WO2010100631A2

WO2010100631A2 - Mine water treatment

Info

Publication number: WO2010100631A2
Application number: PCT/IB2010/050979
Authority: WO
Inventors: Hendrik Willem Van Der Walt
Original assignee: Bacarac Trading 114 (Proprietary) Limited
Priority date: 2009-03-06
Filing date: 2010-03-08
Publication date: 2010-09-10
Also published as: WO2010100631A3; ZA201106788B; AU2010220102A1

Abstract

This invention relates to a method of treating aqueous mine tailings and other aqueous mine residues such as mine water, especially acidic mine water (AMD) from underground workings as well as aqueous residues from reclaimed mining materials, the method including the step of treating the aqueous mine tailings or residue with a biocide/s to kill chemolithotropic bacteria such as Thiobacillic feroxidans and Desulf ovibrio vulgaris. The biocide may be chlorine dioxide, sulphur dioxide, potassium permanganate, tributyl tin oxide (TBTO), cooling tower biocides, hypobromous acid, ozone, and/or ultra violet light. The preferred biocide is a halide, in particular bromine. In accordance with a preferred embodiment of the invention, biocide/s may be applied in a Step A) to the slurry prior to the gold recovery process; in a Step B) to the slurry after the gold recovery process; and optionally in a Step C) to the dump where the slurry has been disposed in water (preferably treated AMD) sprayed on the dump for dust suppression. The composition may be stabilized and comprise an anti-scale agent. Before adding the biocide, the water may undergo neutralization.

Description

MINE WATER TREATMENT

BACKGROUND TO THE INVENTION

The storage of mine tailings and other residues from underground workings have been blamed for a number of serious negative environmental situations, including: the occurrence of dust during windy days, contamination of surface water bodies, contamination of underground water streams and atmospheric contamination by uranium and other toxic substances in the stored tailings.

Another problem in the mining environment is the scaling of water transport pipelines. This is the most costly problem when it comes to the maintaining of pipelines used in pumping wastewater in the treatment processes. The maintaining of an accurate water balance and process control necessitates unobstructed transport through an intricate system of pipeline in and out of reactors or treatment plants. The scaling of pipelines causes difficulties in operating treatment systems at maximum efficiency and at the correct cost efficiency. Under- scale corrosion adds to the problem and the lifetime of steel pipes is shortened and a very high replacement cost is the result.

AMD, or acidic mine drainage, consists of water, made acidic by processes related to the mining of metals that are found in association with the rock that is mined. When the mined rock and the walls of the underground excavations, that contain pyrite (FeS₂), are exposed to oxygen and water, the products are sulphuric acid and sulphates or hydroxides of iron, as illustrated by the following examples:

4FeS₂ + 15O₂ + 7H₂O → 4Fe(OH)₂ + 4H₂SO₄, or

2FeS₂+ 7O₂ + 2H₂O → 2FeSO₄ + 2H₂SO₄, or

4FeS₂ + 15O₂ + 2H₂O → 2Fe₂(SO₄)₃ + 2H₂SO₄ AMD is an acidic mixture into which salts of all of the available metals such as iron are dissolved. AMD is toxic and it has to be cleaned and neutralised. The most widely used treatment process for the removal of the toxic heavy metals from the AMD is the High Density Sludge (HDS) Process. Basically, this is a process where the heavy metals are removed from solution by chemical intervention. During this intervention the metals become insoluble and are then precipitated and phase separation is enforced separating the solids and the treated water. Pumping of the sludge for further treatment or storage is a problem where the sludge, by its very nature, scales the pipes in a very short time. The scaling that occurs in pipelines is shown in Figures 2 and 3. A further, costly, problem is under-scale corrosion.

It is an object of this invention to address these problems.

SUMMARY OF THE INVENTION

According to the invention, there is provided a method of treating aqueous mine tailings and other aqueous mine residues such as mine water, especially acidic mine water (AMD) from underground workings as well as aqueous residues from reclaimed mining materials, the method including the step of treating the aqueous mine tailings or residue with a biocide/s to kill chemolithotropic bacteria such as Thiobacillic feroxidans and Desulfovibrio vulgaris.

The biocide may be chlorine dioxide, sulphur dioxide, potassium permanganate, tributyl tin oxide (TBTO), cooling tower biocides, hypobromous acid, ozone, and/or ultra violet light.

The preferred biocide is a halide, preferably bromine which is preferably administered in the form of hypobromous acid preferably in solution with a pH of 8 to 9.

The hypobromous acid is preferably from a stabilized stock liquid hyprobromous acid solution that has a hypobromous acid concentration of less than 30% (m/m) typically less than 20% (m/m), preferably from 10% to 20% (m/m) (the concentration of hypobromous acid is determined by ion chromatography using a Dionex AD 14 ion exchange column, sodium carbonate - sodium bicarbonate as eluent and suppressed conductivity detection) and contains an amount of stabilizer such as cyanuric acid not exceeding 1 ppm, preferably not exceeding 0.5 ppm, and a pH of 8 to 9, preferably a pH of 8.5 to 8.9, most preferably a pH of 8.8. The stock solution may be a sodium or potassium based hypobromous acid solution, preferably a potassium based hypobromous acid solution.

