AU2017371380B2

AU2017371380B2 - Improved processes for processing of acid mine drainage water

Info

Publication number: AU2017371380B2
Application number: AU2017371380A
Authority: AU
Inventors: Samuel Patrick Costin
Original assignee: Global Aquatica Pty Ltd
Current assignee: Global Aquatica Pty Ltd
Priority date: 2016-12-06
Filing date: 2017-12-05
Publication date: 2023-03-02
Anticipated expiration: 2037-12-05
Also published as: AU2017371380A1; WO2018102850A1

Abstract

A system and method for processing acid mine drainage (AMD) water is disclosed. The system comprises a plurality of cavitation units connected in series with each cavitation unit comprising a water inlet and a water outlet and at least one venturi oxidiser between each water inlet and water outlet and wherein at least a first upstream cavitation unit in the series of cavitation units comprises a feed solution inlet configured to mix a carbonate and/or hydroxide metal salt feed solution with the AMD water in said cavitation unit.

Description

IMPROVED PROCESSES FOR PROCESSING OF ACID MINE DRAINAGE

WATER

PRIORITY DOCUMENT

[ 0001 J The present application claims priority from Australian Provisional Patent Application No. 2016905021 titled "IMPROVED PROCESSES FOR PROCESSING OF ACID MINE DRAINAGE WATER" and filed on 6 December 2016, the content of which is hereby incorporated by reference in its entirety.

INCORPORATION BY REFERENCE

[0002] The following publication is referred to in the present application and its contents are hereby incorporated by reference in their entirety:

[0003] International Patent Application No. PCT/AU2015/000538 (WO 2016/033637) titled

"PROCESSING OF ACID MINE DRAINAGE WATER" in the name of Global Aquatica Pty Ltd.

FIELD

[ 0004 ] The present disclosure relates to processing of acid mine drainage (AMD) water and, more specifically, to a system for and method of processing AMD water.

BACKGROUND

[0005] During wet weather, excess mine site water usually consists of saline groundwater from mining operations and stormwater from rainfall. Conventionally, mine operators run this water into common catchment ponds from which it is either processed and recycled around the mine, or evaporated in ponds. Due to the poor quality of this waste water, Environmental Protection Authorities do not permit release into the catchments except in times of flood in the catchment.

[ 0006] During times of drought, it is normal for groundwater and stormwater flow to reduce significantly and supplying water for mining operations becomes expensive, as water availability to mine reduces significantly if the mine's water source also supplies domestic users. Also, during drought, water quality reduces, which increases the cost of water treatment. [0007] The cause of most mine water degradation is the oxidation of iron sulfide minerals, such as pyrite (FeS₂). Acid is produced by the oxidation of the sulfide to sulphate (reaction A), and by the oxidation and hydrolysis of iron (reaction B):

FeS₂ + 3.50₂ + H₂0 fi Fe² ' + 2S0₄ ^2" + 2tf (A)

Fe^{2 t} + 2.5H₂0 + 0.25O₂ fi Fe(OH)₃(s) + 2H⁺ (B)

[0008] Iron-oxidising bacteria accelerate pyrite oxidation by two mechanisms: direct oxidation, and oxidising Fe² to Fe^3-, which in turn oxidises the sulfide minerals. Direct oxidation is probably most important during initial acidification, when complete hydrolysis of Fe ^t and the resultant precipitation of Fe(OH)₃ are too rapid to allow ferric iron to act as an important oxidant.

10009] As the pH decreases, abiotic oxidation of Fe^{2 ,} slows down dramatically, according to the rate law (reaction C):

-d (Fe^2÷) = k (0₂ (aq)) (Fe² ) (C) dt (H ¹)² where (Fe^2'), (0₂ (aq)), and (FT) are activities, k is the rate constant, and t is time. Below

approximately pH 4, the iron-oxidising bacteria assume the primary role of oxidising Fe²\ thereby allowing reaction B to continue producing acid and ferric hydroxide. Although the reaction stoichiometry remains the same, this is a transition point from the primarily abiotic stage to the partially biological stage. The pH decline typically continues to a stage where the reaction chemistry changes to a biologically-mediated cycle of reactions D and E:

Fe^2÷ + 0.25O₂ + FT fi 0.5Fe³" + 0.5H₂O (D)

FeS₂ + 14Fe³⁺ + 8H₂0 fi 15Fe² + 2S0₄ ^2" + 16H⁺ (E)

[0010] As acidification proceeds and the pH in the immediate vicinity of the pyrite falls to less than 3, the increased solubil ity of iron, and the decreased rate of Fe(OH)₃ precipitation results in increased Fe^{3 t} activity. This is significant because as Fe³ aggressively attacks pyrite, it is reduced to Fe^{2 '} (reaction E) for subsequent reoxidation by iron oxidising bacteria. Oxidation of pyrite by Fe³ ' is about an order of magnitude faster than oxidation by equivalent concentrations of dissolved oxygen, apparently because of different reaction mechanisms at the molecular level. When the pH in the immediate microenvironment of the pyrite falls to approximately 2.5 (often corresponding to a drainage pH of 3.5 to 4.0), bacterial oxidation of Fe²⁺ and reduction of Fe'⁺ by the pyrite (reactions D and E) combine to cause a dramatic increase in acidity and iron concentrations.

[001 1 ] As this solution moves through mine workings or spoils, it undergoes secondary reactions that raise pH, decrease concentrations of iron, and increase the concentrations of other cations. Contact with clays and other aluminosilicates releases aluminium, sodium, potassium, and magnesium, while contact with carbonate minerals releases calcium, magnesium, manganese, and additional iron (siderite). The various effects these reactions have on the chemistiy of the mine drainage water depend on the volume of water , the amount of pyrite oxidised, and the extent and variety of secondary chemical reactions. High sulphate levels also frequently occur, caused by some secondary reactions. The mine drainage is typically acidic; containing high concentrations of dissolved iron, aluminium, and manganese and other metals, namely zinc, nickel, and cobalt.

[0012] As the contaminated mine drainage flows through receiving systems, its toxic characteristics decrease naturally as a result of chemical and biological reactions, and dilution with uncontaminated waters. However, evaporation reverses this effect to a significant extent. Under the aerobic conditions found in most surface waters, iron, aluminium, and manganese precipitate as oxides and hydroxides. A higher proportion of ferric iron indicates it is oxidising from ferrous iron, which hydrolyses and precipitates mainly as iron oxyhydroxides (e.g., FeOOH) or oxyhydroxylsulphates of various compositions and crystallinity. These compounds stain the bottom of many streams orange, often accumulating at sufficient depths to suffocate benthic organisms. The rate of iron precipitation at low pH depends on the activity of the same iron-oxidising bacteria that catalyse pyrite oxidation; the abiotic rate increases a hundredfold for every unit increase in pH, and is also dependent on the amount of oxygen dissolved in the water (See reaction C).

