Field of the Invention

The present invention relates to processes for desorbing mercury from a mercury loaded solid sorbent material, using an eluent solution. Typically, the eluent solution has an alkaline pH and/or comprises an eluent species selected from a sulphide salt, a hydrogen sulphide salt and a cyanide salt. The present invention is particularly useful for the regeneration of solid sorbent materials used in industrial processes for the removal of mercury from water streams, e.g. from water used in gold mining processes.

Background of the Invention

In modern gold mining processes, typically it is necessary to extract gold from complex ores which comprise gold in addition to other metals, including mercury. A common technique for extracting gold from its ores is the cyanide process, wherein leaching of gold is achieved by the addition of cyanide at alkaline pH. Cyanide is a strong lixiviant for gold, and so leaches the gold out of the ore into solution. The gold is typically present in the leaching solution as a gold cyanide complex such as [Au(CN) ₂ ] ^~1 . Silver can also be extracted from its ores using a similar cyanide leaching process. A problem with this process is that cyanide is an equally strong lixiviant for mercury as it is for gold and silver. Accordingly mercury, which is typically present in the ore along with gold or silver, is also leached into the solution. The mercury may be present in the leaching solution for example as Hg(CN) ₂ , [Hg(CN) ₃ ] ^"1 or [Hg(CN) ₄ ] ^"2 . However, typically it is present as [Hg(CN) ₄ ]- ² .

The removal of mercury from mining waters is very important, both on health and safety grounds and on environmental grounds. In particular, mercury volatilisation during extraction processes can be a threat to the health of plant workers, and the presence of mercury in waste waters from mining is of significant environmental concern. Environmental legislation limits the concentration of mercury permitted in waste waters to very low levels in many countries. Accordingly, effective removal of mercury from mining waters is of significant interest to the industry. However, it is important that mercury removal technologies do not remove significant quantities of the gold or silver being mined, to avoid undesirable loss of these materials during processing.

A range of different methods have been employed for mercury removal in this field.

Reference 1 provides a review of different removal technologies, including precipitation with inorganic sulphides or sulphur-based organic compounds; adsorption with activated carbon or crumb rubber; solvent extraction by alkyl phosphorus esters or thiol extractants; ion exchange with isothiouronium groups, thiol resin or polystyrene-supported phosphinic acid; and electrochemical cementation.

Reference 2 describes the removal of mercury from mercury cyanide complexes from the processing streams of gold cyanidation circuits by dissolved air flotation at a laboratory scale. Selective aggregation of mercury was carried out after precipitation of the complexes with sodium dimethyl dithiocarbamate (NaDTC), coagulation with colloidal hydroxides of La and Fe, and flocculation with a polymer. Removal of mercury was achieved by dissolved air flotation of the aggregates formed.

US5599515 describes a method for selectively removing mercury from solutions, preferably solutions containing gold, such as gold cyanide solutions. The method comprises treating the solutions with dialkyldithiocarbamates, preferably potassium dimethyldithiocarbamate, to form stable mercury carbamate precipitates.

Reference 3 describes precipitation of mercury from heap leach solution using a dipotassium salt of 1 ,3-benzenediamidoethanethiol (BDET ² ).

In WO2015/150774, the present inventor describes a process for selectively removing mercury from an aqueous feed solution comprising contacting the aqueous feed solution with a solid sorbent material comprising thiol and/or thiolate. Using the method described in this document, mercury is immobilised on the solid sorbent. WO2015/150774 teaches that the selectivity of the solid sorbent for mercury can be enhanced by the use of free cyanide ions.

Summary of the Invention

Where methods of mercury removal comprise the immobilisation of mercury on a solid sorbent, it would be desirable to enable the solid sorbent to be re-used, by desorbing the immobilised mercury.

Methods employed for desorption of mercury from sorbent materials which comprise thiol or thiolate functional groups typically employ acidic eluents to elute the mercury. For example, Reference 4 describes the use of hydrochloric acid and other complexing compounds (KBr, KSCN, (NH ₂ )2CS and HBr) under acidic conductions for regenerating thiol-functionalised SBA-15 adsorbents of mercury. It is also known to use hydrochloric acid to regenerate mercury adsorbents.

