WO2013040658A1

WO2013040658A1 - Method for controlling emissions

Info

Publication number: WO2013040658A1
Application number: PCT/AU2012/001151
Authority: WO
Inventors: Philip Lloyd Ticehurst
Original assignee: Alcoa Of Australia Limited
Priority date: 2011-09-21
Filing date: 2012-09-21
Publication date: 2013-03-28
Also published as: BR112014006775B1; AU2012313361A1; AU2012313361B2; BR112014006775A2

Abstract

A method for controlling mercury emissions from aqueous alkaline solutions from the Bayer circuit, the method comprising the steps of: introducing a source of copper ions to an aqueous alkaline solution containing sulfide ions; precipitating a copper species; and precipitating a mercury species; thereby facilitating mercury removal from the aqueous alkaline solution.

Description

"Method for Controlling Emissions"

Field of the Invention

The present invention relates to a method for control of mercury emissions from caustic liquor process streams and condensate streams. The method of the present invention has particular application in Bayer process alumina refining.

Background Art

Alumina refineries may emit mercury to the atmosphere from the digestion and evaporation unit processes. Mercury may also be emitted to air from the calcination stacks and stacks from oxalate combustion. Such emissions result from mercury incorporated in the feedstock to the calciners or oxalate furnace. A schematic diagram indicating the major mercury inputs and outputs for an alumina refinery is shown in Figure 1.

Treating mercury emissions from the stacks, particularly from the calcination stacks is undesirable due to the high volumes of gas emitted and the low concentration of mercury in these gas streams.

There is a need to provide a method of controlling mercury emissions that provides a useful alternative to those already known in the industry.

The preceding discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in Australia as at the priority date of the application.

Throughout the specification, unless the context requires otherwise, the word "alumina" will be understood to encompass fully dehydrated alumina, fully hydrated alumina, partially hydrated alumina or a mixture of these forms. Further, the term "aluminium hydroxide" will be understood to encompass fully hydrated alumina, partially hydrated alumina or a mixture of these forms.

Further, throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Disclosure of the Invention

In accordance with the present invention, there is provided a method for controlling mercury emissions from aqueous alkaline solutions from the Bayer circuit, the method comprising the steps of: introducing a source of copper ions to an aqueous alkaline solution containing sulfide ions; precipitating a copper species; and precipitating a mercury species; thereby facilitating mercury removal from the aqueous alkaline solution.

The precipitated copper species may be any form of copper-containing compound. It will be appreciated that the form of the precipitated copper species will be influenced by the components in the aqueous alkaline solution. In one form of the invention, the precipitated copper species is provided as one or more of copper metal, cuprous sulfide or cupric sulfide. Alternatively or additionally, the precipitated copper species is provided as a copper polysulfide. It will be appreciated that the final form or forms of the copper species will depend on a number of conditions including the concentration of copper ions and sulfide ions in the aqueous alkaline solution as well as the presence of reducing agents.

The precipitated mercury species may be any form of mercury-containing compound. It will be appreciated that the form of the precipitated mercury species will be influenced by the components in the aqueous alkaline solution. In one form of the invention, the precipitated mercury species may be provided as one or more of mercury metal, mercurous sulfide or mercuric sulfide. Alternatively or additionally, the precipitated mercury species may be provided as a mercury polysulfide. It will be appreciated that the final form or forms of the mercury species will depend on a number of conditions including the concentration of mercury ions and sulfide ions in the aqueous alkaline solution as well as the presence of reducing agents. In one form of the invention, the copper species and the mercury species may both be a copper mercury mixed sulfide, for example Cu_xHg_yS_z, where x is between 0 and 2, y is between 0 and 2 and z is between 0 and 7. It will be appreciated that the copper mercury mixed sulfide may contain additional ions. It is preferable that the aqueous alkaline solution, prior to the introduction of the source of copper ions, comprises free sulfide ions. Without being limited by theory, it is believed that if the sulfide concentration is sufficiently high, the mercury will be present as the soluble thiomercurate complex anion HgS_x ^2", where x is most commonly 2 but can be 5 or more (Equation 1). It will be appreciated that as Equation 1 represents an equilibrium, a portion of the mercury may remain in solution as Hg²⁺ (aq). Despite this, at high alkalinity and with free sulfide present in molar excess, it is believed that millions of times more mercury can be held in solution as the thiomercurate complex than is typically present in Bayer solutions. Subsequent addition of copper is believed to initially result in the precipitation of copper species as the copper ions react with the free sulfide ions (Equations 2a and 2b). Removal of the free sulfide destabilises the soluble thiomercurate ion and also any transient thiocuprate ion that may form resulting in precipitation of further copper sulfide as well as mercury sulfide (Equations 3a and 3b). Hg²⁺ (aq) + 3S^2" (aq) - HgS₂ ^2' (aq) + S^2' (aq) Equation 1

Cu²⁺ (aq) + S²- (aq) -» CuS (s) Equation 2a

2Cu⁺ (aq) + S^2' (aq) -» Cu₂S (s) Equation 2b

Cu²⁺ (aq) + HgS₂ ^2" (aq) -> CuS (s) + HgS (s) Equation 3a

2Cu⁺ (aq) + HgS₂ ^2" (aq) Cu₂S (s) + HgS (s) Equation 3b It will be appreciated that Equations 1 , 2a and 3a above are presented in a simplified form on the assumption that all species (sulfide, mercury and copper) are present as divalent ions. The applicant believes that these species may, at least in part, be present in other ionic states such as monovalent ions.