[0013] Aluminium generally hydrolyses and precipitates as Al(OH)₃, which is a white particulate. Other aluminium compounds with silica and sulphate can also form, depending on the environmental conditions. Oxidation is not required, and apparently bacterial activity is not a factor. Precipitation of aluminium requires a pH above 4, and is generally observed at a pH of 4.5 or above. Aluminium solids will become soluble, as Al(OH)₄ ^", at pH levels over 8.5.

[0014] This can occur in conventional chemical treatment systems that must increase pH to these higher levels to remove manganese.

[0015 ] Manganese oxidises and hydrolyses to MnOOH or Mn0₂, and precipitates as a black particulate. Ubiquitous manganese-oxidising bacteria can influence the rate of removal, since like iron, oxidation generally precedes precipitation. More important however, is that significant oxidation and precipitation of manganese requires a pH greater than 6, and generally only occurs in passive systems after virtually all of the iron has already precipitated. As a result, manganese removal significantly increases the treatment cost if conventional treatment methods are employed. Manganese precipitation is auto-catalytic; once precipitates form, their presence increases the rate of manganese removal. In conventional chemical treatment systems, the pH is often raised above 9 or 10 to remove manganese to desired levels.

[0016] Acidity can exist as organic acidity associated with dissolved organic compounds, carbon dioxide acidity associated with dissolved carbon dioxide and carbonic acid; proton acidity associated with pH (a measure of free H ¹ ions); and mineral acidity associated with dissolved metals. Mine waters generally have very little dissolved organic carbon, so organic acidity is very low. The amount of dissolved carbon dioxide in the Bowen basin mine drainage is usually very low and usually only contributes significantly to acidity at pH levels > 5. In addition, carbon dioxide acidity can be thought of as temporary, because C0₂-rich waters will degas upon exposure to the atmosphere. The majority of acidity in coal mine drainage usually arises from free protons (manifested in low pH) and the mineral acidity arising from dissolved iron, aluminium, and manganese. These metals are considered acidic because they can undergo hydrolysis reactions that produce FT .

Fe^{2 f} + 0.25O₂ + 1.5H₂0 fi FeOOH + 211 Fe³ + 2H₂0 fi FeOOH + 3 FT Al³ ' + 3H₂0 fi Al(OH)₃ + 3FT Mn²⁺ + 0.25O₂ + 1.5H₂0 fi MnOOH + 2H

[0017] These reactions can be used to calcul ate an estimate of the total acidity of a mine water sample, and to partition the acidity into its various components. The expected acidity of a mine water sample is calculated from its pH and the sum of the milliequivalents of the dissolved acidic metals. For most coal mine drainages, the acidity is calculated as follows,

Acid_cai_c = 50(2Fe²⁺/56 + 3Fe³⁺/56 + 3A1/27 + 2Mn/55 + 1000(10^_p")) where all metal concentrations are in mg/L, and 50 is the equivalent weight of CaCO^, and thus transforming mg/L of acidity into mg/L as CaC0₃ equivalent. Simplifying the equation shows the conversion factors to be applied to each dissolved metal and hydrogen ion concentration (pH):

Acid_cafc = 1.79Fe^2J + 2.68Fe^{3 '} + 5.56A1 + 1.82Mn + 50,000 ( ΚΓ^ρΠ) [0018] Conventional treatment methods for AMD water involve increasing the pH of the water using Ca(OH)₂ to cause metal salts to precipitate out of solution, chemical flocculation of the precipitated metal salts, coagulation and settling, removal of the metal salts by reverse osmosis and removal of suspended solids by filtration. Many other processes need to be installed in addition to the above to protect reverse osmosis membranes used in filtration.

[0019] A system and method for processing AMD water is disclosed in published international patent application WO 2016/033637. The system comprises an aeration station for aerating AMD water and a sulphate reduction station for reducing sulphates present in the oxidised water. However, in practice the metal ion content of AM D water treatment with this system may still be higher than required.

10020] There is a need for new or improved systems and processes for processing AMD water.

SUMMARY

[0021 ] In a first aspect, the present disclosure provides a system for processing acid mine drainage (AMD) water to reduce the content of metal ions in said water, said system comprising a plurality of cavitation units connected in series with each cavitation unit comprising a water inlet and a water outlet and at least one venturi oxidiser between each water inlet and water outlet and wherein at least a first upstream cavitation unit in the series of cavitation units comprises a feed solution inlet configured to mix a carbonate and/or hydroxide metal salt feed solution with the AMD water in said cavitation unit.

10022] The carbonate and/or hydroxide metal salt feed solution can comprise any metal salt that provides an oxidising and/or buffering effect.

[0023] In certain embodiments, the carbonate and/or hydroxide metal salt feed solution comprises an alkali and/or alkaline earth metal salt. The alkali metal may be lithium, sodium and/or potassium. The alkaline earth metal may be magnesium and/or calcium. In specific embodiments, the carbonate and/or hydroxide metal salt feed solution comprises calcium hydroxide. In other specific embodiments, the carbonate and/or hydroxide metal salt feed solution comprises potassium hydroxide. In other specific embodiments, the carbonate and/or hydroxide metal salt feed solution comprises magnesium carbonate. In other specific embodiments, the carbonate and/or hydroxide metal salt feed solution comprises calcium carbonate. In other specific embodiments, the carbonate and/or hydroxide metal salt feed solution comprises potassium carbonate. In other specific embodiments, the carbonate and/or hydroxide metal salt feed solution comprises sodium carbonate. In other specific embodiments, the carbonate and/or hydroxide metal salt feed solution comprises sodium hydroxide. In other specific embodiments, the carbonate and/or hydroxide metal salt feed solution comprises magnesium hydroxide.

[0024] In certain embodiments, the system further comprises a sodium hydroxide and/or magnesium hydroxide feed solution production unit for forming a sodium hydroxide and/or magnesium hydroxide feed solution from AMD water, the sodium hydroxide and/or magnesium hydroxide feed solution production unit comprising: a sulphate reduction station for reducing sulphates present in the AMD water to produce sulphate reduced water, the sulphate reduction station comprising a first bioreactor containing sulphate reducing bacteria; a calcium carbonate precipitation station for precipitating calcium carbonate from the sulphate reduced water and for separating precipitated calcium carbonate therefrom to produce a calcium depleted solution; a sodium carbonate and/or magnesium carbonate conversion station for converting sodium carbonate and/or magnesium carbonate in the calcium depleted solution into sodium hydroxide and magnesium hydroxide respectively.

[0025] In some embodiments, the sodium carbonate and/or magnesium carbonate conversion station comprises a cavitation unit having at least one venturi oxidiser and configured so that calcium depleted solution passing through the cavitation unit is converted into sodium hydroxide and/or magnesium hydroxide feed solution. In other embodiments, the sodium carbonate and/or magnesium carbonate conversion station may be configured to convert sodium carbonate and/or magnesium carbonate in the calcium depleted solution into sodium hydroxide and magnesium hydroxide by degrading the carbonate to oxide using heat, thereby forming hydroxide when mixed with water.