The aqueous tailings/residues may be dosed to provide 0.1 to 1.5, typically 0.5 to 1.5, preferably 0.8 to 1.3, most preferably 0.9 to 1.2 mg free active bromine per L in the slurry/residue prior to disposal.

In accordance with an embodiment of the invention, a slurry from reclaimed material from an existing mine dump is treated with biocide/s prior to and/or after a gold recovery process, preferably prior to and after a gold recovery process; prior to disposal of the slurry on a dump.

In accordance with a preferred embodiment of the invention, biocide/s may be applied: in a Step A) to the slurry prior to the gold recovery process; in a Step B) to the slurry after the gold recovery process; and optionally in a Step C) to the dump where the slurry has been disposed in water (preferably treated AMD) sprayed on the dump for dust suppression.

In Step A), a hypobromous acid solution may be added to the slurry at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 1 to 10, preferably 3 to 6, most preferably 5kg per metric ton slurry.

In Step B), a hypobromous acid solution may be added to the slurry at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 1 to 10, preferably 5 to 8, most preferably 7kg per metric ton slurry.

In Step C), a hypobromous acid solution may be added to the slurry at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 1 to 10, preferably 2 to 4, most preferably 3kg per metric ton slurry.

In accordance with another embodiment of the invention, a slurry from a mining operation is treated with biocide/s prior to and/or after a slurry concentration step, preferably prior to and after the slurry concentration step; prior to disposal of the slurry on a dump.

In this embodiment of the invention, the biocide/s may be applied: in a Step A) to the slurry prior to a concentration step; in a Step B) to the slurry after the concentration step; and optionally in a Step C) to the dump where the slurry has been disposed in water (preferably treated AMD) sprayed on the dump for dust suppression .

In accordance with another embodiment of the invention, water (such as treated AMD) for use in dust suppression on a mine dump is treated with biocide/s prior to spraying the water onto the dump.

The biocide/s may be added to the water at a at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 1 to 10, preferably 2 to 4, most preferably 3kg per metric ton slurry.

A further embodiment of the invention is a method of treating acid mine drainage (AMD) solution which has been neutralized, for example treated in a High Density Sludge (HDS) Process, the method including the step of treating the solution with a biocide/s to kill chemolithotropic bacteria such as Thiobacillic feroxidans and Desulfovibrio vulgaris.

The hypobromous acid is preferably from a stabilized stock liquid hyprobromous acid solution that has a hypobromous acid concentration of less than 30% (m/m) typically less than 20% (m/m), preferably from 10% to 20% (m/m) (the concentration of hypobromous acid is determined by ion chromatography using a Dionex AD 14 ion exchange column, sodium carbonate - sodium bicarbonate as eluent and suppressed conductivity detection) and contains an amount of stabilizer such as cyanuric acid not exceeding 1 ppm, preferably not exceeding 0.5 ppm, and a pH of 8 to 9, preferably a pH of 8.5 to 8.9, most preferably a pH of 8.8.

The stock solution may be a sodium or potassium based hypobromous acid solution, preferably a potassium based hypobromous acid solution.

The neutralized AMD solution may treated to provide 0.1 to 1 , preferably 0.5 to 0.9, preferably 0.6 to 0.8, most preferably 0.5 to 0.7 mg free active bromine per L water, after addition to the solution.

The biocide/s may be added to the neutralized AMD solution at a at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 1 to 10, preferably 2 to 8, most preferably 3 to 5 mg per L AMD solution.

Preferably, the neutralized AMD solution is treated with the biocide/s in combination with an anti-sealant.

The anti-sealant is an aqueous product for scale prevention in pipelines transporting water with a high solids content, for example a product containing organophosphonate and possibly a silica dispersant.

The anti-sealant should be alkaline, with pH = 7-11, preferably 8-11, more preferably 10-11 , most preferably 10.

The anti-sealant may include an organophosphonate (such as 1-hydroxyethane-1 , 1- diphosphonic acid (HEDP)) and/or sodium hexametaphosphate (SHMP), high alkaline acrylate or any silica dispersant/acrylate.

The organophosphonate may be dosed at 1x10^"6 to 1x10^"3 mg/L, preferably 1.5x10^"5 to 1.5X10^"4 mg/L, more preferably 3x10^"5 to 5x10^"5 mg/L.