However, the present inventors have found that the use of acidic eluents is incompatible with a process in which cyanide is or may be present, since the acidic conditions lead to the production of highly toxic hydrogen cyanide gas. As described above, cyanide may be present in gold mining feeds where cyanide has been used as a lixiviant to extract gold from its ore. Alternatively or additionally, cyanide may be used to enhance the selectivity of sorbents for mercury over other metal ions, e.g. as described in WO2015/150774.

Therefore, there remains a need for alternative methods for the regeneration of sorbent materials for the removal of mercury. In particular, there remains a need for methods for desorption of mercury from sorbent materials which are compatible with mercury removal processes where cyanide may be present.

Accordingly, the present invention provides a process for desorbing mercury from a mercury-loaded solid sorbent material, the process comprising contacting the

mercury-loaded solid sorbent material with an eluent solution, wherein the eluent solution has an alkaline pH. Typically, the eluent solution comprises an eluent species, which is typically a salt.

The present inventors have found that certain eluent species are particularly convenient for use at alkaline pH. Accordingly, the present invention also provides a process for desorbing mercury from a mercury-loaded solid sorbent material, the process comprising contacting the mercury-loaded solid sorbent material with an eluent solution comprising an eluent species, wherein the eluent species is one or more selected from a sulphide salt, a hydrogen sulphide salt and a cyanide salt. Typically, the eluent solution has an alkaline pH.

The present invention also provides use of an eluent solution comprising an eluent species for desorbing mercury from a mercury-loaded solid sorbent material. Typically, the eluent solution has an alkaline pH, and/or the eluent species is a salt, e.g. one or more selected from a sulphide salt, a hydrogen sulphide salt and a cyanide salt.

Typically, the eluent solution has a pH of 10 or more. Typically, the solid sorbent material comprises thiol and/or thiolate functional groups. After the mercury has been desorbed, the solid sorbent material may be reused as a mercury sorbent, e.g. for selectively removing mercury from an aqueous solution comprising mercury. The process for desorbing mercury may therefore be a process for regenerating a mercury loaded solid sorbent material.

Brief Description of the Drawings

Figure 1 shows the amount of mercury eluted with the acid eluents from mercury-loaded sorbent material as determined in Example 1 .

Figure 2 shows the amount of mercury eluted with a range of non-acidic eluents from mercury-loaded sorbent material as determined in Example 1 .

Figures 3 and 4 show the elution of mercury using sodium hydrogen sulphide (NaSH) with varying concentrations as determined in Example 3.

Figures 5 and 6 show the elution of mercury using sodium sulphide (Na ₂ S) with varying concentrations as determined in Example 3. Figure 7 shows the effect of flow rate on the rate of mercury elution as determined in Example 3.

Figure 8 shows how the elution progresses in a column with sodium sulphide as the eluent. Figure 9 shows the outlet concentration of mercury and other metals during a load cycle as determined in Example 4.

Figure 10 shows results form Example 4, where it is demonstrated that mercury

breakthrough for each cycle is highly consistent over the three cycles showing no loss of capacity. Detailed Description

Preferred and/or optional features of the invention will now be set out. Any aspect of the invention may be combined with any other aspect of the invention unless the context demands otherwise. Any of the preferred and/or optional features of any aspect may be combined, either singly or in combination, with any aspect of the invention unless the context demands otherwise.

In the present invention, an eluent solution is used to desorb mercury from a mercury-loaded solid sorbent material. The eluent solution has a neutral or alkaline pH. Typically, the eluent solution has a pH of at least 8, at least 9, at least 10, at least 10.5 or at least 1 1 . For example, the pH of 2M Na ₂ S is 14, the pH of 1 M Na ₂ S is 13.6, the pH of 0.1 M Na ₂ S is 13.2, the pH of 1 M NaSH is 12.8, the pH of 0.1 M NaSH is 1 1 .9 and the pH of 0.1 M NaCN is 1 1 .5.

The eluent solution comprises an eluent species. Typically, the eluent solution is an aqueous solution. The eluent solution may comprise water. Typically the eluent species is soluble in water.

The eluent species may comprise a sulphide salt or a hydrogen sulphide salt. The salt may include the anion S ^2~ , or the anion [SH] ^~ . Typically, the salt comprises a metal cation, for example selected from an alkali metal ion and an alkaline earth metal ion. Typically, salts comprising cations of other metals are not sufficiently soluble in water to be used

conveniently in the present invention. The salt may have the formula M ₂ S or MSH, where M is an alkali metal, such as lithium, sodium or potassium. In some embodiments, it may be preferred that the alkali metal is sodium or potassium, particularly sodium. The salt may have the formula MS or M(SH) ₂ , where M is an alkaline earth metal, such as magnesium or calcium.