[0026] Also disclosed herein is a method for processing acid mine drainage (AMD) water to reduce the content of metal ions in said water, the method comprising: passing AMD water to be treated and a carbonate and/or hydroxide metal salt feed solution through a first cavitation unit comprising at least one venturi oxidiser under conditions to oxidise the solution and reduce the content of metal ions in the water to produce a first reduced metal ion content solution; passing the first reduced metal ion content solution and, optionally a carbonate and/or hydroxide metal salt feed solution through a second cavitation unit comprising at least one venturi oxidiser under conditions to oxidise the solution and reduce the content of metal ions in the water to produce a second reduced metal ion content solution; and, optionally, passing the second reduced metal ion content solution and, optionally, a carbonate and/or hydroxide metal salt feed solution through at least one further cavitation unit under conditions to oxidise the solution and reduce the content of metal ions in the water to produce treated water.

[0027] The carbonate and/or hydroxide metal salt feed solution can comprise any metal salt that provides an oxidising and/or buffering effect.

[0028] In certain embodiments, the carbonate and/or hydroxide metal salt feed solution comprises an alkali and/or alkaline earth metal salt. The alkali metal may be lithium, sodium and/or potassium. The alkaline earth metal may be magnesium and/or calcium. In specific embodiments, the carbonate and/or hydroxide metal salt feed solution comprises calcium hydroxide. In other specific embodiments, the carbonate and/or hydroxide metal salt feed solution comprises potassium hydroxide. In other specific embodiments, the carbonate and/or hydroxide metal salt feed solution comprises potassium hydroxide.

[0029] In other specific embodiments, the carbonate and/or hydroxide metal salt feed solution comprises magnesium carbonate. In other specific embodiments, the carbonate and/or hydroxide metal salt feed solution comprises calcium carbonate. In other specific embodiments, the carbonate and/or hydroxide metal salt feed solution comprises potassium carbonate. In other specific embodiments, the carbonate and/or hydroxide metal salt feed solution comprises sodium carbonate. In other specific embodiments, the carbonate and/or hydroxide metal salt feed solution comprises sodium hydroxide. In other specific embodiments, the carbonate and/or hydroxide metal salt feed solution comprises magnesium hydroxide.

[0030] In certain embodiments, the method further comprises a step of producing a sodium hydroxide and/or magnesium hydroxide feed solution from AMD water, the method comprising: reducing sulphates present in the AMD water sulphate reducing bacteria to produce sulphate reduced water; precipitating calcium carbonate from the sulphate reduced water; separating precipitated calcium carbonate from the sulphate reduced water to produce a calcium depleted solution; converting sodium carbonate and/or magnesium carbonate in the calcium depleted solution into sodium hydroxide and magnesium hydroxide respectively.

[003 1 ] In certain embodiments, the sodium carbonate and/or magnesium carbonate in the calcium depleted solution is converted into sodium hydroxide and magnesium hydroxide respectively by passing the calcium depleted solution through a cavitation unit having at least one venturi oxidiser and configured so that calcium depleted solution passing through the cavitation unit is converted into sodium hydroxide and/or magnesium hydroxide feed solution. In other embodiments, the sodium carbonate and/or magnesium carbonate in the calcium depleted solution is converted into sodium hydroxide and magnesium hydroxide by degrading the carbonate to oxide using heat, thereby forming hydroxide when mixed with water.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Embodiments of the presently disclosed systems and methods will now be described, by way of example only, with reference to the following drawings, in which:

[0033 ] Figure 1 is a schematic view of an embodiment of a system for processing acid mine drainage water according to the present disclosure;

[0034] Figure 2 is a schematic view of an embodiment of a sodium hydroxide and/or magnesium hydroxide feed solution production unit according to the present disclosure;

[0035] Figure 3 is a schematic view of an embodiment of a sodium hydroxide and/or magnesium hydroxide feed solution production unit according to the present disclosure;

[0036] Figure 4 is a top view of a venturi oxidiser;

[0037] Figure 5 is a cross-sectional view of the venturi oxidiser taken along line 5-5 of Figure 4;

[0038] Figure 6 is a cross-sectional view of the draught tube of the venturi oxidiser of Figure 4 taken along line 6-6 of Figure 5;

[0039] Figure 7 is a schematic view of an embodiment of a sulphate reduction station;

[0040] Figure 8 is a side view of an embodiment of a tank for use in the sulphate reduction station shown in Figure 7; and

[0041 ] Figure 9 is a side view (left) and an end view (right) of a series of connected tanks for use in the sulphate reduction station shown in Figure 7.

DESCRIPTION OF EMBODIMENTS

10042] The present disclosure arises from the inventor's further development of the system for processing acid mine drainage (AMD) water that is disclosed in published international patent application WO 2016/033637. Specifically, the inventor has developed an improved system and method for processing AMD water that provides more effective removal of metal salts. Whilst the present system and method have been developed primarily for processing AMD water, and will be described hereinafter with reference to this application, it will be appreciated that they are not limited to this application and may also be used for treatment of aqueous industrial waste products from manufacturing processes, water containing sulphates and metals for example.

[0043] Shown in Figure 1 is a system 10 for processing AMD water 12 to reduce the content of metal ions in the AMD water. The system 10 comprises a plurality of cavitation units 14a, 14b, 14c, 14d connected in series with each cavitation unit comprising a water inlet 16a, 16b, 16c, 16d and a water outlet 18a, 18b, 18c, 18d and at least one venturi oxidiser 20 (see Figures 4 to 6) between each water inlet 16 and water outlet 18. At least a first upstream cavitation unit 14a in the series of cavitation units comprises a feed solution inlet 22 configured to mix a carbonate and/or hydroxide metal salt feed solution 24 with the AMD water 12. Treated water 26 having a reduced metal ion content is produced by the system 10.

[0044] Treated water 26 having a reduced metal ion content produced by system 10 may be further treated using any treatment techniques known in the art. For example, treated water 26 may be further treated via a bacterial sulphate reduction station such as the one described in WO 2016/033637.