In accordance with a preferred embodiment of the invention, the anti-sealant comprises an alkaline aqueous solution containing 10-20, typically 15 mg/L organophosphonate active ingredient (such as 1-hydroxyethane-1, 1-diphosphonic acid), preferably with an acrylate base and containing a silica dispersant. This solution may be dosed at 1 to 10, preferably 2 to 6, typically 3 to 5 mg/L

BRIEF DESCRIPTION OF THE DRAWING

FIGURE 1 is a schematic flow diagram of a process according to an embodiment of the invention;

FIGURES 2&3 are photographs showing scaling that occurs in pipes transporting treated AMD; and

FIGURES 4&5 are photographs showing the benefits of treating pipes transporting treated AMD that has been treated in accordance with a method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Problems associated with stored tailings can be attributed to microbiological activity, for example by chemolithotropic bacteria, such as Thiobacillus ferroxidans and Desulfovibrio vulgaris, during disposal and storage of mine wastes. Biological activity underground before and after mining operations is a major contributing factor in the dissolution of metals in underground water. Even during the treatment process, these organisms are part of either the removed sludge or of the treated water.

During the storage of the sludge the microbial activity slows down or may even enter into a period of inactivity. However, with the addition of food sources (carbonaceous material) during further sludge storage and/or the growing of vegetation on the mine dump these microbes are activated and during their life cycle the metals are brought back into the food chain and this results in the dissolution of the metals. These metals leach through the mine dump and re-enter the soil. This is a major cause of subterranean water pollution.

This invention relates to the treatment of mine tailings and other residues such as mine water from underground workings, with an effective amount of a biocide to kill chemolithotropic bacteria such as Thiobacillic feroxidans and Desulfovibrio vulgaris. The biocide may be chlorine dioxide, sulphur dioxide, potassium permanganate, tributyl tin oxide (TBTO), cooling tower biocides, hypobromous acid, ozone, and ultra violet light. The preferred biocide is bromine. The bromine may be from bromine tablets available from the Dead Sea Bromine Group, but is preferably from a stabilized stock liquid hyprobromous acid solution as described in PCT patent publication no. VVO 02/070404, the content of which is incorporated herein by reference.

The preferred stabilised stock hypobromous acid solution has a hypobromous acid concentration of less than 30% (m/m) typically less than 20% (m/m), preferably from 10% to 20% (m/m) (the concentration of hypobromous acid is determined by ion chromatography using a Dionex AD 14 ion exchange column, sodium carbonate - sodium bicarbonate as eluent and suppressed conductivity detection) and contains an amount of cyanuric acid as a stabiliser not exceeding 1 ppm, preferably not exceeding 0.5 ppm. Typically, the stock solution has a pH of 8 to 9, preferably a pH of 8.5 to 8.9, most preferably a pH of 8.8.

The hypobromous acid stock solution according to the invention is produced by combining a solution containing bromide ions with a solution containing hypochlorous acid. The solution containing bromide ions may be formed by dissolving a bromide source in water. The bromide source may be selected from sodium bromide, potassium bromide or lithium bromide, preferably potassium bromide. A bromide ion solution may formed by dissolving potassium bromide in water, to provide a 37% solution. The hypochlorous acid solution may be prepared from an alkali or alkaline earth metal hypochlorite selected from sodium hypochlorite, potassium hypochlorite, magnesium hypochlorite, lithium hypochlorite and calcium hypochlorite. Usually, such a solution has a pH of about 14. The pH of the solution is lowered to below 7.5, by adding hydrochloric acid, to provide a hypochlorous acid solution preferably with a pH of 7.41 , and 3.5%, by weight, available chlorine. The hypochlorous acid and bromide solutions are then combined in quantities to provide the required concentration of hypobromous acid. A formula for this reaction is set out below:

HOC* + NaBr → HOBr + NaCC

A stabiliser, in the form of cyanuric acid (dissolved in water which has been heated to 4O⁰C) is then added immediately to the hypobromous acid solution so formed. A small amount of the cyanuric acid, i.e. less than 1 ppm, preferably less than 0.5 ppm is added. Other stabilizers that may be used include:

Sulfamic acid, CAS 5329-14-6

Sodium Sulfamate, CAS 13845-18-6

Potassium Sulfamate, CAS 13823-50-2 benzenesulfonamide, CAS 98-10-2 urea CAS 57-13-6 ammonia CAS 7664-41-7 thiourea, CAS 62-56-6 creatinine CAS 60-27-5

2,4-lmidazolidinedione, CAS 461-72-3 (an alkyl hydantoin)

1-amino-2-hydroxyethane CAS 141-43-5 (monoethanolamine)

2,2'-dihydroxydiethylamine CAS 111-42-2 (diethanolamine) sulfanilamide CAS 63-74-1 imidodicarbonicdiamide CAS 108-19-0 (biuret)

1 ,3,5-triazine-2,4,6(1 H,3 H,5 H)triimine CAS 108-78-1 (melamine).

In accordance with an embodiment of the invention and with reference to Figure 1 , an existing mine dump 10 is reclaimed by removing tailings via a slurry pump station 12 and supplying the tailings to a gold recovery process 14. The gold recovery process 14 is typically a carbon in pulp (CIP) process, which yields gold 16 and a slurry waste 18 which is typically dumped at a mega dump site 20. The mega dump site 20 is under-laid with a bentonitic clay isolation layer 22 (500mm thick x 4% active clay). Water 24 sprayed on the dump 20 for dust suppression. Water for the dust spraying and for forming the slurry is obtained from treated mine water 26, typically neutralized AMD.

