incurred are ash disposal - Valuable metals are produced such as cobalt, nickel and MgCO 3 - Aluminium present in the ash can be used for further sulphate removal 25 - The MgCO3 can be used to control the pH in a process in accordance with the invention in which Ca(HS) 2 is dosed to metal-containing water. In such a process, less gypsum will precipitate and metal sulphides can then be easier recovered. 30 It is to be appreciated that, although dissolved sulphate removal from the water is effected in the stage 132 through the addition of BaCO 3 and C0 2 , primary sulphate removal from the water is associated with metal removal from the water. In other words, by removal of metals, primarily Cat and Mg 2 , from the water, 18 sulphates are also automatically removed. Thus, the metals that are linked to sulphates in neutral (or acid) waters, are mainly Ca and Mg. Ca and Mg, as also emerges from the reactions taking place in reaction stage 102, do not precipitate with sulphides. Instead, Mg is precipitated with Ca(OH)2, which is thereafter 5 recovered in downstream processing (reaction stage 157). Ca is precipitated as CaCO 3 (by means of the carbonate in BaCO3), in the reaction stage 132. Furthermore, removal of alkali earth metals, particularly Ca 2 +, is essential for the production of BaS, which in turn is converted to BaCO 3 which is used in the 10 process. The importance of metals removal can be highlighted further in an embodiment (not shown), in which Ca(HS) 2 is produced from gypsum, in a Ca(HS) 2 reaction stage into which the gypsum line 196 then leads, is considered. Typically, 15 Ca(HS) 2 can then be recovered as a by-product, or can be added to the metal containing water for metal precipitation. However, Ca(HS) 2 cannot be recovered adequately from the water unless (i) Ca is removed downstream of reaction unit 118, as CaS by means of CaSO 4 precipitate (achieved through Ca(OH) 2 addition in the stage 20 118), to a SO 4 level of typically 2000mg/f or the equivalent Ca level of 800mg/f; (ii) Ca is removed further to lower levels by means of CaCO 3 by dosing BaCO 3 in the stage 132 (to precipitate CaCO and BaSO 4 ) and CO 2 (to adjust the pH back to 8.3 bearing in mind that Mg removal is effected by 25 dosing with Ca(OH) 2 to a pH of 11.5). The metals removal process of the invention is even more attractive if one can remove the valuable metals, such as Ni and Co, separately from the less valuable metals such as Fe2+, Fe 3 " and Al 3 +. This is achieved in the process of Figure 3.

19 In Figure 2, all metals, including Fe* and Fe+, are treated with sulphide to precipitate the metals as sulphides, excepts A 3 and Mg 2 + which precipitate as metals hydroxides. As the metals precipitate, the Ca 2 +, coming from CaCO 3 or Ca(OH) 2 , that was dosed, results In gypsum precipitation. There is thus a 5 mixture of MS, M(OH) 3 and gypsum present in the water, H 2 S0 4 is used to leach out the metals from the gypsum. The negative side of this approach Ia more sludge needs to be handled and more chemicals are used, eg H 2 80 4 . This is avoided in the process of Figure 3. Thus, in Figure 3, reference 10 numeral 300 generally indicates a process according to yet another embodiment of the invention, for treating metal-containing water. In Figure 3, stages and lines which are the same as, or similar to, those of Figure 2, are indicated with the same reference numerals. 15 In the process 300, the water transfer line 116 that leads from the reaction stage 102, passes into a settler 302. It will be appreciated that, since none of the complex Ca(HS) 2 Is present in the stage 102, no precipitation of metals as sulphides will take place in the stage 102. Instead, in the stage 102, only 20 lower value metals present In the water, such as Fe 2 +, Fe 3 ' and A13*, will precipitate out by means of CaCO 3 , as M(OH) 3 where M is thus Fe or Al. CaCO 3 is not essential, and, instead, any other alkali such as lime, MgCO3 or the like can be used In the stage 102 to achieve precipitation of M(OH) 3 , provided that the pH in the stage 102 Is kept sufficiently low to prevent 25 precipitation of more valuable metals. When CaCO 3 is used, Fe 2 + must first be oxidized, eg In the manner taught in US 6419834, which Is hence incorporated herein by reference. The Fe(OH) 3 and AI(OH) 3 that precipitate out are removed, in a settler 304, 30 along a withdrawal line 306. The water passes from the settler 304, along a line 308, into a reaction stage 310 which is fitted with a stirrer 312. Ca(HS) 2 is added to the water in the stage 310, along a line 314.