[0045] Each cavitation unit 14a, 14b, 14c, and 14d comprises a venturi oxidiser 20 which draws atmospheric pressure air into a nozzle at a high velocity and generates cavitation within the air stream to disassociate the water and form hydroxyl and hydrogen radicals. The hydroxyl radicals are strongly oxidising:

H₂0 fi H^* + 'OH

[0046] Chlorides in the water dissociate to form a calcium cation and a chlorine ion:

CaCl fi Ca + Cl^"

[0047] The chlorine ion may combine with the hydroxyl radical:

CI^" + ^*OH fi [CIOH]- Reaction rate = 4.3 x 10⁹ M ¹ s ¹

[0048] Chlorine has a strong affinity to oxygen, resulting in any one of six oxide states. When the chloride dissociates, and is then mixed with oxygen, the unstable chlorine hydroxide may form a stable chlorine oxide state:

[CIOH]- fi cr + 'OH cr + cr fi ci₂ ^"

2C1₂ ^~ + 0₂ fi 2d- + 0₂

2d- + 0₂ fi 2C10₂

10049] AMD water contains significantly more sulphates than chlorides. The sulphate radical resulting from the dissociation is known to have a reaction time several orders of magnitude slower than chloride to hydroxide:

SO₄ - + OH^" fi S0₄ ^2" + OH Reaction rate = 1.4 x 10⁷ M ^_I s ^_I

[0050] The hydroxide molecules that are temporarily bonded to the chlorine may then dissociate and bond to metals such as iron and aluminium and/or combine with the oxygen from the air stream to form metal oxides. Metals in AMD water exist in the form of sulphates. Therefore the metals may bond with the oxygen to form oxides and the cations previously bound to chloride then bond to sulphate as the chlorides have already been consumed as chlorine oxides. The hydroxides may then reform as water. In this way, passage of the AMD water through successive cavitation units 14a, 14b, 14c, and 14d results in successive formation of metal oxides and is associated with an increase in pH at each unit. At least some of the metal oxides formed are removed from the water at each cavitation unit 14a, 14b, 14c, and 14d. In practice, it is found that the metal ion content of the water can be reduced by about 4g/L at each unit 14a, 14b, 14c, and 14d. It is postulated that the precipitating metals coagulate with suspended solids such as silica as they fall in the cavitation unit 14. As a result, the dissolved and suspended solids such as silica, which is not soluble in water, are both removed by this process. The silica is commonly present in acid mine drainage water as a suspended solid from the breakdown of aluminosilicates (clay) by acid. It is these silicates that destroy reverse osmosis membranes used in prior art systems.

[0051 ] The number of cavitation units 14 in the system 10 can be varied for different starting waste waters. However, based on the discussion above it is clear that one or more cavitation units 14 are required to reduce the metal ion content of the water and the more cavitation units 14 in the system the more metal ions will be removed from the water. In practice, the inventor has found that four cavitation units 14a, 14b, 14c, and 14d is a suitable number for treating AMD water containing 20 g/L of metals. However, water with higher metal content may require more cavitation units to be connected in series, such as 5, 6, 7, 8, 9 or 10 units. Notably, on the same site a lime dosing plant of the type known in the art can only remove 4.8 g/L in total. [0052] A suitable venturi oxidiser 20 for use in the cavitation units 14a, 14b, 14c, and 14d is shown in Figures 4 to 6. The venturi oxidiser 20 comprises an annular vessel 82 for receiving pressurised water through two diametrically opposed inlets 84. A draught tube 86 of circular cross-section extends through the vessel 82, such that the draught tube 86 defines an inner wall of the annular vessel 82. Both ends of the draught tube 86 are open, with one of the ends defining an inlet port 88 and the other end defining an outlet port 90. A plurality of venturi inlets 92 are provided in the draught tube 86 inside the vessel 82. The venturi inlets 92 are perpendicular to the draught tube 86. In the illustrated embodiment, the venturi inlets 92 are aligned. However, the venturi inlets 92 may be offset to reduce coalescing of the aerosolised water as it passes through the draught tube 86. Pressurised water from the vessel 82 enters the draught tube 86 through the venturi inlets 92. The venturi inlets 92 have a length L configured to cause water entering the draught tube 86 to dissociate within the tube.

[0053] The applicant has found that the length L of the venturi inlets 92 is an important factor in disassociating the water into a water vapor gas along with hydroxyl radicals and hydrogen before the vaporised and dissociated water combines with the oxygen from the air. These radicals are known to be extremely strong oxidants. Moreover, the resultant water vapor gas advantageously reacts very quickly with oxygen.

[0054] The feed solution inlet 22 is in the form of an injection point 96 in inlet port 88 through which the carbonate and/or hydroxide metal salt feed solution 24 and/or any additional buffer or other chemicals is injected for mixing with the AMD water 12 from the venturi inlets 92.

[0055] The ports of the venturi oxidiser 20 can be aligned to produce a direction of spray producing a spiral effect. This rotation induced by the angular velocity of the spray increases the volume of air drawn into the draught tube, and increases the reaction time thereby increasing the oxidising capacity of the cavitation units 14a, 14b, 14c, and 14d. The oxidised, atomized water/high velocity air mix is blown out of the cavitation unit 14a into cavitation unit 14b, and so on. Further oxidation of the water occurs during its flight through the air. The oxidation of the water increases its pH, which results in the precipitation of significant amounts of metal salts at a pH as low as 5. For example, iron enters the cavitation zone as ferrous sulphate and exits as ferrous hydroxide in the form of a gas (water vapour). Immediately after cavitation, this gas is combined with oxygen from the air in the draught tube 86 and continues to react as it falls into a tank. The carbonate and/or hydroxide metal salt solution 24, such as sodium hydroxide, may be added to the water/air mix or directly added to the water in the tank.

Preferably the carbonate and/or hydroxide metal salt is added when the water is in the fonn of a gas to increase the reaction rate. It has been found in practise that the addition of an oxidising and/or buffering carbonate and/or hydroxide metal salt causes the ferrous hydroxide to precipitate from the water to form a sludge. No metals will precipitate as settleable suspended solids if no metal salt is added. Analysis of the water after cavitation and prior to the addition of carbonate and/or hydroxide metal salt (e.g. NaOH) shows reduced dissolved iron in the water. A lesser added concentration of carbonate and/or hydroxide metal salt (e.g. NaOH) will cause the iron to precipitate as ferric hydroxide and a greater concentration of carbonate and/or hydroxide metal salt (e.g. NaOH) will cause the iron to precipitate as ferrous hydroxide. The resulting water in the tank has less than 0.4 mg/L of dissolved oxygen. Retention of the water within the tank for a given duration will cause further reduction of oxygen in the water to less than 0.1 mg/L as the iron changes from ferrous to ferric state. This anoxic state is critical to the efficient functioning of the anaerobic sulphate reducing bacteria in following steps.

10056] Thus, in use, pressurised water is injected into the annular vessel 82 through inlets 84 and is disassociated and oxidised as it passes through the venturi inlets 92 and into the draught tube 86, where it mixes w^rith air drawn into the draught tube 86 through inlet port 88. The carbonate and/or hydroxide metal salt feed solution 24 may be added to the air in the draught tube or directly into the water standing in the tank.