In accordance with the invention, the slurry is treated with a biocide 28.

The biocide is applied: in a Step A) to the slurry prior to the CIP process 14; in a Step B) to the slurry after the CIP process 14; and in a Step C) to the mega dump 20 in water 24 sprayed on the dump 20 for dust suppression.

The preferred biocide 28 is hypobromous acid having a hypobromous acid concentration from 0.5% to 5% (m/m), typically 0.5% to 2% (m/m). In accordance with an embodiment of the invention, the hypobromous acid is obtained from a stabilised stock liquid hypobromous acid solution which contains about 13% (m/m) hypobromous acid, an amount of cyanuric acid not exceeding 0.5 ppm, and having a pH of 8.8. This stock solution is diluted at a station 30, possibly using treated mine water 26, to provide a 10% solution, and a hypobromous acid concentration of 1.3% (m/m).

In Step A), the 1.3 % (m/m) stabilized liquid hypobromous acid solution is added to the slurry 12 at a dosage of 5kg per metric ton slurry. The slurry 18 is treated at this step with 0.5 to 1.5, typically 0.9 mg/L free bromine which results in 0.35 to 0.65 mg/L free bromine in the slurry.

In Step B), the 1.3 % (m/m) stabilized liquid hypobromous acid solution is added to the slurry 12 at a dosage of 7kg per metric ton slurry. The slurry 18 is treated at this step with 0.5 to 1.5, typically 1 mg/L free bromine which results in 0.9 to 1.2 mg/L free bromine in the slurry.

In Step C), the 1.3 % (m/m) stabilized liquid hypobromous acid solution is added to the water 24 at a dosage of 3kg per metric ton water. The water 24 is treated at this step with 0.05 to 0.5, typically 0.1 mg/L free bromine which results in 0.05 to 0.75 mg/L free bromine in the water.

Contrary to other biocides, bromine has no deleterious effects when it is present at low concentrations in potable water. Furthermore, the dosages as recommended in accordance with the present invention will have very little presence (if any) in waters directly in contact with a leachate. With reference to Example 3, tests that have been carried out indicate that sludge dosed with bromine does not have a negative impact on the absorption rate of gold in the CIP Process. In fact, it has been found that treatment with the biocide helped to keep the surface area of the activated carbon free of blinding (caused by bacteria) for a longer period: thereby increasing the gold/carbon ratio and thus presenting a better yield per m² of activated carbon surface area.

An added benefit of using hypobromous acid in Step C in the water sprayed on the mega dump 20, is that the hypobromous acid acts as a strong anti-sealant and assists with keeping the spray nozzles scale free. The present spraying of the mine dumps has shown that the blocking of nozzles presents a major problem. The blockage causes the spray to exit the nozzle as a small jet and not a mist. This causes water erosion which washes the sand away and the wetting is also limited to small areas with little or no dust suppression.

Slurry from a mining operation may be treated in a similar manner. The biocide is applied: in a Step A) to the slurry prior to concentration of the slurry in an evaporation dam; in a Step B) to the slurry after the concentration of the slurry in the evaporation dam; and in a Step C) to the dump where the slurry has been disposed in water sprayed on the dump for dust suppression.

In Step A), 1.3 % (m/m) stabilized liquid hypobromous acid solution is added to the slurry at a dosage of 5 kg per metric ton slurry. The slurry 18 is treated at this step with with 0.5 to 1.5, typically 0.9 mg/L free bromine which results in 0.35 to 0.65 free bromine in the slurry.

In Step B), 1.3 % (m/m) stabilized liquid hypobromous acid solution is added to the slurry at a dosage of 7kg per metric ton slurry. The slurry 18 is treated at this step with 0.5 to 1.5, typically 1 mg/L free bromine which results in 0.9 to 1.2 mg/L free bromine in the slurry.

In Step C), 1.3 % (m/m) stabilized liquid hypobromous acid solution is added to the water at a dosage of 3kg per metric ton water. Accordingly, the water 24 is treated at this step with 0.05 to 0.5, typically 0.1 mg/L free bromine which results in 0.05 to 0.75 mg/L free bromine in the water.

In the formation of AMD, bacteriological processes play a vital role. The bacteria: Thiobacillic Feroxidans and Desulfovibrio vulgaris have been noted and studied internationally and many of the metal dissolution in underground flooded areas have been ascribed to their metabolic functions. This microbial life also exists in the sludge formed during the treatment process. According to a further aspect of the invention, it has been determined that bacterial activity causes excessive amounts of biomass and this mass serves as good nuclei for the initial scale formation. It has been found that the removal of bacteria from AMD limits the amount of scale forming nuclei (biomass) and thus limits scaling in pipes and also extends the time for scale formation.