The present invention enables the user to solubilise the mercury present in solution and control where in the Bayer circuit, the thiomercurate anion dissociates and the mercury released as insoluble mercury species such as Hg, Hg₂S and HgS.

Advantageously, the method of the present invention confers the ability to recover mercury from an aqueous alkaline solution and thereby reduce emissions of mercury from an alumina refinery. For example, the method of the present invention may be used to reduce mercury emissions from calcination stacks by reducing the amount of mercury in the aqueous alkaline solution and thereby reducing the amount of mercury precipitated with aluminium hydroxide.

In one form of the invention, the aqueous alkaline solution contains sufficient sulfide to solubilise the mercury as the thiomercurate complex. Alternatively, it may be necessary to add a source of sulfide ions to the solution to solubilise the mercury.

High temperature alkaline digestion of sulfide minerals produces sulfide ion species. It will be appreciated that the amount which is produced and how much survives depends on the relative amounts of sulfide producing species and oxidising species in the bauxite and the temperature of digestion.

The method of the invention may comprise the further step of: adding a source of sulfide ions to the aqueous alkaline solution.

Preferably, the step of adding a source of sulfide ions to the aqueous alkaline solution further comprises the step of: adding sufficient sulfide ions such that the aqueous alkaline solution contains free sulfide. >

Without being limited by theory, it is believed that the amount of sulfide relative to the desired mercury concentration required depends, at least in part, on the temperature. At higher temperatures, greater amounts of sulfide are needed to suppress the mercury vapour pressure and prevent mercury exiting as vapour. The vapour pressure of mercury is known to increase more than exponentially with temperature.

It is believed that below 100 °C, a sulfide/mercury molar ratio (moles of sulfide per moles of mercury) of about 4 is required. It is believed that between 00 °C and 150 °C, a sulfide/mercury molar ratio of about 8 is required. Finally, it is believed that above 150 °C, a sulfide/mercury molar ratio of greater than 8 is required. Data shows that 500 ppb mercury can be held in solution by 10 ppm sulfide when flashing a liquor from 250 °C to 00 °C, representing a molar ratio of about 125:1. While it is advantageous to have an excess of sulfide ions, it will be appreciated that the greater the amount of free sulfide present, the greater the amount of copper that needs to be added to the solution to induce precipitation of the mercury species.

The method of the invention may comprise the further step of: adding a solid support or coagulant to the aqueous alkaline solution.

The method of the invention may comprise the further step of: introducing a source of copper ions to the aqueous alkaline solution in combination with a solid support or coagulant.

The solid support or coagulant may be provided in the form of red mud, iron hydroxide or as part of a filter, for example, a sand filter. The solid support or coagulant may assist the precipitation of mercury species and subsequent occlusion of mercury species with the solid support or coagulant.

Without being limited by theory, it is believed that the solid support or coagulant can coat the precipitated copper and mercury species, and assist in particle agglomeration as well as the adherence of them onto red mud residue. It is believed that iron hydroxide may be particularly advantageous in this regard. It is further believed that coating of the precipitated copper and mercury species in iron hydroxide can reduce the teachability of the copper and/or mercury over time.

A convenient way of introducing iron hydroxide to the system is by the addition of a solution of iron (III) chloride.

The method of the invention may comprise the further step of: measuring the concentration of copper ions in the aqueous solution.

The concentration of copper ions in the aqueous alkaline solution may be measured by direct or indirect means. In one form of the invention, the concentration of copper is indirectly monitored by reference to the redox potential of the solution.

The concentration of copper ions in the aqueous alkaline solution may be monitored throughout the Bayer circuit and further copper ions added to the aqueous solution as required to precipitate copper species and/or mercury species or to precipitate further copper species and/or mercury species.

The step of: introducing a source of copper ions to an aqueous alkaline solution containing sulfide ions; may be repeated.

The source of copper ions may be introduced to the aqueous alkaline solution at more than one location.

The method of the invention may comprise the further step of: measuring the concentration of sulfide ions in the aqueous solution. The concentration of sulfide ions in the solution may be monitored throughout the Bayer circuit and further sulfide ions added to the aqueous solution as required to maintain an appropriate level of free sulfide.

Where the method of the invention comprises the step of: adding a source of sulfide ions to the aqueous alkaline solution, the step of: adding a source of sulfide ions to the aqueous alkaline solution, may be repeated.

The source of sulfide ions may be introduced to the aqueous alkaline solution at more than one location. The source of copper ions and the source of sulfide ions may be added to the aqueous alkaline solution at different locations in the Bayer circuit or at the same location as each other. The source of copper ions and the source of sulfide ions may be added to the aqueous alkaline solution at different times to each other or concurrently.

The steps of: precipitating copper species; and precipitating mercury species; may occur at different locations in the Bayer circuit or at the same location as each other.

Without being limited by theory, it is believed that it is possible to control the precipitation rates of the copper species and mercury species such that they precipitate at different locations in the Bayer circuit.