In some embodiments, the eluent is a sulphide salt. In some embodiments, the eluent is sodium sulphide (Na ₂ S).

The eluent species may comprise a cyanide salt. The salt may include the anion CN\ Typically, the salt comprises a metal cation, for example selected from an alkali metal ion and an alkaline earth metal ion. Typically, salts comprising cations of other metals are not sufficiently soluble in water to be used conveniently in the present invention. The salt may have the formula MCN, where M is an alkali metal, such as lithium, sodium or potassium. In some embodiments, it may be preferred that the alkali metal is sodium or potassium, particularly potassium. The salt may have the formula M(CN) ₂ , where M is an alkaline earth metal, such as magnesium or calcium. As demonstrated in the Examples below, altering the concentration of the eluent species can affect the rate and efficiency of mercury elution. Where the rate of mercury elution is increased, more of the mercury is eluted for the same number of bed volumes of eluent solution passed through the bed of mercury-loaded solid sorbent material. Where the efficiency of mercury elution is increased, more of the mercury is eluted for the same number of moles of eluent species. In general, the present inventors have found that as the concentration of eluent species increases, the rate of mercury elution increases but the efficiency of mercury elution decreases. Typically it is preferable to increase the rate of mercury elution in commercial processes, particularly where the eluent species is re-used. However, the skilled person will understand that in some embodiments it may instead be preferable to maximise the efficiency of mercury removal.

Typically, the concentration of the eluent species in the eluent solution is 0.2 M or greater, e.g. 0.4 M or 0.5 M or greater. As demonstrated in the Examples below, where the concentration of eluent species Na ₂ S is 0.1 M, no significant elution of mercury is observed. In contrast, significant elution of mercury is seen using 0.5 M Na ₂ S solution. In some embodiments, it may be preferable that the concentration of eluent species in the eluent solution is 1 M or greater, 1 .5 M or greater or 2 M or greater. This may be particularly advantageous where the eluent species is a hydrogen sulphide salt, e.g. NaSH. As demonstrated in the Examples below, the rate of mercury elution increases significantly between 1 M and 2M concentrations of NaSH. However, a 0.9M solution of NaSH provides an adequate rate of mercury elution coupled with excellent efficiency, so operating at lower NaSH concentrations may be preferred in some circumstances.

In some embodiments, the concentration of the eluent species in the eluent solution is 0.6 M or greater, for example 0.65 M or greater, 0.7 M or greater, 0.75 M or greater, 0.8 M or greater, 0.85 M or greater, 0.9 M or greater, 1 M or greater, 1 .1 M or greater, 1 .2 M or greater, 1 .3 M or greater, 1 .4 M or greater or 1 .5 M or greater. In some embodiments, the concentration of the eluent species in the eluent solution is 2 M or greater, for example 2.1 M or greater, 2.2 M or greater, 2.3 M or greater, 2.4 M or greater, 2.5 M or greater, 2.6 M or greater, 2.7 M or greater, 2.8 M or greater, 2.9 M or greater or 3 M or greater.

There is no particular preference for the upper limit of the concentration of eluent species. Typically, the upper limit is the maximum solubility of the eluent species in the eluent solution under the conductions used for elution. The concentration of eluent species may be 15M or less, 13M or less, 10M or less or 5M or less. The solubility limit of certain eluent species is listed below.

Na ₂ S: 2.4M at 20 ^e C

NaSH: 8.9M at 20 ^e C

KCN: 1 1 M at 25 ^e C

NaCN: 13M at 25 ^e C

After contact with the mercury-loaded solid sorbent material, the eluent solution includes some desorbed mercury, and accordingly may be considered to be a mercury-loaded eluent solution. The mercury-loaded eluent solution may conveniently be disposed of as a solution. Alternatively, in the present invention the mercury may conveniently be removed from the eluent solution. In this way, the eluent solution may be regenerated to allow it to be re-used for desorption of mercury from a mercury-loaded solid sorbent material. Additionally, the removal of mercury from the mercury-loaded eluent solution can permit convenient disposal or reuse of the mercury.