An in-line testing plant was set up at a gold mine to treat AMD that was neutralized by lime precipitation in the High Density Sludge (HDS) Process to a pH of 10-12, typically about 11.5. Mild steel test coupons were inserted at strategic places in the flow line and the process water was pumped at a fixed volume over the coupons. The water was treated with a combination of hypobromous acid and an anti-sealant.

There are many types of anti-sealants (also known as scale inhibitors) disclosed in the prior art. For example U.S. Pat. No. 4,563,284 (Amjad), issued Jan. 7, 1986, discloses an effective threshold amount of a phosphonocarboxylic acid and a telomeric phosphinocarboxylic acid that contains features of both phosphonates and polyacrylates. U.S. Pat. No. 4,762,621 (Masler, III et al.), issued Aug. 9, 1988, discloses a scale inhibitor comprising a copolymer of an acrylic acid and a lower alkyl ester of itaconic acid. U.S. Pat. No. 4,784,774 (Amjad et al.), issued Nov. 15, 1988, discloses a scale inhibitor containing a homopolymer of maleic acid or a copolymer of a monounsaturated monooarboxylic or dioarboxylic acid or salt thereof containing 3 to 5 carbon atoms and a phosphonoalkane carboxylic acid. U.S. Pat. No. 4,952,327 (Amjad et al.), issued Aug. 28, 1990, discloses a scale inhibitor obtained by adding to an aqueous medium 0.5 to 500 ppm of a copolymer containing at least one of each of the following three monomers: (a) monounsaturated carboxylic acids as well as salts and anhydrides thereof such as acrylic acid, methacrylic acid, or maleic acid; (b) acrylamidoalkane sulfonic acids and salts thereof, such as 2- acrylamido-2-methylpropane sulfonic acid (AMPS. RTM.), a registered trademark of the Lubrizol Corporation; and (c) styrene sulfonic acid and its salts. U.S. Pat. No. 4,652,377 (Amjad), issued Mar. 24, 1987, discloses a scale inhibitor comprised of a polyacrylic acid, phytic acid, and a phosphonocarboxylic acid containing at least one phosphono group, at least two carboxylic groups, and a hydrocarbon chain of at least two carbon atoms. United States Patent 4713195 discloses a threshold scale inhibitor for aqueous solutions containing calcium and magnesium scale forming ions comprises a mixture of three components: a maleic acid or anhydride homopolymer, an organophosphonate and a sulfonated styrene- maleic acid copolymer.

In accordance with the present invention, the anti-sealant should be alkaline, with pH = 7-11 , preferably 8-11 , more preferably 10-11 , most preferably 10. The anti-sealant may comprise a combination of organophosphonate (such as 1-hydroxyethane-1 , 1-diphosphonic acid (HEDP)) and/or sodium hexametaphosphate (SHMP), high alkaline acrylate or any silica dispersant/acrylate.

In accordance with a preferred embodiment of the invention, the anti-sealant comprises an alkaline aqueous solution (pH=10) containing 15 mg/L organophosphonate active ingredient (1-hydroxyethane-1 , 1-diphosphonic acid) with an acrylate base and containing a silica dispersant. The anti-sealant may be dosed at 1 to 10, preferably 2 to 6, typically 3 to 5 mg/L (i.e the organophosphonate is dosed at 1.5x10^"5 to 1.5X10^"4 mg/L, preferably 2x10^"5 to 6x10^"5 mg/L, typically 3x10^"5 to 5x10^"5 mg/L) together with a dosing of a 1.3 % (m/m) stabilized liquid hypobromous acid solution described above at a rate of 3 to 5 mg/L (which provides 0.5 to 0.7 mg/L active bromine in the solution). Results of tests conducted on neutralized AMD from/in pipes on a mine are illustrated in Figures 4 and 5, and Table 1 below. Table 1

(Stabilised hypobromous acid solution addition = 3.5 mg/L)

The anti-sealant and stabilized liquid hypobromous acid solution combination performed synergistically and produced excellent scale preventative results at a lower cost than a higher dosing rate of the anti-sealant alone and the results were far better.

Example 1

Initial test work done using a pilot dump (20 tons of typical mine tailings) and treating the new sludge additions with biocides yielded good results.

The following biocides were tested:

Chlorine dioxide

Sulphur dioxide

Potassium permanganate

Tributyl tin oxide

Bromine Tablets (Dead Sea Bromine Group)

Variety of water treatment (cooling tower) biocides

Stabilised Bromine Liquid (Detox)

Qzone

Ultra violet Applications

Most of the applications had an initial kill rate of higher than 90% but, surprisingly, only three maintained a sterile environment for longer than 2 weeks:

Tributyl tin oxide, Bromine Tablets (BCH) and Stabilised Bromine Solution.

When the sterilised sand was re-infected with bacteria, only the Bromine (tablets and liquids) treated sand stayed bacteria free. After 5 weeks with no additional treatment but with sterilised sludge addition no metal leaching had taken place in the sand that was sterilised using Stabilised Bromine.