It will be appreciated that where the invention includes the step of adding a source of sulfide ions to the aqueous alkaline solution, the sulfide ions may originate from any source that provides sulfide ions in alkaline solutions.

In one form of the invention, the sulfide ions are provided in the form of sodium polysulfide. The sodium polysulfide is preferably added as an aqueous solution of a concentration between about 10 - 90% ^w/_w. More preferably/sodium polysulfide is added as an aqueous solution of a concentration between about 20 - 60% ^w/_w. In a specific form of the invention, sodium polysulfide is added as an aqueous solution of a concentration of about 40% ^w/_w. In an alternate form of the invention, the sulfide ions are provided in the form of any sulfide salt soluble under caustic conditions including sodium sulfide or sodium hydrogen sulfide. In another form of the invention, the sulfide is provided in the form of an organosulfide compound.

Preferably, the method comprises the further step of: maintaining sufficient sulfide concentration in the alkaline solution such that there is free sulfide in the aqueous alkaline solution.

The concentration of the free sulfide in the aqueous, alkaline solution may-be— measured by any means known in the art including the redox potential of the solution. For example, the applicant has discovered that the redox potential of the aqueous alkaline solution may be a convenient way to indirectly monitor the sulfide concentration. Preferably, the free sulfide is present in sufficient quantities to retain a redox potential in the alkaline solution of below minus 600 mV measured by Ag electrode relative to Ag/AgCI electrode (i.e. below Eh of - 380 mV). It is understood that higher free sulfide concentrations result in more negative redox potentials.

A redox potential of a freshly aerated solution below minus 600 mV measured by Ag electrode relative to Ag/AgCI electrode is believed to indicate the presence of trace free sulfide. The sulfide can be quantified by titrating a suitable aliquot containing 5 μg to 400 g sulfide ion to an end point of minus 540 mV measured by Ag electrode relative to Ag/AgCI electrode using 0.003 Ag⁺ ion. Sulfide has a half life of some hours in cold aerated Bayer liquor in the absence of iron ions. Interfering reducing compounds such as iron ions and organic matter are exceptionally air sensitive. Commercially available SAOB buffer (EDTA base) or ascorbic acid can also be used but care is required so that interfering substances are not also preserved. Consequently, samples should be taken with as little exposure to air as possible to preserve the sulfide but the sample should then be exposed to air immediately before titration to prevent ferrous ion and other reducing compounds, like aldehydes, interfering when titrating with silver ion. Without being limited by theory, it is believed that the amount of sulfide required is dependent at least, on the amount of mercury that is required to be stabilised and the temperature of the process at that point.

Preferably, the method more specifically comprises the step of: maintaining the free sulfide concentration in the alkaline solution to retain a redox potential in the alkaline solution prior to the step of precipitating mercury species of less than -540 mV.

Preferably, the method more specifically comprises the step of: maintaining the free sulfide concentration in the alkaline solution to retain a redox potential in the alkaline solution prior to Jhe step_^oLprecipitating mercury species of less than -600 mV. Without being limited by theory, it is believed that a redox potential of less than - 600 mV after vigorous aeration, measured by Ag electrode relative to Ag/AgCI electrode should be sufficient at the end of the precipitation stage of the Bayer circuit to prevent Hg precipitating onto aluminium hydroxide. However, it is believed that a eightfold molar excess of sulfide (lower mV) will be required in low temperature digestion and even higher levels may be needed in high temperature digestion to suppress the mercury vapour pressure and prevent mercury exiting as vapour (or conversely lower levels of Hg will remain in solution for a given amount of free sulfide as the temperature is raised). The aqueous alkaline solution may be provided in the form of any process stream from the Bayer circuit with pH of 7 or more including Bayer process liquor, dilute alkaline process streams found in Bayer refineries such as residue wash water and condensate streams. It is expected that most process streams in the Bayer circuit would have a pH of about 11 or higher. In addition to reducing mercury precipitation with alumina and subsequent mercury emissions due to calcination , the method of the present invention may be used to reduce mercury emissions from oxalate furnace stacks by reducing the amount of mercury transferred into the oxalate kiln..

In one form of the invention where the Bayer process includes the steps of: digestion of bauxite with alkaline solution; liquid-solid separation to provide a residue and a green liquor; precipitation of aluminium hydroxide from the green liquor to provide aluminium hydroxide and spent liquor; and calcination of the aluminium hydroxide to provide alumina, the sulfide ions are added to the alkaline solution prior to or during the step of digestion of bauxite. It will be appreciated that as the spent liquor from which aluminium hydroxide is precipitated is recycled to digest further bauxite, the term 'prior to digestion' in this context means at any point in the Bayer process after precipitation of aluminium hydroxide. It will be understood that the spent liquor may undergo treatment before being recycled to digestion. Such treatment steps may include purification (for example, oxalate anion removal) and concentration by evaporation.

In a further form of the invention, the sulfide ions are added to the spent liquor directly prior to digestion. In the present context, the term 'directly' is taken to mean after the spent liquor has been treated and before the spent liquor is added to a digester.

In a further form of the invention, the sulfide ions are added to the alkaline solution concurrent with the step of digestion of bauxite.

Preferably, the method comprises the further step of: maintaining sufficient sulfide concentration in the alkaline solution during the step of digestion of bauxite such that there is free sulfide in the aqueous alkaline solution after the step of digestion of bauxite.