[0057] It will be appreciated that the presently disclosed system 10 achieves considerable cost savings and environmental benefits due to not requiring imported chemicals to remove metal salts from AMD water. Furthermore, the high quality of treated water produced by the system 10 is useable to replace purchased water sources for mineral processing and dust suppression. Depending on the chemistry of the raw water, it may also be used for commercial food crop watering, livestock watering and, with disinfection, as drinking water.

10058] The carbonate and/or hydroxide metal salt feed solution 24 can be any metal salt that provides an oxidising and/or buffering effect. In certain embodiments, the carbonate and/or hydroxide metal salt comprises an alkali and/or alkaline earth metal salt. The alkali metal may be lithium, sodium and/or potassium. The alkaline earth metal may be magnesium and/or calcium. In specific embodiments, the carbonate and or hydroxide metal salt feed solution comprises calcium hydroxide. In other specific embodiments, the carbonate and or hydroxide metal salt feed solution comprises potassium hydroxide. In other specific embodiments, the carbonate and/or hydroxide metal salt feed solution comprises magnesium carbonate. In other specific embodiments, the carbonate and or hydroxide metal salt feed solution comprises calcium carbonate. In other specific embodiments, the carbonate and/or hydroxide metal salt feed solution comprises potassium carbonate. In other specific embodiments, the carbonate and/or hydroxide metal salt feed solution comprises sodium carbonate. In other specific embodiments, the carbonate and/or hydroxide metal salt feed solution comprises sodium hydroxide. In other specific embodiments, the carbonate and/or hydroxide metal salt feed solution comprises magnesium hydroxide. [0059] The carbonate and/or hydroxide metal salt feed solution 24 can be obtained from any suitable source, including from commercial sources of alkali and/or alkaline earth metal salts such as sodium hydroxide and magnesium hydroxide. However, obtaining alkali and/or alkaline earth metal salts from commercial sources does add significant cost to the process and, advantageously, in certain embodiments of the present disclosure shown in Figure 2 the system 10 further comprises a sodium hydroxide and/or magnesium hydroxide feed solution production unit 50 for forming a sodium hydroxide and/or magnesium hydroxide feed solution 24 from AMD water 12. The sodium hydroxide and/or magnesium hydroxide feed solution production unit 50 comprises a sulphate reduction station 52 for reducing sulphates present in the AMD water 12 to produce sulphate reduced water 54. The sulphate reduction station comprises a bioreactor containing sulphate reducing bacteria. The sodium hydroxide and/or magnesium hydroxide feed solution production unit 50 also comprises a calcium carbonate precipitation station 56 for precipitating calcium carbonate from the sulphate reduced water 54 and for separating precipitated calcium carbonate 58 therefrom to produce a calcium depleted solution 60. The sodium hydroxide and/or magnesium hydroxide feed solution production unit 50 also comprises a sodium carbonate and/or magnesium carbonate conversion station 62 for converting sodium carbonate and/or magnesium carbonate in the calcium depleted solution 60 into sodium hydroxide and magnesium hydroxide respectively. The sodium carbonate and/or magnesium carbonate conversion station 62 comprises a plurality of cavitation units 64 having at least one venturi oxidiser (such as venturi oxidiser 20) and configured so that calcium depleted solution 60 passing through the cavitation unit 64 is converted into sodium hydroxide and/or magnesium hydroxide feed solution 24. The molecular shearing produced by the cavitation within the venturi oxidiser produces high localised temperatures of ΙΟ,ΟΟΟ , pressures of 1000 atmospheres and hydroxyl radicals resulting in the thermal degradation of carbonate to oxide, the oxides form into hydroxides as the oxides react with the water.

[0060] The sodium hydroxide and/or magnesium hydroxide feed solution production unit 50 may comprise an aeration station (not shown) for aerating AMD water 12 upstream of the sulphate reduction station 52. The aeration station may comprise a spray unit configured to direct the AMD water 12 into a first reservoir, such as a pond or tank. The spray unit may comprise a venturi oxidiser to oxidise the AMD water and provide oxidised water. The reservoir may be elongate and the AMD water 12 may be sprayed into one end of the reservoir. The water may be retained in the reservoir for a predetermined period to facilitate precipitation of metals in the reservoir. The inlet to the sulphate reduction station 52 may be at an opposing end of the reservoir. Baffles may be provided in the reservoir. The iron transforms from ferrous to ferric state in the reservoir, and the retention period in the pond may be timed to remove the maximum amount of oxygen from the water. An anaerobic state is required by the sulphate reducing bacteria. An anaerobic state can also be achieved by other suitable means. [0061 ] The water entering the sulphate reduction station 52 is anoxic and the sulphate reduction station 52 is sealed to maintain an anoxic environment. The pH in the sulphate reduction station 52 may be higher than the pH of AMD water 12 entering the aeration station. The pH in the sulphate reduction station 52 may be between 4 and 9 and in some cases between 6 and 8.

10062] The bioreactor in the sulphate reduction station 52 may contain a food source for the bacteria. The food source may comprise hydrogen, carbon, nitrogen and phosphorous. The food source may comprise carbohydrate formed by enzymatic hydrolysis of lignocellulosic biomass. The carbohydrate may be in the form of a solid that is insoluble in water. The food source may be disinfected before being injected into the bioreactor. The carbohydrate formed from the enzymatic hydrolysis of lignocellulosic biomass may be glucose. A glucose utilising sulphate reducing bacteria may be used to reduce the sulphates to sulphides in the bioreactor. The glucose utilising sulphate reducing bacteria may be capable of oxidising the glucose to carbon dioxide. The carbon dioxide may react with the water forming carbonic acid: C0₂ (aq) + H₂0→ H₂C0₃ (aq). The carbonic acid may lose up to two protons, forming carbonate. The carbonate may combine with the cations released by the bacteria to form metal carbonate salts, such as CaC0₃.

[0063] A first sensor may be provided for measuring chemical oxygen demand (COD) or a concentration of total organic carbon (TOC) in the source of water being treated. A second sensor may be provided for measuring a concentration of S0₄ ^2" in the water output from the aeration station. Alternatively, a third sensor may be provided for monitoring a COD/S0₄ ^2" ratio in the bioreactor. A processor responsive to the first and/or second and/or third sensor(s) may control flow of water from the aeration station into the bioreactor and/or flow of the food source into the bioreactor to achieve a desired COD /S0₄ ^2" ratio in the bioreactor. The desired COD/S0₄ ^2" ratio in the bioreactor may be selected to inhibit precipitation of metal salts in the bioreactor. Also, the desired COD/S0₄ ^2" ratio may reduce the growth of alternative micro-organisms. The bioreactor may comprise a plurality of tanks ] 02 connected in series, as described in detail later. The plurality of tanks may be connected by overflows. A first of the tanks may be a stabilising tank for regulating unsteady flow received from the aeration station. The first tank may also stabilise the water chemistry. The first tank may overflow into a second tank. The second tank may overflow into a third tank. At least the second in the series of tanks may contain the sulphate reducing bacteria and the food source. A sludge removal valve 1 10 may be provided in the tanks 102. Submerged structures may be provided in the tanks to facilitate retention of the bacteria. The bioreactor may be sealed to prevent release of gases generated therein. The bioreactor may comprise a gas outlet for controlled exhaust of gases generated in the bioreactor. A pump may be provided in the gas outlet pipe or in the bioreactor to draw the gases toward the gas outlet. The pump may be a passive pump, such as a venturi, or may be an active pump. Alternatively, the bioreactors may be pressurised to cause the gas to flow from the sulphate reducing bioreactors into one or more hydrosulphide oxidising bioreactors, as described later.