Example 2 - Preparation of Stabilised Stock Potassium Based Hypobromous Acid Solution

132,5£ of a sodium hypochlorite solution having 15% available as chlorine, at a pH of 14.5 was mixed with 365.51 of water to provide a hypochlorous acid solution with a pH of 14.2. The pH of this solution is lowered to 7,5 by adding 14,6 g/l of hydrochloric acid (10%), to provide a hypochlorous acid stock solution having a free chlorine content of 3.5% by weight.

150 kg of potassium bromide was dissolved in 350£ of water to provide a 30%, by weight, potassium bromide stock solution having a pH of 6.9.

A 13% (m/m) potassium based hypobromous acid solution was prepared by mixing 113,234 of the hypochlorous acid stock solution described above with 214,011! of the sodium bromide stock solution, described above, (i.e. the potassium bromide and hypochlorous acid stock solution mentioned above are mixed at a ratio of 1.89 : 1 to form a solution containing 13% (m/m) hypobromous acid at a pH of 8.8 and 7% (m/m) potassium. 130,9mg of cyanuric acid (dissolved in water heated to 4O⁰C) is then added immediately to the solution to provide a concentration of cyanuric acid of 0.4 ppm.

Example 3

In the following Examples:

All tests were done at room temperature to emulate actual plant conditions Where applicable ASTM Test Methods were used toi determine values Freshly taken AMD and Mine Dump sand were used

The bacterial activity is expressed as Total Bacterial Count (TBC) TBC was done on a sample of demineralised water leachate. The 200 ml leachate sample was collected over 30 minutes.

Example 3A- Determination of Kill Rate

Table 2 below shows the results of tests conducted on AMD using the stabilized hypobromous acide solution of Example 2. In this example, AMD and Sand were mixed to a Density of 1.35 Table 2

Example 3B - Determination of Immunity

Table 3 shows the results of tests conducted on AMD which has already been treated using the stabilized hypobromous acid solution of Example 2, and which is then re-infected with bacteria. In this example:

AMD and Sand were mixed to a density of 1.47

Total bacterial count (TBC) was taken at start of tests

Stabilized hypobromous acid was added at the correct level as indicated and TBC taken

1 L of AMD was added to the test mixture

TBC taken at the indicated level

Table 3

Example 3C

Table 4 shows the results of tests conducted on impact of the addition of the stabilized hypobromous acid solution of Example 2 on the absorption rate of gold in the CIP Process. In this example:

AMD and Sand were mixed to a Density of 1.06. The sand was analysed for gold concentration.

Stabilized hypobromous acid solution was added to the correct concentration.

The pH of the mixture was adjusted to 11.5.

This mixture was passed through a 120 mm deep bed of activated carbon.

The gold concentration was measured after each run.

Table 4

Claims

CLAlMS

1. A method of treating aqueous mine tailings, aqueous mine residues, or aqueous residues from reclaimed mining materials, the method including the step of treating the tailings/residues with biocide/s to kill chemolithotropic bacteria.

2. The method as claimed in claim 1, wherein the chemolithotropic bacteria include Thiobacillic feroxidans and/or Desulfovibrio vulgaris.

3. The method as claimed in claim 1 or 2, wherein the biocide/s is/are chlorine dioxide, sulphur dioxide, potassium permanganate, tributyl tin oxide (TBTO), cooling tower biocides, hypobromous acid, ozone, and/or ultra violet light.

4. The method as claimed in claim 3, wherein the biocide is a halide.

5. The method as claimed in claim 4, wherein the halide is bromine.

6. The method as claimed in claim 5, wherein bromine is administered in the form of hypobromous acid.

7. The method as claimed in claim 6, wherein the hypobromous acid is from a stock liquid hyprobromous acid solution that has a hypobromous acid concentration of less than 30% (m/m) (the concentration of hypobromous acid is determined by ion chromatography using a Dionex AD 14 ion exchange column, sodium carbonate - sodium bicarbonate as eluent and suppressed conductivity detection), with a pH of 8 to 9.

8. The method as claimed in claim 7, wherein the stock liquid hypobromous acid solution is stabilized and contains an amount of stabilizer not exceeding 1 ppm.

9. The method as claimed in claim 8, wherein the stabilizer is cyanuric acid.

10. The method as claimed in any one of claims 6 to 9, wherein the hypobromous acid solution is sodium or potassium based.

11. The method as claimed in claim 10, wherein hypobromous acid solution is potassium based.

12. The method as claimed in any one of claims 5 to 11 , wherein the aqueous tailings/residues is dosed to provide 0.1 to 1.5 mg free active bromine per L in the slurry/residue prior to disposal.

13. The method as claimed in claim 12, wherein the aqueous tailings/residues is dosed to provide 0.5 to 1.5 mg free active bromine per L in the slurry/residue prior to disposal.

14. The method as claimed in claim 13, wherein the aqueous tailings/residues is dosed to provide 0.8 to 1.3 mg free active bromine per L in the slurry/residue prior to disposal.