20 In the stage 314, and by means of the Ca(HS) 2 addition with the Ca(HS) 2 dissolving in the water, MS precipitation, where M is Mn 2 +, Ni 2t and/or Co 2 , takes place. 5 Water passes from the stage 310 along a line 316, into a settling stage 318, from which MS are withdrawn along a line 320, into a filtration or clarifying stage 322. MS are withdrawn from the stage 322 along a line 324, while water is recycled, by means of a line 326, back to the line 316. 10 Water from the stage 318 passes, by means of a line 328, into the stage 118. Precipitated CaSO 4 and Mg(OH) 2 pass, by means of the line 128, from the settler or clarifier 126, to a kiln 330. The kiln 330 is similar to the kiln 150, with coal entering it along a line 152, and typically operates at about 1000*C, with the 15 production of C0 2 , which is withdrawn along a line 332, and CaS and MgO, which are withdrawn along a line 334. The CaS and MgO pass into a reaction stage 336, entering the stage along line 334, where they react with CO 2 and H 2 0, to produce CaCO 3 , which is withdrawn 20 as a product along a line 342 and Ca(HS) 2 which is withdrawn along a line 344. The Ca(HS) 2 line 314 leads from the line 344. The line 344 leads into a reaction stage 346, where the Ca(HS) 2 is reacted with C0 2 , which enters the stage 346 along a line 348, to produce CaCO 3 which is 25 withdrawn along the line 110, and H 2

S/CO

2 which is withdrawn along a line 351. In the stage 346, Mg(OH 2 ) is converted to Mg(HC0 3

)

2 which passes from the stage 346 along a line 351, into a reaction stage 352.

CO

2 enters the stage 352 along the line 332, while H 2

S/CO

2 is withdrawn along a 30 line 354. The H 2 0 line 340 leads from the stage 352, as does a MgCO 3 line 356. MgCO 3 is thus produced as a by-product in the stage 352. The H2S/CO2 lines 166, 350 and 354 all lead into the Pipco stage 214.

C:%N RPoODCOD&M4 4360UU_1. DOC-10012 21 The BaS(aq)/Ca(OH) 2 line 160 from the stage 157, leads into a separation stage 360, where Ca(OH) 2 is separated from BaS, with Ca(OH) 2 being withdrawn along the line 122, and an aqueous component containing dissolved BaS passing, along 5 a line 362, into the stage 162. In the stage 132, BaCO 3 is thus added for removal of dissolved CaSO 4 by precipitation of BaSO 4 and CaCOs 3 Dissolved sulphate removal using BaCO 3 is slow, and hence the water is contacted with excess BaCO 3 , eg using a column 10 reactor. The reaction is faster when CO 2 is dosed for pH adjustment in the same reactor in which BaCO 3 is dosed. High pH water needs to bypass the stage 132, eg using a bypass line 366, and be contacted with BaCO3 treated water, with the aim that SO 4 after lime treatment will 15 precipitate soluble Ba to less than 1mg/f. Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and comprising", will be understood to imply the inclusion of a stated integer or step or 20 group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an 25 acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Claims (12)