Preferably, the method more specifically comprises the step of: maintaining the free sulfide concentration in the alkaline solution during the step of digestion of bauxite to retain a redox potential in the alkaline solution of less than -540 mV.

Preferably, the method more specifically comprises the step of: maintaining the free sulfide concentration in the alkaline solution during the step of digestion of bauxite to retain a redox potential in the alkaline solution of less than -600 mV.

It will be appreciated that the desired redox potential may differ for different alkaline solutions.

It will be appreciated that quantity of sulfide added to the alkaline solution will be influenced by the quantity of free sulfide already present in the alkaline solution. Where the sulfide ions are added to the alkaline solution prior to or concurrent with the step of digestion of bauxite, the step of: introducing a source of copper ions to the aqueous⁷ alkaline solution, may occur prior to or concurrent with the step of digestion of bauxite and after or concurrent with the step of introducing a source of sulfide ions to the aqueous alkaline solution.

Where the sulfide ions are added to the alkaline solution prior to or concurrent with the step of digestion of bauxite, the step of: introducing a source of copper ions to the aqueous alkaline solution, may occur after the step of digestion of bauxite and prior to or concurrent with the step of liquid-solid separation.

Said forms of liquid-solid separation may include sand separation, mud thickening, mud washing and security filtration

Where the step of introducing a source of copper ions to the aqueous alkaline solution occurs prior to or concurrent with the step of liquid-solid separation, the alkaline solution advantageously comprises red mud solids. The red mud solids can assist with the precipitation and capture of copper species and/or mercury species and/or mixed copper and mercury species due to the large surface area of red mud (mainly hydrated iron oxides and other metal hydroxides).

The method may further comprise the step of transferring at least a portion of the precipitated mercury species to a refinery residue area.

The method of the present invention may be used to reduce mercury emissions from the digestion and evaporation unit processes by solubilising the mercury present in the condensable and non condensable gaseous emissions from these unit processes with an aqueous alkaline solution containing sulfide and then removing the mercury by copper addition.

Condensate streams in the Bayer circuit that may be treated using the method of the invention include condensate streams formed in the evaporation of spent liquor and in the digestion of bauxite. The concentration of spent liquor by evaporation and the digestion of bauxite can provide non-condensable vapour streams and condensable vapour streams. Both streams can contain mercury.

The method of the invention may comprise the further step of: contacting an aqueous alkaline solution containing sulfide with non- condensable gases from the Bayer process.

The aqueous alkaline solution containing sulfide may be a solution suitable for use as seal water in a vacuum pump used to remove non condensable gases from the Bayer process such as residue wash water,

The method of the invention may comprise the further step of: contacting an aqueous alkaline solution containing sulfide with condensed vapour from the Bayer process.

Where the method comprises the step of: contacting an aqueous alkaline solution containing sulfide ions with non- condensable gases from the Bayer process, the method preferably comprises the further step of: introducing a source of copper ions to the aqueous alkaline solution containing sulfide ions after contacting the aqueous alkaline solution containing sulfide ions with the non-condensable gases.

Preferably, the step of: introducing a source of copper ions to the aqueous alkaline solution containing sulfide ions after contacting the aqueous alkaline solution containing sulfide ions with the non condensable gases, is performed in conjunction with the addition of a solid or coagulant to assist in the deposition of mercury and also to coat the sulfide particles with iron hydroxide in order to reduce the subsequent teachability of the copper and/or mercury where the solid or coagulant may be added either concurrent with or subsequent to copper ion addition. The method of the invention may comprise the further step of: contacting an aqueous alkaline solution containing sulfide with cojTdjensable gases-from the Bayer process^

Where the method comprises the step of: contacting an aqueous alkaline solution containing sulfide ions with condensable gases from the Bayer process, the method preferably comprises the further step of: introducing a source of copper ions to the aqueous alkaline solution containing sulfide ions after contacting the aqueous alkaline solution containing sulfide ions with the condensable gases. Preferably, the step of: introducing a source of copper ions to the aqueous alkaline solution containing sulfide ions after contacting the aqueous alkaline solution containing sulfide ions with the condensable gases, is performed in conjunction with the addition of a solid or coagulant to assist in the deposition of mercury and also to coat the sulfide particles with iron hydroxide in order to reduce the subsequent leachability of the copper and/or mercury where the solid or coagulant may be added either concurrent with or subsequent to copper ion addition.

Where the method comprises the step of addition of a solid or coagulant to assist in the deposition of mercury where the solid or coagulant may be added either concurrent with or subsequent to copper ion addition may further comprise the step of: adding solid iron hydroxide to the alkaline solution.

Where the method comprises the step of adding solid iron hydroxide to the alkaline solution the method may further comprise the step of: forming the solid iron hydroxide in situ through addition of iron ions to the alkaline solution. The present invention confers the advantage of reducing mercury emissions from non condensable vapours emitted from the digestion or evaporation unit processes without the need to treat the vapour.

In accordance with the present invention, there is provided alumina produced by any one of the Bayer processes described hereinabove.

In accordance with the present invention, there is provided an apparatus for the production of alumina by any one of the Bayer processes described hereinabove.