15. The method as claimed in claim 14, wherein the aqueous tailings/residues is dosed to provide 0.9 to 1.2 mg free active bromine per L in the slurry/residue prior to disposal.

16. The method as claimed in any one of the preceding claims, wherein a slurry from reclaimed material from a mine dump is treated with biocide/s prior to and/or after a gold recovery process, prior to disposal of the slurry on a dump.

17. The method as claimed in claim 16, wherein the slurry is treated prior to and after a gold recovery process; prior to disposal of the slurry on a dump.

18. The method as claimed in claim 17, wherein biocide/s is/are applied: in a Step A) to the slurry prior to the gold recovery process; in a Step B) to the slurry after the gold recovery process; and optionally in a Step C) to the dump where the slurry has been disposed in water sprayed on the dump for dust suppression.

19. The method as claimed in claim 18, wherein, in Step A), a hypobromous acid solution is added to the slurry at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 1 to 10kg per metric ton slurry.

20. The method as claimed in claim 19, wherein, in Step A), the hypobromous acid solution is added to the slurry at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 3 to 6kg per metric ton slurry.

21. The method as claimed in claim 20, wherein, in Step A), the hypobromous acid solution is added to the slurry at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 5kg per metric ton slurry.

22. The method as claimed in any one of claims 18 to 21 , wherein, in Step B), a hypobromous acid solution is added to the slurry at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 1 to 10kg per metric ton slurry.

23. The method as claimed in claim 22, wherein, in Step B), the hypobromous acid solution is added to the slurry at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 1 to 10, preferably 5 to 8, most preferably 7kg per metric ton slurry.

24. The method as claimed in claim 23, wherein, in Step B), the hypobromous acid solution is added to the slurry at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 7kg per metric ton slurry.

25. The method as claimed in any one of claims 18 to 24, wherein, in Step C), a hypobromous acid solution is added to the slurry at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 1 to 10kg per metric ton slurry.

26. The method as claimed in claim 25, wherein, in Step C), the hypobromous acid solution is added to the slurry at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 2 to 4kg per metric ton slurry.

27. The method as claimed in claim 26, wherein, in Step C), the hypobromous acid solution is added to the slurry at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 3kg per metric ton slurry.

28. The method as claimed in any one of claims 1 to 15, wherein a slurry from a mining operation is treated with biocide/s prior to and/or after a slurry concentration step, prior to disposal on a dump.

29. The method as claimed in claim 28, wherein the slurry is treated prior to and after the slurry concentration step; prior to disposal of the slurry on a dump.

30. The method as claimed in claim 29, wherein the biocide/s is/are applied: in a Step A) to the slurry prior to a concentration step; in a Step B) to the slurry after the concentration step; and optionally in a Step C) to the dump where the slurry has been disposed in water sprayed on the dump for dust suppression .

31. The method as claimed in claim 30, wherein, in Step A), a hypobromous acid solution is added to the slurry at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 1 to 10 5kg per metric ton slurry.

32. The method as claimed in claim 31 , wherein, in Step A), the hypobromous acid solution is added to the slurry at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 3 to 6kg per metric ton slurry.

33. The method as claimed in claim 32, wherein, in Step A), the hypobromous acid solution is added to the slurry at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 5kg per metric ton slurry.

34. The method as claimed in any one of claims 30 to 33, wherein, in Step B), a hypobromous acid solution is added to the slurry at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 1 to 10kg per metric ton slurry.

35. The method as claimed in claim 34, wherein, in Step B), the hypobromous acid solution is added to the slurry at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 1 to 10, preferably 5 to 8, most preferably 7kg per metric ton slurry.

36. The method as claimed in claim 35, wherein, in Step B), the hypobromous acid solution is added to the slurry at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 7kg per metric ton slurry.

37. The method as claimed in any one of claims 30 to 36, wherein, in Step C), a hypobromous acid solution is added to the slurry at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 1 to 10kg per metric ton slurry.

38. The method as claimed in claim 37, wherein, in Step C), the hypobromous acid solution is added to the slurry at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 2 to 4kg per metric ton slurry.

39. The method as claimed in claim 38, wherein, in Step C), the hypobromous acid solution is added to the slurry at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 3kg per metric ton slurry.

40. The method as claimed in any one of claims 1 to 15, wherein water for use in dust suppression on a mine dump is treated with biocide/s prior to spraying the water onto the dump.

41. The method as claimed in claim 40, wherein a hypobromous acid solution is added to the slurry at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 1 to 10kg per metric ton slurry.

42. The method as claimed in claim 41 , wherein the hypobromous acid solution is added to the slurry at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 2 to 4kg per metric ton slurry.

43. The method as claimed in claim 42, wherein the hypobromous acid solution is added to the slurry at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 3kg per metric ton slurry.

44. A method of treating acid mine drainage (AMD) solution which has been neutralized, the method including the step of treating the solution with a biocide/s to kill chemolithotropic bacteria.

45. The method as claimed in claim 44, wherein the AMD has been neutralized in a High Density Sludge (HDS) Process.

46. The method as claimed in claim 44 or 45, wherein the chemolithotropic bacteria are Thiobacillic feroxidans and Desulfovibrio vulgaris.