[0064] The gas from the sulphate reduction station 52 utilising sulphate reducing bacteria may consist of 1 mol hydrogen sulphide and 1 mol carbon dioxide. Addition of a limited volume of air to chemolithotrophic sulphide oxidising bacteria in the above step causes the hydrogen sulphide or hydrosulphide gas to be converted into elemental sulphur and water as the bacteria utilise the carbon dioxide as the electron donor. The only remaining gas from the process may be nitrogen from the introduced air. Thus, the sulphate reduction station 52 may be connected to a sulphide oxidation station 48 (Figure 3). A conduit may extend between the sulphate reduction station 52 and the sulphide oxidation station 48 to facilitate transfer of gases generated in the sulphate reduction station 52 to the sulphide oxidation station 48. The sulphide oxidation station 48 may comprise at least one tank containing water. The water in the sulphide oxidation station 48 may have a pH of between 6 and 9 and in some embodiments of around 8.5. Sulphide oxidising bacteria may be present in the water in the sulphide oxidation station 48. The sulphide oxidising bacteria may be chemolithotrophic. An inlet may be provided in the sulphide oxidation station 48 for controlled introduction of air into at least one tank. Oxygen concentration into at least one tank may be controlled by controlling the amount of air introduced through the inlet. The sulphide oxidation station 48 may also include a gas outlet for returning nitrogen from the sulphide oxidation station 48 to the sulphate reduction station 52.

[0065] In another embodiment, the sulphide oxidation station 48 may further comprise a second bioreactor. A conduit may extend between the first bioreactor and the second bioreactor to facilitate transfer of gases generated in the first bioreactor to the second bioreactor. The second bioreactor may comprise at least one tank containing water containing carbonate. The carbonate may be produced by the carbon dioxide oxidation by the sulphate reducing bacteria. The water in the second bioreactor may have a pH of between 6 and 9 and in some embodiments of around 8.5. Sulphide oxidising bacteria may be present in the water in the second bioreactor. The sulphide oxidising bacteria may be aerobic. The bacteria may also utilise carbonate as the electron donor. An inlet may be provided in the second bioreactor for controlled introduction of air into at least one tank. Oxygen concentration into at least one tank may be controlled by controlling the amount of air introduced through the inlet. The second bioreactor may also include a gas outlet for returning carbon dioxide and nitrogen from the second bioreactor to the first bioreactor.

[0066] A portion of the water output from the sulphide oxidation station 48 may be fed into the second bioreactor and/or directed onto acidity producing rock heaps and/or used as a buffer for water fed into the first bioreactor. [0067] Calcium carbonate is precipitated from the sulphate reduced water 54 at the calcium carbonate precipitation station 56 to produce a calcium depleted solution 60. The calcium carbonate precipitation station 56 may comprise a settling reservoir downstream of the sulphate reduction station 52. Some bacteria will flow out of the sulphate reduction station 52 into the settling reservoir. In the settling reservoir, the bacteria consume the remaining DOC, which results in a chemical environment that causes metal salts to precipitate and form sludge.

[0068] The contaminants in the bioreactor 52 are principally compounds that would not oxidise, such as CaS0₄, MgS0 , MnS0 , and Na₂S0₄. As the bacteria reduce the sulphates, the compounds change from sulphates to carbonates due to the carbon in the water. The carbonates are inhibited from precipitating inside the bioreactor 52 by maintaining a required residual concentration of DOC in the water. An example of the formation of carbonates from sulphates for the calcium cation:

CaS0₄ + 2(CH₂0) fi CaC0₃ + H₂S + C0₂ + H₂0 where "CH20" represents a carbon source capable of donating an electron to the bacteria.

[0069] The increased pH generated by the reactions within the bioreactor 52 facilitates precipitation of the carbonates as sludge in the settl ing reserv oir at the calcium carbonate precipitation station 56 downstream of the bioreactor 52 after the bacteria consume the remaining DOC. The dead organics in sludge in the settling reservoir inhibit the precipitated carbonates from forming hard scales.

[0070] Precipitated calcium carbonate 58 can be separated from the calcium depleted solution 60 by settling, filtration, centrifugation, etc. For example, a filtration station comprising a sand filter can be provided downstream of the calcium carbonate precipitation station 56. A water quality sensor may be provided at the downstream end of the filtration station. A controller responsive to the water quality sensor may cause the water passing the filtration station to be returned to the reservoir of the aeration station for reprocessing if the sensor detects that the quality of the water is below a predetermined level. The controller causes the water passing the filtration station to be directed to a storage reservoir for recycling if the sensor detects that the quality of the water is above a predetermined level.

[ 0071 J The precipitation of heavy mass carbonate sludges permits separation of the sludges from the water without the assistance of flocculants using thickeners. This substantially increases the shear resistance of the resulting sludge, pennitting it to be safely stored at substantially increased heights.

[0072] Where aerobic sulphide oxidating bacteria are used, the carbonate containing water may be stored in the hydrosulphide oxidising tanks as electron donors for the bacteria. [0073] After removal of the calcium carbonate the calcium depleted solution 60 is fed to the sodium carbonate and/or magnesium carbonate conversion station 62 where sodium carbonate and/or magnesium carbonate in the calcium depleted solution 60 are converted into sodium hydroxide and magnesium hydroxide, respectively. This is effected using a cavitation unit 64 having at least one venturi oxidiscr and configured so that calcium depleted solution 60 passing through the cavitation unit 64 is converted into sodium hydroxide and/or magnesium hydroxide feed solution 24. Again, formation of hydroxyl radicals combined with high localised temperature and pressure as described previously results in conversion of the carbonate salts remaining in the calcium depleted solution 60 to hydroxides.

10074 ] Magnesium hydroxide has a low solubility in water resulting in the formation of a magnesium hydroxide sludge in the base of the tank receiving water from the carbonate conversion station 62.

[0075] This magnesium hydroxide sludge may be fed into the feed solution inlet 22 into the draught tube of the oxidiser to cause the precipitation of the metal hydroxide sludge.

[0076] The calcium and magnesium depleted water containing sodium hydroxide may be filtered using a membrane reverse osmosis process. This process will produce a sodium hydroxide concentrate sludge. This sludge can be fed into the feed solution inlet 22 into the draught tube of the oxidiser to cause precipitation of the metal hydroxide sludge.