Brief Description of the Drawings

The present invention will now be described, by way of example only, with reference to four embodiments thereof, and the accompanying drawings, in which :-

Figure 1 is a schematic flow sheet of the Bayer Process circuit showing the major mercury inputs and outputs;

Figure 2 is a schematic flow sheet showing how a method in accordance with a first embodiment of the present invention may be utilised in a Bayer Process circuit;

Figure 3 is a schematic flow sheet showing how a method in accordance with a second embodiment of the present invention may be utilised in a Bayer Process circuit;

Figure 4 is a schematic flow sheet showing how a method in accordance with a third embodiment of the present invention may be utilised in a Bayer Process circuit; and

Figure 5 is a plot of copper addition against redox potential.

Best Mode(s) for Carrying Out the Invention Those skilled in the art will appreciate that the invention described herein is amenable to variations and modifications other than those specifically described. It is to be understood—that the invention includes- all such variations—and— modifications. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features.

By way of example, the method of the present invention is described in the context of the control of mercury emissions from process streams of Bayer alumina refining, although such should not be seen as limiting the generality of the foregoing description.

The invention focuses on the removal of mercury in the Bayer process -by careful control of addition of sulfide or sulfide-containing compounds and copper ions to facilitate precipitation of mercuric sulfide whilst substantially minimising the problems associated with the control of mercury oxidation in the presence of organic compounds. By careful manipulation of the Bayer process chemistry through the amount and location of sulfide and copper ion addition, the speciation and hence the fate and transport of mercury in the Bayer process can be more accurately predicted and controlled. Figure 1 shows a schematic flow sheet of the Bayer process circuit for a refinery using a single digestion circuit in accordance with a first embodiment of the present invention. The Bayer process comprises the steps of:

Milling 11 and pre-desilication 13 of bauxite 14; digestion 12 of bauxite 14 in a caustic solution; liquid-solid separation 16 of the mixture to residue 18 and green liquor 20; filtration 22 of the liquor 20; precipitation 23 of the filtered liquor 20; cooling of the liquor 20 to cause aluminium hydroxide 24 precipitation; separating aluminium hydroxide 24 and spent liquor 26; recycling spent liquor 26 to digestion 12 with some or all of the spent liquor

26 going through evaporation 28 or oxalate removal 29; and calcining 30 the aluminium hydroxide 24 to alumina. As shown in Figure 1 , mercury emission 32 can occur at various locations in the circuit.

In accordance with one embodiment of the invention, as shown in Figure 2, the sulfide 34 as an aqueous solution containing sodium polysulfide at a concentration of about 40% Sodium polysulfide by weight, is added to the Bayer liquor. The addition of sulfide 34 may occur at one or more points in the process including for example, prior to digestion 12, to maximise residence time and facilitate maximum solubilisation of mercury prior to liquid-solid separation 16 as shown in Figure 2. Alternatively or additionally, the sulfide 34 may be added to the spent liquor 26 before or after treatment steps of oxalate removal 29 and evaporation 28.

The sulfide concentration is maintained such that that there is sufficient free sulfide in the aqueous alkaline solution to solubilise mercury at the desired locations, for example after aluminium hydroxide precipitation and after bauxite digestion. The amount of sulfide that needs to be added may depend upon several factors including the agitation method used in precipitation, the propensity for sulfide to be produced or consumed in digestion and the sulfide source used.

The addition of sulfide 34 causes the mercury to be solubilised as a mercury sulfide complex. Copper is subsequently added 38 to consume excess free sulfide facilitating precipitation of copper species and mercuric species.

It will be appreciated that the dosing of sulfide 34 to liquor may be influenced, among other factors, by the extent of exposure to air in the precipitation process and the temperature of the aqueous alkaline solution at the addition point.

The copper concentration and the sulfide concentration may be monitored at various locations throughout the circuit and additional sulfide and/or copper added as required.

Sulfide 34 may be added to the spent liquor after removal of the precipitated aluminium hydroxide. Addition of sulfide at this location will assist in stabilising any mercury remaining in the spent liquor and reduce mercury precipitation with oxalate. In a many Bayer refineries, oxalate in the spent liquor is precipitated and removed from the spent liquor. Some of the oxalate is retained to be used as seed for further precipitation and some of the oxalate may be destroyed in a kiln. If mercury precipitates with the oxalate it will be released to the atmosphere should the oxalate be thermally destroyed.

Sulfide 34 may also be added to the liquor after removal of residue and prior to precipitation of aluminium hydrate. Sulfide addition at this point may complex any mercury that was not removed by copper addition 38. Complexation of such residual mercury has been demonstrated to inhibit mercury precipitation with aluminium hydroxide. If mercury is precipitated with aluminium hydroxide it will be released to the atmosphere at calcination 30. Alternatively, the sulfide ions 34 may be added to the spent liquor after oxalate removal 29 or after evaporation 28 and prior to digestion 12.

Preferred locations for the addition of copper ions 38 are after digestion 12 and before liquid solid separation, or during the liquid solid separation stage. It is desirable that the mercury species precipitate with red mud and be combined with red mud removal 18. Alternatively, or, additionally, copper ions 38 may be introduced in the form of copper ions applied to a solid support. For example, copper ions applied to the filters 22, such that copper and mercuric sulfide precipitate as the mixture passes through the filter.