1. 2. A process according to Claim 1, wherein A is Ca, so that the 15 complex is Ca(HS) 2 or Ca(HS)(HCO 3 ).
2. 3. A process according to Claim 2, wherein formation of the complex Ca(HS)(B) is effected by passing the aqueous component through a bed of particulate CaS while simultaneously treating it with CO 2 . 20
3. 4- A process according to Claim 2, wherein formation of the complex Ca(HS)(B) is effected by adding the acid or CO 2 to a slurry comprising CaS mixed with water, to form the complex and to lower the pH of the slurry thereby to increase the dissolved sulphide concentration CaS, and then mixing the resultant 25 treated aqueous component with the metal-containing water.
4. 5. A process according to Claim 4, wherein CO 2 is added to the slurry, thereby dissolving CaS, with dissolved Ca(HS) 2 complex forming and CaCO 3 precipitating, in accordance with reaction (1): 30 CO2 + 2CaS + H20 -Ca(H S)2 + CaCO3(1 VWrWORDIUW UTMa"UU-1. L, 1IOW2U12 23
5. 6. A process according to Claim 5, which includes removing at least some of the CaCO 3 from the treated aqueous component, before mixing the treated aqueous component with the metal-containing water. 5 7. A process according to Claim 5 or 6, wherein sufficient carbon dioxide is added to the CaS slurry so that the dissolved Ca(HS) 2 complex that is formed is at or near its maximum concentration in the slurry, at the temperature of the slurry; with the process including thereafter heating at least some of the slurry, thereby precipitating Ca(HS) 2 as a solid since its solubility in water is less at 10 elevated temperatures than at low temperatures; and recovering the precipitated solid Ca(H S) 2 as a by-product.
6. 8. A process according to any one of Claims 4 to 7 inclusive, wherein the admixing of the aqueous component with the metal-containing water is 15 effected in a plurality of reaction stages through which the metal-containing water passes sequentially and to each of which aqueous component is introduced, with each reaction stage being at a different pH, so that different metals precipitate at successively higher pH values in successive reaction stages. 20 9. A process according to Claim 8, wherein a gypsum crystallization inhibitor is added to the metal-containing water upstream of at least one of the reaction stages.
7. 10. A process according to any one of Claims 4 to 9 inclusive, wherein 25 the metal-containing water is acidic, with the process then including pre-treating the acidic metal-containing water, before it is admixed with the treated aqueous component, with CaCO 3 in a pre-treatment stage, to neutralize free acid in the water and to precipitate any Fe 3 + and Al 3 * as Fe(OH) 3 and AI(OH) 3 respectively, and with CO 2 being generated. 30
8. 11. A process according to Claim 10, wherein the acidic metal-containing water contains sulphuric acid as well as iron and aluminium as dissolved metals, with neutralization proceeding in accordance with reaction (7), so as to neutralize 24 sulphuric acid in the water and to precipitate iron and aluminium as Fe(OH) 3 and AI(OH) 3 respectively, and with CO 2 being generated: H 2 SO 4 +CaCO 3 - CaSO 4 + CO 2 + H 2 0 (7) 5 with a sludge comprising CaSO 4 , Fe(OH) 3 and AI(OH) 3 being produced, and with at least part of the CO 2 generated in the pre-treatment stage in accordance with reaction (8) being used to reduce the pH of the aqueous component: 10 CaSO 4 + 2C -- + CaS + 2CO 2 (8)
9. 12. A process according to any one of Claims 4 to 11 inclusive, which includes the preliminary step of producing the slurry by reacting, in a CaS production stage, gypsum (CaSO 4 ) with a reductant at elevated temperature, to 15 produce CaS and CO 2 in accordance with reaction (8); and admixing the CaS with water, to obtain the slurry: CaSO 4 + 2C -+ CaS + 2CO 2 (8) 20 13. A process according to any one of Claims 4-to 12 inclusive, which includes, in a polishing stage which takes place after the treatment of the water with the slurry, contacting the water with Fe(OH)3 to remove residual sulphides therefrom. 25 14. A process according to Claim 13, which includes, when it is desired to remove gypsum from the water, adding lime to the polishing stage to stimulate gypsum crystallization, in accordance with the reaction (10): Ca 2t +SO4 + 2H 2 0 -- CaSO4.2H 2 0 (10) 30
10. 15. A process according to Claim 1, wherein A is Ba. U:UhFOrDiUWUYlbM4 aDU.1 .uO1 oaaQ12 25
11. 16. A process according to Claim 15 wherein, in a BaS production stage, barium sulphate is reacted with a reductant to produce BaS which is then admixed with water to obtain the aqueous component in the form of a slurry, in accordance with reactions (11) and (12): 5 BaSO 4 + 2C BaS + 2CO 2 (11) BaS + CO 2 + H 2 0 -4 BaCO 3 + H 2 S (12)
12. 17. A process substantially as hereinbefore described, with reference to the 10 accompanying drawings. 15