[0077] The calcium, magnesium and sodium depleted hydroxide rich water may be combined with an embodiment of the sulphide oxidising bioreactor to utilise the carbon dioxide in the biogas from the sulphate reduction. The carbon dioxide is added to the calcium, magnesium and sodium depleted hydroxide water to produce carbonates. The resulting carbonate water may be added to the treated water from the cavitation units 14a, 14b, 14c and 14d to reduce the concentration of remaining compounds without detrimentally affecting aqueous ecology in receiving streams.

[0078] The sodium hydroxide and/or magnesium hydroxide feed solution 24 can be used in system 10. Alternatively, in addition, the system 10 and/or sodium hydroxide and/or magnesium hydroxide feed solution production unit 50 may be configured so that at least some of the sodium hydroxide and/or magnesium hydroxide feed solution 24 and/or water from any one of bioreactor 52 and/or calcium carbonate precipitation station 56 may be removed from the system to be stored and/or used as treated AMD water. In other words, not all of the solution 24 needs to be fed back into system 10 and some can be stored and/or used as treated AMD water. The system 10 may further comprise a filtration station downstream of the cavitation units 14a, 14b, 14c, and 14d. The filtration station may comprise a media filter, such as a sand filter, and/or a mesh filter. A water quality sensor may be provided at the downstream end of the filtration station. The controller may be responsive to the water quality sensor to direct the water passing the filtration station based on the detected water quality. The controller may be responsive to the water quality sensor and may cause the water to be returned to the system if the sensor detects that the quality of the water is below a predetermined level. The controller may cause the water to be directed to a storage reservoir for recycling if the sensor detects that the quality of the water is above a predetermined level.

[0079] The sulphate reducing bacteria in the sulphate reduction station 52 chemically 'reduce' the sulphate in the AMD water 12 to sulphide. As the metals are removed before bacterial reduction, highly toxic metal sulphides are not produced. Metal sulphide toxicity is a major limiting factor on bacterial treatment processes. However, the sulphide anions must bond with a cation if they are to be removed from the sulphate reduction station 52. Hence, with the lack of metals, an alternative cation is required. Advantageously, the conditions within the sulphate reduction station 52 can be adjusted to produce hydrogen. Hence, as the bacteria release the sulphides through their membranes, these anions bond with hydrogen, forming H₂S, HS^" and S₂.

[0080] Hydrogen sulphide (H₂S) is substantially less toxic to the bacteria than metal sulphides.

Sulphide can exist in different forms such as the most lethal non-dissociated form, H₂S (decreasing in concentration from pH 5 to 8), less lethal HS^" (increasing from pH 6 to 9); S₂ (increasing from pH 8 to above 10) where it is the sole sulphide compound.

[0081 ] The literature indicates there exists substantial disagreement on the concentration of the above compounds that present toxicity to the sulphate reduction bacteria (SRB). For example, it is reported that an increase of the pH in the range 6.8 to 8.5 could lead to toleration of higher sulphide levels. The toxicity to total sulphides reported falls in the range of 64 to 2059 mg/L and those for the non- dissociated H₂S vary from 57 to 550 mg/L.

10082] The present inventor has found that stripping of the sulphide gas results in substantially improved SRB activity, which in turn increases the pH, which in turn converts the sulphides from the more toxic H₂S to the less toxic HS- form. Removing the sulphides maintained a concentration in the sulphate reduction station 52 of 476 mg/L which is close to the inhibitory range reported by several researchers. Increase in the amount of nutrient is reported by researchers to also assist sulphide removal. It must be kept in mind that the SRB were more efficient as the pH increased.

[0083] The present inventor found that if the concentration of H₂S is kept below 40 to 50 mg/L microbial conversion rates of more than 65 g S0₄ L ¹ d^"1 are achieved and bacterial conversion efficiencies for sulphate removal of 95% can be maintained. This compares well with the 3 g S0₄ L ¹ d^"1 experienced with other temporary bacterial treatment processes for AMD. Concentrations of total sulphides at pH 8 of 1200 mg/L were not toxic to the SRB as minimum H₂S exists. [0084] In summary, this 50 mg/L concentration of H₂S occurs if: a) The sulphides are constantly removed to maintain a constant total sulphide concentration in the bioreactors of 476 mg/L; b) A constant COD/S04 ratio of 0.67 is maintained; and c) pH is > 7.6, preferably above 8.0 thereby forming a minimum of lethal H₂S, preferring hydrosulphide (HS^~).

[0085] The biogas produced in the sulphate reduction station 52 consists of hydrosulphide in the form of a gas combined in equal mol with carbon dioxide. This gas is produced by the oxidation of the carbohydrate by the bacteria. For reasons discussed, it is important that this biogas is removed as it is produced, otherwise the resulting toxicity will limit the productivity of the bacteria.

[0086] The present inventor has trialled removing gaseous sulphides by circulating nitrogen gas through the sulphate reduction station 52. Given the very large volumes of nitrogen required in a full size plant this process is not considered viable for typical water volumes that can be treated using the processes described herein. Introducing metal filings into the sulphate reduction station 52 to form insoluble metal sulphides was partially successful but imposes the problem of transporting metal filings over large distances, and a problem exists in eradicating the toxic resultant product. Membrane degassers could be used, but the membranes are likely to become blocked by the slime from the bacteria in the water, eventually becoming inoperable. Additives are available to reduce sliming, but these also inhibit the efficiency of the sulphate reduction station 52.

[0087] An advantageous solution is shown in Figure 7 and comprises a system 100 for removing gases directly from the sulphate reduction station 52 by reducing the pressure in the bioreactor oft eh sulphate reduction station 52 causing the gases to separate from the water. The biogas can then be pumped to the sulphide oxidation station 48. Alternatively, the water from the sulphate reduction station 52 can be pumped to separate vacuum vessels.

[0088] The water in the sulphate reduction station 52 contains sulphides and carbon dioxide. The pressure of the water in each bioreactor is reduced to approximately 18 kPa (absolute) by vacuum pumps. To ensure constant pressure between all bioreactors all extracted biogas is combined into a single set of vacuum pumps. It has been found experimentally that SRB are not affected by partial vacuum. [0089] When commissioning, bioreactor pipe tanks 102 are filled with AMD water 12 to be treated. The pipe tanks 102 are laid horizontal with the centre of the pipe raised with a gas offtake 104 at the top of the pipe 106 located midway along the pipe tank length, as shown in Figure 8. The first bioreactor pipe tank 102 shall be higher than the next tank and so on to permit the water to flow under gravity through the three pipe stages whilst still maintaining the partial vacuum.

[0090] When the bioreactor pipe tanks 102 are full of water, and sulphate reduction has been occurring for 48 hours, the vacuum pump is started and adjusted to a suction pressure of about 18 kPa (absolute).