In a second embodiment of the invention, the mercury removal steps may be applied to the evaporation or digestion vapour circuit as shown in Figure 3. In many Bayer refineries, spent liquor 26 is concentrated by evaporation 28 prior to being reused in digestion 12. Both the digestion and evaporation processes are carried out well above atmospheric temperature and pressure and thus provide vapour 40 which comprises condensable 42 and non-condensable 44 components. The present invention provides for treatment of both the condensable 42 and non-condensable 44 components of the vapour stream.

The non-condensables 44 that are passed to a vapour pump 46 can contain mercury. It is known to use water 48 to seal such a vapour pump 46. Addition of sulfide 34 to the seal water 48 prior to entering the vapour pump 46 can solubilise any mercury from the non condensables in the seal water as the thiomercurate complex. On exiting the vapour pump 46, copper ions 38 can be added to the used seal water 50 in the mixing tank 52 to precipitate copper species and mercury species as shown in Figure 3. The resulting slurry 53 is passed to solid liquid separation 54 where mercury 32 is removed from the residue 56

Mercury can also be present in the condensable vapour 42. Addition of sulfide 34 to the condensed vapour 42 can solubilise any mercury present in the condensed stream as the thiomercurate complex.

The condensed stream 58 may be combined with the seal water 50 as shown in Figure 3 or they may be treated separately. If desired or required, a coagulation medium, for example a source of ferric ions 39 may be added in a coagulation step 41. The ferric ions 39 will coagulate the precipitation of copper species and mercuric species through the formation of iron (III) hydroxide to aid filtration downstream.

It will be appreciated that embodiments one and two of the present invention may be combined as shown in Figure 4 to provide a third embodiment wherein the condensate treatment circuit is reflected as 60. In accordance with a further embodiment of the invention, the mercury removal steps may be applied downstream to the evaporation or digestion vapour circuit. As shown in Figure 5, a separate treatment circuit is used to treat a mixture of the vacuum seal water 50 and the condensed stream 52 from the digestion condenser 62 and the evaporation condenser 64. Sulfide 34 as an aqueous solution containing sodium polysulfide at a concentration of about 40% sodium polysulfide by weight, is added to the mixture of the vacuum seal water 50 and the condensed stream 58 from the digestion condenser 62 and the evaporation condenser 64 and it transferred to reaction tank 66. The addition of sulfide 34 may occur at one or more points in the process including for example, in the reaction tank 66, or in the digestion condenser 62 and the evaporation condenser 64 in order to maximise residence time and facilitate maximum solubilisation of mercury in reaction tank 66.

The sulfide concentration throughout the mercury removal process is maintained such that that there is sufficient free sulfide in the aqueous alkaline solution to solubilise mercury at the desired locations, for example in the reaction tank 66. The amount of sulfide that needs to be added may depend upon the process conditions, such as, for example the agitation method used in precipitation, the propensitv for sulfide to be produced or consumed in digestion or the sulfide source used.

The addition of sulfide 34 causes the mercury to be solubilised as a mercury sulfide complex. Copper sulphate 38 is added to the underflow of reaction tank 66 which is then subsequently passed to a second reaction tank 70 where excess free sulfide is consumed in order to facilitate the precipitation of copper species and mercuric species.

The underflow of the second reaction tank 70 is in the form of a slurry which contains precipitated copper species and mercuric species and is then transferred to a third reaction tank 72 to further facilitate the precipitation of copper species and mercuric species. If required, additional copper 38 may be added to the underflow of second reaction tank 70 in order to consume any residual free sulfide species.

The underflow of the third reaction tank72 is then passed to a settling tank 74 where it is mixed with bulk residue solids from other parts in the Bayer circuit.

It will be appreciated that in certain situations where the settling tank 74 may not be available, that a coagulation medium, for example a source of ferric ions 76, can be added to the second reaction tank 70 underflow in order to coagulate the precipitation of copper species and mercuric species through the formation of iron (III) hydroxide to aid filtration downstream.

The following examples serves to more fully describe the manner of using the above-described invention, as well as to set forth the best modes contemplated for carrying out various aspects of the invention. It is understood that this example in no way serves to limit the true scope of this invention, but rather is presented for illustrative purposes.

Example 1 A pilot plant trial was conducted using a pilot apparatus connected to a refining operation. Overflow of refinery liquid-solid separation 16 was directed to the pilot feed tank and the flow rate adjusted to provide a residence time in the feed tank of approximately the same as that iri the refinery filter feed tanks. The average residence time was in the vicinity of twenty minutes.

No additional sulfide ion was added, as measurements confirmed sufficient mercury in solution to test the effectiveness of copper as a removal agent and the sulfide level was sufficient to solubilise the mercury as the thiomercurate complex as indicated by a liquor redox potential of less than -600mV measured by Ag electrode relative to Ag/AgCI electrode. A single dose of copper ions (as copper sulfate pentahydrate solution) were added to the filter feed tank in order to increase the copper levels to those expected at a steady state condition. Additional copper was added as required to maintain a target copper dose to start sulfide ratio. Several copper dose rates were used to determine the optimum does required.