In particular, mercury may be removed from the mercury-loaded eluent solution by precipitation (e.g. selective precipitation) of a mercury species (e.g. a compound comprising mercury). This may be carried out by reducing the pH of the mercury-loaded eluent solution. This precipitation is particularly convenient where the eluent species is a sulphide or hydrogen sulphide salt, such as a sodium hydrogen sulphide or sodium sulphide. For example, the mercury may be eluted as mercury sulphide. In order to reduce the pH, an acid or an acidic salt may be added to the mercury-loaded eluent solution. Any compound suitable for reducing the pH below 10 may be used, and the nature of the compound is not particularly limited. Strong acids such as HCI, HBr, HI or sulphuric acid may be used. Alternatively, salts such as sodium hydrogen carbonate, sodium hydrogen sulphate, ammonium chloride, or mono or disodium phosphate may be used. Alternatively, weak acids such as formic acid, acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, oxalic acid, lactic acid, malic acid, citric acid, benzoic acid or carbonic acid may be used.

The pH of the eluent solution is typically reduced to about pH 9. The pH may be reduced to pH 10.5 or below, pH 10 or below, or pH 9.5 or below. Typically, reducing the pH of about 9 is preferred since this maximises the precipitation of Hg. However, typically the pH is not reduced below 8.5 or to below 8, to reduce the risk of HCN or H ₂ S being generated. In some embodiments, the pH of the eluent solution is reduced to a pH of at least 8, such as at least 8.1 , at least 8.2, at least 8.3, at least 8.4, at least 8.5, at least 8.6, at least 8.7, at least 8.8, at least 8.9 or at least 9. In some embodiments, the pH of the eluent solution is reduced to a pH of 10.5 or below, such as 10.4 or below, 10.3 or below, 10.2 or below, 10.1 or below, 10 or below, 9.9 or below, 9.8 or below, 9.7 or below, 9.6 or below or 9.5 or below.

For example, the pH of the eluent solution may be reduced to a pH in the range 8 to 10.5, such as 8.1 to 10.4, such as 8.2 to 10.3, such as 8.3 to 10.2, such as 8.4 to 10.1 , such as 8.5 to 10, such as 9 to 10, such as 9 to 9.5.

The eluent solution may be re-used after the mercury has been removed from it (e.g. by precipitation). Accordingly, the process of the present invention may further comprise contacting the regenerated eluent solution with a mercury loaded solid sorbent material to desorb mercury from the solid sorbent material. The cycle of mercury removal and eluent regeneration may be repeated one or more times, e.g. two or more, five or more, or ten or more times.

Following regeneration of the eluent solution, typically the concentration of mercury in the eluent solution is less than 100 ppm, less than 50 ppm, less than 20 ppm, less than 10 ppm, less than 5 ppm or less than 1 ppm. Prior to regeneration, the concentration of mercury in the eluent solution may be at least 500 ppm, at least 1000 ppm or at least 10,000 ppm.

The process of the present invention is particularly suitable for the regeneration of solid sorbent materials used for the selective removal of mercury from aqueous feeds. In particular the solid sorbent materials may be employed in the selective removal of mercury from aqueous feeds which comprise mercury in addition to one or more precious metals. For example, the solid sorbent material may be employed in the removal of mercury in a precious metal treatment process, e.g. in a precious metal mining process.

As the skilled person will understand, the term precious metals includes gold, silver and the platinum group metals (which are platinum, palladium, rhodium, iridium, osmium and ruthenium). In particular, the solid sorbent material may be employed in the removal of mercury from an aqueous feed solution comprising gold and/or silver.

The aqueous feed solution may comprise cyanide. Conveniently, the solid sorbent material may be employed in selectively removing mercury from processing waters for gold and/or silver cyanidation processes typically employed to extract gold and/or silver from their ores. For example, the aqueous feed solution may by the solution produced directly from a cyanide heap leach step, or it may have been subjected to further processing following the leach step, such as contact with activated carbon. The solid sorbent material may be employed for selectively removing mercury from processing waters prior to an electrowinning step, e.g. immediately prior to an electrowinning step. As the skilled person will understand, electrowinning refers to electrodeposition of metal from a solution, typically metal which has been extracted from its ore into the solution.

Accordingly, in a further preferred aspect, the present invention provides a process for removing mercury from an aqueous feed solution, comprising

(i) contacting the aqueous feed solution with a solid sorbent material capable of sorbing (e.g. adsorbing) mercury to provide mercury loaded solid sorbent material; and

(ii) regenerating the solid sorbent material by contacting it with an eluent solution to desorb the mercury.