47. The method as claimed in any one of claims 44 to 46, wherein the biocide/s is/are chlorine dioxide, sulphur dioxide, potassium permanganate, tributyl tin oxide (TBTO), cooling tower biocides, hypobromous acid, ozone, and/or ultra violet light.

48. The method as claimed in claim 47, wherein the biocide is a halide.

49. The method as claimed in claim 48, wherein the halide is bromine.

50. The method as claimed in claim 49, wherein bromine is administered in the form of hypobromous acid.

51. The method as claimed in claim 50, wherein the hypobromous acid is from a stock liquid hyprobromous acid solution that has a hypobromous acid concentration of less than 30% (m/m) (the concentration of hypobromous acid is determined by ion chromatography using a Dionex AD 14 ion exchange column, sodium carbonate - sodium bicarbonate as eluent and suppressed conductivity detection), with a pH of 8 to 9.

52. The method as claimed in claim 51, wherein the stock liquid hypobromous acid solution is stabilized and contains an amount of stabilizer not exceeding 1 ppm.

53. The method as claimed in claim 52, wherein the stabilizer is cyanuric acid.

54. The method as claimed in any one of claims 50 to 53, wherein the hypobromous acid solution is sodium or potassium based.

55. The method as claimed in claim 54, wherein hypobromous acid solution is potassium based.

56. The method as claimed in any one of claims 49 to 55, wherein the neutralized AMD solution is treated to provide 0.1 to 1 mg free active bromine per L AMD solution.

57. The method as claimed in claim 12, wherein the neutralized AMD solution is treated to provide 0.5 to 0.9 mg free active bromine per L AMD solution.

58. The method as claimed in claim 13, wherein the neutralized AMD solution treated to provide 0.6 to 0.8 mg free active bromine per L AMD solution.

59. The method as claimed in claim 14, wherein the neutralized AMD solution is treated to provide 0.5 to 0.7 mg free active bromine per L AMD solution.

60. The method as claimed in any one of claims 44 to 50, wherein a hypobromous acid solution is added to the neutralized AMD solution slurry at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 1 to 10mg per L AMD solution.

61. The method as claimed in claim 60, wherein the hypobromous acid solution is added to the neutralized AMD solution at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 2 to 8mg per L AMD solution.

62. The method as claimed in claim 61 , wherein the hypobromous acid solution is added to the neutralized AMD solution at a dosage of, or equivalent to, a 1.3 % (m/m) hypobromous acid solution at 3 to 5mg per L AMD solution.

63. The method as claimed in any one of claims 44 to 62, wherein the neutralized AMD solution is treated with a biocide in combination with an anti-sealant solution.

64. The method as claimed in claim 63, wherein the anti-sealant solution is alkaline, with pH = 7-11.

65. The method as claimed in claim 64, wherein the pH of the anti-sealant solution = 8- 11.

66. The method as claimed in claim 65, wherein the pH of the anti-sealant solution = 10- 11.

67. The method as claimed in claim 66, wherein the pH of the anti-sealant solution = 10.

68. The method as claimed in any one of claims 63 to 67, wherein the anti-sealant solution includes an organophosphonate and/or sodium hexametaphosphate (SHMP), high alkaline acrylate or any silica dispersant/acrylate.

69. The method as claimed in claim 68, wherein the anti-sealant solution includes an organophosphonate.

70. The method as claimed in claim 69, wherein the organophosphonate is 1- hydroxyethane-1 , 1-diphosphonic acid (HEDP).

71. The method as claimed in claim 69 or 70, wherein the organophosphonate is dosed at 1x10^"6 to 1x10^"3 mg/L

72. The method as claimed in claim 71 , wherein the organophosphonate is dosed at, preferably 1.5x10^"5 to 1.5x10^"4 mg/L.

73. The method as claimed in claim 72, wherein the organophosphonante is dosed at 3x10-⁵ to 5x10-⁵ mg/L.

74. The method as claimed in claim 69, wherein the anti-sealant solution contains 10-20 mg/L organophosphonate active ingredient.

75. The method as claimed in claim 74, wherein the anti-sealant solution contains 15 mg/L organophosphonate active ingredient.

76. The method as claimed in claim 74 or 75, wherein the organophosphonate is is 1- hydroxyethane-1 , 1-diphosphonic acid.

77. The method as claimed in any one of claims 74 to 76, wherein the anti-sealant solution includes an acrylate base.

78. The method as claimed in any one of claims 74 to 77, wherein the anti-sealant solution includes a silica dispersant.

79. The method as claimed in any one of claims 74 to 78, wherein the anti-sealant solution is dosed at 1 to 10 mg/L.

80. The method as claimed in claim 79, wherein the anti-sealant solution is dosed at 2 to 6 mg/L.

81. The method as claimed in claim 48, wherein the anti-sealant solution is dosed at 3 to 5 mg/L.