Samples were taken directly after the filter feed tank and filtered through a 0.2 μιη polysulfone filter. The redox potential was determined immediately after sampling and aeration using a hand-held electrode and sub-samples taken for mercury, copper and sulfide determinations. The results are provided in Table 1. below:

Table 1 : Hg Measurements

The zero copper dose measurements show a redox potential due to sulfide sufficiently negative to solubilise the mercury as the thiomercurate complex at relatively high levels. Copper addition at 0.008 mM per litre produced an approximately 1 :1 reduction in sulfide suggesting formation of CuS. This copper , dose was not sufficient tojremo e the ^re^uired_^amount of sulfide to destabilise the thiomercurate complex, therefore the mercury remained in solution. Note that the redox potential remained well below -600mV. Copper addition at 0.015 mM per litre produced a reduction in sulfide levels but not at the 1. level expected for CuS formation, this was attributed to co- precipitation of CuS and CU2S. This copper dose was not sufficient to remove the required amount of sulfide to destabilise the thiomercurate complex, therefore the mercury remained in solution while the redox potential remained well below - 600mV.

Copper addition at 0.031 mM per litre produced a corresponding reduction in sulfide levels to below the detection limit of the measurement while consuming all of the added copper. However the mercury remained in solution as the thiomercurate complex confirming that only very low levels of sulfide were sufficient to solubilise mercury under the test conditions.

Copper addition at 0.062 and 0.092 mM per litre induced a significant change in the mercury levels in the output solution with a corresponding increase in the redox potential of the solution indicating removal of free sulfide. In this particular example, a copper dose rate sufficient to produce a redox potential greater than approximately - 570 mV resulted in mercury removal from solution. Therefore having established the redox potential at which mercury is removed the copper dose may be readily controlled based on redox potential to achieve the minimum copper addition level required. The example described above shows significant mercury removal due to copper addition largely in the absence of red mud solids or any other precipitating or coagulating reagent. It is expected that the presence of such solids will enhance mercury precipitation such that lower mercury levels in the output could be readily achieved. Further, although the test described above was performed using Bayer liquor with TC approximately 200 g/L, (NaOH represented as a₂C0₃ equivalents) it is expected that the redox chemistry demonstrated would apply to other alkaline solutions in a Bayer refinery. According to the. method of removing mercury from sulfide containing solutions by copper ion addition, targeting a particular solution redox potentJaJ_^wouJa^e^pected_ to be_{^ .}readily_a_dap_tejdJQ__m.ore_dilu.te alkaline solutions. Example 2

A mixture of refinery residue lakewater and condensate from the digestion condenser was prepared to approximately simulate the conditions created by the used seal water 50 and condensed vapour 42 as shown in Figure 3. The pH of the test solution was approximately 12.7 and the redox potential of the freshly aerated solution as measured by Ag electrode relative to Ag/AgCI electrode indicated an absence of free sulfide in solution.

Sulfide was added to the solution of refinery residue lakewater and condensate as an aqueous solution of sodium polysulfide ( a₂S₄) a portion of which would dissociate to free sulfide. The addition of the sulfide resulted in a corresponding redox potential decrease, indicating that the addition of free sulfide impacts the redox potential in this system.

Cupric ions (Cu²⁺) were added in 0.0015 mM increments. The measured redox potential is plotted against the addition of cupric ions in Figure 6. The addition of the Cupric ions (Cu2+) caused an increase in redox potential consistent with removal of free sulfide from solution. The addition of sulfide and cupric ions as described above was repeated with fresh aliquots of sodium sulfide and cupric ions to further confirm the effect.

This data in Figure 6 clearly demonstrates that the redox potential of the freshly aerated solution measured by Ag electrode relative to Ag/AgCI electrode may be used to indicate the free sulfide level. Further, it is clear that free sulfide may be removed by the addition of copper ions. As such, the copper dose may be readily controlled based on redox potential to achieve the copper addition level required.

Example 3 A separate treatment.circuit was implemented downstream to the evaporation and digestion vapour circuit in order to treat a mixture of the vacuum seal water and the condensate from digestion and evaporation, as shown in Figure 5.

Sulfide was added to each of the digestion condenser 62 and the evaporation condenser 64 as an aqueous solution of sodium polysulfide (Na₂S₄) a portion of which would dissociate to free sulfide. No additional sulfide ion was added to the reaction tank 66, as measurements confirmed that the sulfide level was sufficient to solubilise the mercury as the thiomercurate complex. Samples from each of the condensers were taken and the free sulfide was measured using standard titration techniques.

A single dose of copper ions (as copper sulfate pentahydrate solution) was added to the second reaction tank 70 in order to consume the free sulfide. Copper was added in slight stoichiometric excess so that ail the free sulfide ions were consumed. The copper dose was controlled to ensure that no more than 2 ppm free copper remained in the solution. Several copper dose rates were used to determine the optimum dose required.