Steps (i) and (ii) are typically repeated (i.e. allowing the solid sorbent materials to be re- used). Steps (i) and (ii) are typically repeated one or more, e.g. two or more, five or more, or ten or more times. The eluent solution may be regenerated by removing the mercury from it (e.g. by precipitation) as described herein.

The sorbent materials useful in the processes of the present invention typically comprise thiol and/or thiolate functional groups. It is believed that it is these thiol or thiolate functional groups which interact with the mercury to sorb (e.g. adsorb) it onto the sorbent materials. Typically, the sorbent materials will comprise mercury adsorbing moieties comprising thiol or thiolate functional groups, immobilised on a solid support. As the skilled person will understand, the sorbent materials are typically adsorbent materials.

The nature of the mercury adsorbing moieties is not particularly limited in the present invention. The mercury adsorbing moieties should comprise one or more thiol or thiolate functional groups. As the skilled person will understand, a thiol functional group is -SH, and a thiolate functional group is -S , which is typically associated with a positively charged counter ion. For example, sodium thiolate is -S ^~ Na ⁺ . Thiolate functional groups may be preferred where the sorbent material would otherwise be hydrophobic, as the presence of thiolate functional groups may enhance wetting by the aqueous feed solution and/or the aqueous eluent solution.

For example, the mercury adsorbing moieties may have the structure of Formula I or Formula II below:

Formula I

Formula II in which L is a linker group, and M ⁺ is a counter ion, such as a metal counter ion. For example, it may be an alkali metal counter ion such as Na ⁺ or K ⁺ . As the skilled person will understand, the wobbly line indicates attachment of the linker group to the solid support.

Preferably, the linker group is a non-hydrolysable linker group. The term non-hydrolysable linker group includes linker groups which are not typically hydrolysed under aqueous conditions. This means that the thiol or thiolate functional group is not readily detached from the solid support, in use.

The structure of the linker group is not particularly limited in the present invention. The linker group may be, for example, a Ci to Ci ₅ hydrocarbon moiety, optionally including one or more ether or thioether groups. The term hydrocarbon moiety is intended to include saturated or unsaturated, straight or branched optionally substituted hydrocarbon chains, optionally including one or more optionally substituted cyclic hydrocarbon groups, such as cycloalkylene, cycloalkenylene and arylene groups, including groups where one or more ring carbon atoms are replaced by a heteroatom, such as a heteroatom selected from O, N and S. As the skilled person will readily understand, the linker group is a divalent group attached both to the solid support and to the thiol or thiolate functional group. For example, the linker group may be selected from:

-Ri -, wherein Ri is Ci to Ci ₅ (e.g. Ci to C10 or Ci to C ₅ ) straight or branched, optionally substituted alkylene or alkenylene moiety;

-R2-X-R2-, wherein each R ₂ is independently Ci to C10 (e.g. Ci to C ₅ ) straight or branched, optionally substituted alkylene or alkenylene moiety and wherein X is selected from O and S; and -R3-Y-R3-, wherein each R ₃ is independently present or absent and when present is independently selected from Ci to C10 (e.g. Ci to C ₅ ) straight or branched, optionally substituted alkylene or alkenylene moiety, and -R4-X-R4- wherein each R ₄ is independently Ci to C ₅ (e.g. Ci to C ₃ ) straight or branched, optionally substituted alkylene or alkenylene moiety, wherein Y is selected from cycloalkylene, cycoalkenylene, arylene, in which one or more ring carbon atoms are replaced by a heteroatom selected from O, N and S, and wherein X is selected from O and S.

It may be preferred that Ri is Ci to C10 branched or unbranched, optionally substituted alkylene or alkenylene moiety. It may be preferred that Y is selected from C ₄ to C ₆ cycloalkylene and C ₄ to C ₆ arylene. It may be preferred that X is O.

Suitable mercury adsorbing moieties include those according to one of Formula III, Formula IV or Formula IV below:

Formula IV

Formula V wherein:

each of R ₅ , Re, and R ₇ , is independently Ci to C10 (e.g. Ci to C ₅ ) straight or branched alkylene or alkenylene, optionally substituted with up to four functional groups selected from -OR10, -SR10, -S ^~ M ⁺ and -NR10R10;

each R ₈ and R ₉ is independently selected from Rn -X-Rn and Ci to C10 (e.g. Ci to C ₅ ) straight or branched alkylene or alkenylene, optionally substituted with up to four functional groups selected from -OR10, -SR10, -S ^~ M ⁺ and -NR10R10;

each Rio is independently H or Ci to C ₅ alkyl;

each Rii is independently Ci to C ₅ straight or branched alkylene or alkenylene, optionally substituted with up to four functional groups selected from -OR10, -SR10, -

Ri is a C ₅ or C ₆ cycloalkyl, cycloalkenyl or aryl ring;

each X is independently S or O;

R ₈ may optionally be absent;

R ₉ may optionally be absent; and

any SH group may instead be S ^~ M ⁺ , wherein M is a counter ion, such as a metal counter ion (e.g. alkali metal counter ion such as Na or K).

Further suitable mercury adsorbing moieties include those according to one of Formula VI, Formula VII or Formula VIII below:

Formula VI

Formula VII

Formula VIII wherein:

each n is independently 1 to 10, more preferably 1 to 5 or 2 to 5;

each m is independently 0 to 10, more preferably 0 to 5, 0 to 3, 1 to 5, or 1 to 3; Ri2 and Ri ₃ are each independently selected from SH, NH ₂ or OH, provided that at least one of R12 and Ri ₃ is SH;

p is 0 or 1 ; and

any SH group may instead be S ^~ M ⁺ wherein M is a counter ion, such as a metal counter ion (e.g. alkali metal counter ion such as Na or K).

As used herein, the term optionally substituted includes moieties wherein one two, three, four or more hydrogen atoms have been replaced with other functional groups. Suitable functional groups include -ORi ₀ , -SRi ₀ , -S ^~ M ⁺ and -NRi ₀ Rio wherein each Ri ₀ is independently H or Ci to C ₅ alkyl.

Examples of suitable mercury adsorbing moieties include

As discussed above, typically the sorbent materials of the present invention comprise mercury adsorbing moieties comprising thiol or thiolate functional groups, immobilised on a solid support. The nature of the solid support material is not particularly limited. Typically, the support material is in the form of particles such as powder, granules or fibres.

Where the support is a fibre, typically the fibre diameter (e.g. number average fibre diameter, e.g. determined by microscope counting of a representative sample of fibres) is about 0.05mm. For example, the fibre diameter may be in the range from 0.01 mm to 0.1 mm, more preferably 0.03mm to 0.07mm. The fibre length is not particularly limited. Short fibres having a length (e.g. number average length, e.g. determined by microscope counting of a representative sample of fibres) of about 0.3mm may be particularly suitable, e.g. in the range from 0.1 -1 mm. Longer fibres, e.g. up to about 50mm may also be suitable. Fibres may be formed into pads or papers using techniques known to the skilled person such as wet laying.

Where the support is a granule or powder, typical number average particle diameters (e.g. determined by microscope counting of a representative sample of particles, e.g. taking the maximum particle dimension as the diameter) are in the range from 0.1 mm to 0.5mm, but this is not particularly limited. For example, diameters ranging from 0.01 mm or 0.05mm to 1 mm are suitable, although smaller and larger particles are also appropriate.

The solid support may be formed of polymer material, which may optionally be substantially non-porous. Suitable polymer materials include organic polymer materials. Particularly preferred are hydrocarbon polymers such as polyolefin materials. Particularly suitable polyolefins are polyethylene, polypropylene, polybutylene etc. Other hydrocarbon polymers such as polystyrene are also suitable. Alternative solid supports materials include silica.

It will be understood that in some cases it may be preferable to activate the surface of the support to facilitate immobilisation of the functional groups. Suitable surface activation techniques will be known to those skilled in the art, including for example plasma treatment, corona discharge and flame treatment.

Suitable adsorbents are available from Johnson Matthey Pic, and include Smopex® adsorbents, especially Smopex 1 1 1 and Smopex 1 12, and QuadraSil® adsorbents, especially QuadraSil-MP and Silica FS1A. Examples

Example 1 - Batch Elution of Hg

Different thiol-functionalised materials were loaded with mercury under batch conditions, set out in Table 1 below:

Table 1

The materials were mixed for 18 hours in a roller mixer with a synthetic solution of known concentration of mercury cyanide, K ₂ Hg(CN) ₄ , and the pH was adjusted using sodium hydroxide. The pH and the Hg concentration used are shown in Table 1 above.