Samples from each of the second reaction tank 70 and the third reaction tank 72 were taken and measurements of the free Cupric ions (Cu²⁺) concentrations were analysed by standard titration techniques. The redox potential was also analysed immediately after sampling and aeration using a hand-held electrode and sub- samples taken for copper and sulfide determinations. The results are provided in Table 2 below:

Table 2: Sulfide and Copper Measurements

The Cu²⁺ concentration was then directly correlated to the redox potential. When Cu²⁺ is in excess, free sulfide is not present and thus relatively high mV values for the tank solution are recorded (no free sulfide remains). However, when Cu is depleted, the recorded mV is low as there is insufficient copper to consume all of the sulfide as indicated by a liquor redox potential of less than -600mV: It is understood that higher free sulfide concentrations result in more negative redox potentials. This data clearly demonstrates that the redox potential of the freshly aerated solution measured by Ag electrode relative to Ag/AgCI electrode may be used to indicate the free sulfide level. This redox potential is then determinative of whether sufficient Cu²⁺ is being dosed to consume the free sulfide in order to facilitate the precipitation of the mercuric species. As such, the copper dose may be readily controlled based on redox potential to achieve the copper addition level required.

Further, as demonstrated by Figure 7, it is clear that free sulfide as measured in the treatment tank may be removed by the addition of copper ions. This demonstrates that the copper ion addition facilitates the mercury removal, where mercury was present as the thiomercurate complex.

Claims

A method for controlling mercury emissions according to claim 1 , wherein the precipitated copper species is one or more of copper metal, cuprous sulfide, cupric sulfide or copper polysulfide. ^;

A method for controlling mercury emissions according to any one of the preceding claims, wherein the precipitated mercury species is one or more of mercury metal, mercurous sulfide, mercuric sulfide or mercury polysulfide.

A method for controlling mercury emissions according to claim 1 , wherein the copper species and the mercury species are a copper mercury mixed sulfide.

A method for controlling mercury emissions according to any one of the preceding claims, wherein the aqueous alkaline solution, prior to the introduction of the source of copper ions, comprises free sulfide ions.

A method for controlling mercury emissions according to any one of the preceding claims, wherein the method comprises the further step of: adding a source of sulfide ions to the aqueous alkaline solution.

A method for controlling mercury emissions according to any one of the preceding claims, wherein the method comprises the further step of: measuring the concentration of copper ions in the aqueous solution and repeating the step of: introducing a source of copper ions to an aqueous alkaline solution containing sulfide ions.

A method for controlling mercury emissions according to any one of the preceding claims, wherein the method comprises the step of: measuring the concentration of sulfide ions in the aqueous solution. and repeating the step of: adding a source of sulfide ions to the aqueous alkaline solution, required to maintain an appropriate level of free sulfide.

A method for controlling mercury emissions according to any one of the preceding claims, wherein the source of copper ions and the source of sulfide ions are added to the aqueous alkaline solution at different locations in the Bayer circuit or at the same location as each other.

10. A method for controlling mercury emissions according to any one of the preceding claims, wherein the source of copper ions and the source of sulfide ions are added to the aqueous alkaline solution at different times to each other or concurrently.

11. A method for controlling mercury emissions according to any one of the preceding claims, wherein the steps of: precipitating copper species; and precipitating mercury species; occur at different locations in the Bayer circuit or at the same location as each other. 12. A method for controlling mercury emissions according to any one of the preceding claims, wherein the aqueous alkaline solution is any process stream from the Bayer circuit with pH of 7 or more including Bayer process liquor, dilute alkajine_^ process ..^'streams found-in Bayer refineries suclras- residue wash water and condensate streams.

13. A method for controlling mercury emissions according to any one of the preceding claims, comprising the further steps of: digestion of bauxite with aqueous alkaline solution; liquid-solid separation to provide a residue and a green liquor; precipitation of aluminium hydroxide from the green liquor to provide aluminium hydroxide and spent liquor; and calcination of the aluminium hydroxide to provide alumina, wherein the sulfide ions are added to the alkaline solution prior to or during the step of digestion of bauxite. 14. A method for controlling mercury emissions in a Bayer process according to claim 13, wherein the method comprises the further step of: maintaining sufficient sulfide concentration in the alkaline solution during the step of digestion of bauxite such that there is free sulfide in the aqueous alkaline solution after the step of digestion of bauxite. 15. A method for controlling mercury emissions in a Bayer process according to claim 13 or 14, wherein the step of: introducing a source of copper ions to the aqueous alkaline solution, occurs prior to or concurrent with the step of digestion of bauxite and after or concurrent with the step of introducing a source of sulfide ions to the aqueous alkaline solution.

16. A method for controlling mercury emissions in a Bayer process according to claim 13 or 14, wherein the step of: introducing a source of copper ions to the aqueous alkaline solution, occurs after the step of digestion of bauxite and prior to or concurrent with the step of liquid-solid separation.

17.

Bayer process according to claim 1 , wherein the method comprises the further step of: contacting an aqueous alkaline solution containing sulfide with condensed and/or non-condensable gases from the Bayer process.

A method for controlling mercury emissions in a Bayer process according to claim 17, wherein the method comprises the further step of: introducing a source of copper ions to the aqueous alkaline solution after contacting the non-condensable gases with the alkaline solution containing sulfide.

A method for controlling mercury emissions in a Bayer process according to claim 18, wherein the step of: introducing a source of copper ions to the aqueous alkaline solution after contacting the non condensable gases with the alkaline solution containing sulfide, is performed in conjunction with the addition of a solid or coagulant to assist in the deposition of mercury.

A method for controlling mercury emissions in a Bayer process, substantially as hereinbefore described with reference to any one of examples 1 to 3.

A method for controlling mercury emissions in a Bayer process, substantially as hereinbefore described with reference to any one of figures 1 to 7.