After the loading, the resins were filtered, dried and the moisture content was analysed using an infrared balance. The metal content of the solid was calculated by difference of the mercury concentration of the initial solution from the final solution as determined by

ICP-OES. The calculated metal loadings are listed in Table 1 above.

Elution tests were carried out for each material as follows: 200 mg (dry weight) of material was weighed into a 50 mL centrifuge tube and combined with 20 mL of the eluent. Table 2 below shows a list of tested eluents: Table 2

The samples were mixed on a roller mixer at 50 revolutions per minute (rpm) for 2 hours at room temperature. The eluent was then collected by filtration (0.45 μηι syringe filter) and analysed by ICP-OES. The percentage metal eluted was determined from the calculated metal content of the initial loaded solid and the amount of metal in the eluent as calculated from the final eluent concentration.

Figure 1 shows the amount of mercury eluted with the acid eluents. Concentrated hydrochloric acid removes >95% of the mercury from all four resins, but it becomes less effective at lower concentrations. At 1 M HCI concentration no elution is observed. 2 M hydrobromic acid is also effective at eluting mercury, as also shown by Arencibia et al

(Reference 4). However, as discussed above acid eluents are unsuitable for use in systems where cyanide may be present, for example for the removal of mercury from gold mining water. Figure 2 shows the amount of mercury eluted with a range of non-acidic eluents, and shows that both potassium cyanide (KCN) and sodium sulphide (Na ₂ S) gave elution of mercury with sodium sulphide generally giving >90% removal of mercury. Both of these compounds are compatible with a cyanide stream and therefore offer a considerable advantage over the acidic systems.

Example 2 - Batch Elution of Column-loaded Hg

To show that the elution can also be applied to material loaded in a column environment, a batch of thiol functionalised resin was added to a column and treated using a mixed metal stream as follows: the initial material was rinsed with 3 bed volumes (BVs) of 1 M sodium hydroxide solution and then a mixed metal solution was flowed through the material for 120 BVs. To ensure complete loading within an acceptable time period an exaggerated concentration of mercury was used. The concentration of the mixed metal load solution was 2,500 ppm Hg, 190 ppm Au, 135 ppm Ag as the cyanide salts. The final metal content of the loaded solid sorbent material is given in Table 3 below. The metal content was determined by ICP analysis of the dissolved final dry material.

Table 3

The metal content is given as a weight content with respect to the total mass of resin.

Elution from the solid material was carried out in batch mode and was as expected. It was demonstrated that sodium cyanide, sodium sulphide and sodium hydrogen sulphide are capable of eluting mercury from the resins under basic (high pH) conditions.

Table 4 shows the percentage of metal eluted from the loaded material (as shown in Table 3 above). 1 wt% of material was added to 20ml_ of eluent solution and left for two hours at room temperature.

Table 4

Example 3 - Column Elution: Effect of Elution Conditions

To study the effect of elution conditions a thiol functionalised resin was loaded into a column and a mercury solution (3,000 ppm) was pumped through it for 60 BVs (bed volumes). The outlet solution was collected, analysed by ICP-OES and the resin metal loading was calculated by difference. The resin bed was rinsed with 3 BVs of deionised water and then with the eluent solution. The eluate was collected, diluted with sodium sulphide solution to stabilise and analysed by ICP-OES. Thiol functionalised resin was used for each test condition. The loading across the samples was calculated as 412 ± 16 (mgHg/gresin).

Figure 3 shows the elution of mercury using sodium hydrogen sulphide (NaSH) with varying concentrations. Increasing the concentration of eluent improves the rate of elution. At low concentrations the amount of mercury eluted is more efficient per mole of eluent (as shown in Figure 4). Likewise, Figure 5 shows the elution with sodium sulphide (Na2S). Elution with sodium sulphide shows the same trend as sodium hydrogen sulphide with an increased rate of elution at higher concentration but at a lower efficiency against total moles of eluent pumped (Figure 6). Elution is poorly effective at 0.1 M concentration of sodium sulphide. By comparing Figure 3 and Figure 5 we can see that using sodium sulphide as the eluent gives more rapid elution than sodium hydrogen sulphide. Comparing Figure 4 and Figure 6 shows that the reagent usage is also much more efficient when using sodium sulphide compared to sodium hydrogen sulphide. Overall whilst both sodium hydrogen sulphide and sodium sulphide work for the high pH elution, sodium sulphide is preferred as it provides both a more rapid and more efficient elution. Table 5 shows that the final mercury content of the solid was less than 7 wt% in all cases. With the best eluent systems tested (2 M Na ₂ S or 6.2 M NaSH) the final mercury content was less than 0.3 wt%.

Table 5

Figure 7 shows that the flow rate has no significant effect on the rate of mercury elution when compared on a total volume pumped basis. Obviously the flow rate will affect the total time required to complete the elution although this is limited by the maximum flow rate permitted for any given material. Figure 8 shows how the elution progresses in a column with sodium sulphide as the eluent. The mercury loaded resin has a pale colouration. Initially the sulphide solution causes the appearance of a black band, thought to be a fine mercury sulphide precipitate, this is then dissolved in the sulphide solution and carried out of the column leaving the regenerated pale coloured adsorbent. This gives the appearance of the black band slowly moving up the resin bed. The bed becomes completely clear and regenerated after passing 2-3 BVs (bed volumes) of eluent.

Example 4 - Material Cycling

To show that the material can be reused after desorption of the mercury, a mixed metal stream was loaded with the mixed metal solution, rinsed with deionised water (3 BVs), eluted with 1 M sodium sulphide (3 BVs) and rinsed with deionised water (3 BVs), and the entire cycle was repeated three times. To ensure complete loading within an acceptable time period an exaggerated concentration of mercury was used. The concentration of the load solution was 2,000 ppm Hg, 155 ppm Au, 140 ppm Ag as the cyanide salts. Figure 9 shows the outlet concentration during the load cycle. We can see that gold breaks through quickly due to the high mercury selectivity under these cyanide containing process conditions. Silver is adsorbed but then displaced by mercury. Mercury is strongly adsorbed and only begins to breakthrough after 30 to 40 BVs, with full breakthrough after 90 BVs at a loading of 370 mgHg/gresin. As discussed, following the load cycle the material was rinsed with water, eluted with sodium sulphide and then rinsed again with deionised water to remove the sodium sulphide solution from the column. This regenerated resin was then subjected to an repeated identical load cycle. Figure 10 shows how the mercury

breakthrough for each cycle is highly consistent over the three cycles showing no loss of capacity.

Example 5 - Precipitation of Mercury Sulphide

By decreasing the pH of a mercury sulphide loaded sodium sulphide solution, the mercury sulphide can be caused to precipitate. To confirm that this could be applied to the mercury loaded column eluate, a sample of the eluate was acidified and filtered. The dissolved mercury concentration was measured before and after acidification.

Initially to test the precipitation of mercury sulphide from the sodium sulphide eluate, a solution of mercury sulphide in sodium sulphide was prepared by dissolving mercury sulphide in sodium sulphide. The results are shown in Table 6. This shows that decreasing the pH to 8.9 leads to the mercury precipitating to less than 1 ppm in the final solution, from an initial concentration of 36,700 ppm. The final mercury concentration was slightly higher (8.1 ppm) when the pH was only dropped to pH 9.7. Both strong acids (sulphuric acid) and salts such as sodium hydrogen carbonate could be used to decrease the pH from the initial pH 13.6. When using strong acids the pH could be decreased much further causing more rapid precipitation of the mercury sulphide, although this then leads to the production of large quantities of hydrogen sulphide which may be disadvantageous.

Table 6

Sample left over the weekend to precipitate

References

1. Miller, J. D., Alfaro, E., Misra, M., & Lorengo, J. (1996). Mercury control in the cyanidation of gold ores. Pollution Prevention for Process Engineers, Engineering Foundation, 151 -64

2. Tassell F. et al (1997). Removal of Mercury from Gold Cyanide Solution by Dissolved air Flotation, Minerals Engineering, Vol.10, No. 8, 803-81 1

3. Metlock et al (2002). Advanced Mercury Removal from Gold Leachate Solutions Prior to Gold and silver Extraction: A Field Study from an Active Gold Mine in Peru, Envirn. Sci.

Technol. 2002, 36, 1636-1639

4. Arencibia et al (2010). Regeneration of Thiol-Functionalised Mesostructured Silica Adsorbents of Mercury, Applied Surface Science 256 (2010) 5453-5457