100911 A vacuum pump 108 is used to create the partial vacuum inside the bioreactor pipe tanks 102 and also create sufficient pressure to overcome the hydrostatic head inside the sulphide bioreactor tank.

10092] In practice it was established that if the total sulphide concentration is limited to 476 mg/L, at a pH 7.7 - 8.0, it will have a minimal effect on the efficiency of the SRB. By Henry's law, if the pressure inside the sulphate reduction station 52 is maintained at 18.3 kPa, then the concentration of total sulphides will be 476 mg/L. Therefore a vacuum pump is selected that will produce a vacuum of 18.3 kPa (183 mbar) absolute. As the sulphides are toxic to the bacteria and the C0₂ is not, they are the determining component of the two part biogas.

[0093] Water will convert from a liquid to a vapour at or near zero pressure absolute. As the temperature of the water increases, conversion to vapour will occur at a higher absolute pressure. For example, water exposed to the ambient heat in Australia will typically reach approximately 35 degrees C when shaded, higher if exposed to the sun. At 35 deg C the vapour pressure of water is

approximately 6 kPa absolute.

[0094] By Henry's law, the lower the pressure (absolute) of the sulphate reduction station 52, the lower the concentration of biogas. Conversely, it must be remembered that determination of vacuum pump pressure, vacuum vessel pressure and indicated pressure on gauges are approximate. It is essential that the water in the bioreactor does not turn into vapour, both for biochemical and engineering reasons. Therefore a sufficient 'buffer' must exist between the water vapour pressure at the site and the actual partial vacuum in the sulphate reduction station 52. Therefore, approximately 18 kPa is preferred, which is indicative by Henry's law (and confirmed experimentally) of a concentration of 476 mg/L of total sulphides and 450 mg/L of carbon dioxide in the bioreactor. As described above this will permit the operation of the bioreactor at a sulphide concentration below the toxicity level of the bacteria. [0095] It has also been confirmed experimentally that partial vacuum does not reduce the efficiency of the bacteria to reduce sulphate.

[0096] The bacteria must be contained in a sealed tank 102. It is essential the tank is sealed to:

• Control the bacterial environment for maximum efficiency;

• Safely contain the hydrogen sulphide produced; and

• Allow for application of a partial vacuum to extract the biogas.

[0097] The tanks 102 can be formed from polyethylene pipes with end caps. Long (12m for example) pipes permit the water under treatment to flow in one direction, from one end of the pipe to the other, thus preventing 'short circuiting' of the water.

[0098] The pipe tanks 102 can be filled with beads which form the home for the bacteria. The pipe tanks 102 can be stacked vertically, causing the treatment path between tanks to be accomplished by gravity, deleting the need for pumps.

[0099] In a conventional tank operating at atmospheric pressure, stirring is important to ensure the sulphide gas is effectively removed. Otherwise bubbles of biogas may become trapped on the beads killing the bacteria that live on the bead. In a conventional tank this is achieved using pumps or mixers to recirculate the flow to the bottom of the tank, causing uplift and turbulence. In the present case, it is believed that the uplift forces of the rising gas are sufficient to cause the required turbulence.

[00100] As described earlier, the alkalinity generated in the sulphate reduction station 52 causes the calcium sulphate to convert to calcium carbonate which has a low solubility in water. It has been found in practise that the calcium carbonate occurs in the sulphate reduction station 52 when the EPS around the bacteria becomes saturated with cations, causing the calcium cation to enter the alkaline aqueous solution, forming a carbonate.

100101 ] In a conventional tank this precipitated CaC0₃ is prevented from settling in the bottom of the tank by the stirring action described above. In tanks 102 no such stirring exists, and the rising gases may not provide sufficient uplift forces to prevent settlement. Therefore, the pipe tank 102 is raised in the centre of the pipe and sufficient slope exists either end of the tank to cause the calcium carbonate to move down the base of the pipe and collect at the pipe ends. Sludge valves 1 10 can be placed at either end of each tank 102 to permit the removal of precipitated calcium carbonate. [00102] An alternative arrangement is to have many vertical pipe tanks, interconnected. This would permit the calcium carbonate to fall to the bottom for collection. The vertical pipes may be laid on the same level, utilising the differential vacuum (caused by the draw of water from the final tank) to draw the water through the pipes instead of gravity.

[00103] A standard water pump (not shown) can be used to pressurise the water to more than atmospheric pressure as it draws it from the third pipe tank 102. This water then enters the sodium hydroxide and/or magnesium hydroxide feed solution production unit 50 for conversion of the carbonates into hydroxides for the removal of the magnesium and sodium.

[00104] Also disclosed herein is a method for processing acid mine drainage (AMD) water to reduce the content of metal ions in said water, the method comprising: passing AMD water to be treated and a carbonate and/or hydroxide metal salt feed solution through a first cavitation unit comprising at least one venturi oxidiser under conditions to oxidise the solution and reduce the content of metal ions in the water to produce a first reduced metal ion content solution; passing the first reduced metal ion content solution and, optionally a carbonate and/or hydroxide metal salt feed solution through a second cavitation unit comprising at least one venturi oxidiser under conditions to oxidise the solution and reduce the content of metal ions in the water to produce a second reduced metal ion content solution; and, optionally, passing the second reduced metal ion content solution and, optionally, a carbonate and/or hydroxide metal salt feed solution through at least one further cavitation unit under conditions to oxidise the solution and reduce the content of metal ions in the water to produce treated water.

[00105 ] In certain embodiments, the method further comprises a step of producing the sodium hydroxide and/or magnesium hydroxide feed solution from AMD water, the method comprising: reducing sulphates present in the AMD water sulphate reducing bacteria to produce sulphate reduced water; precipitating calcium carbonate from the sulphate reduced water; separating precipitated calcium carbonate from the sulphate reduced water to produce a calcium depleted solution; converting sodium carbonate and/or magnesium carbonate in the calcium depleted solution into sodium hydroxide and magnesium hydroxide respectively.

[00106] In certain embodiments, the sodium carbonate and/or magnesium carbonate in the calcium depleted solution is converted into sodium hydroxide and magnesium hydroxide respectively by passing the calcium depleted solution thorough a cavitation unit having at least one venturi oxidiser and configured so that calcium depleted solution passing through the cavitation unit is converted into sodium hydroxide and/or magnesium hydroxide feed solution. [00107] In certain embodiments, not all of the sodium hydroxide and/or magnesium hydroxide feed solution generated in the cavitation unit is used as the hydroxide and/or magnesium hydroxide feed solution and some is used and/or stored as treated AMD water.

[00108 ] The above described system and method, either in portion or whole, are expected to negate the requirement for imported chemicals, substantially reduce wastes, and reduce the environmental effects of the extraction of metals from sulphide ore, substantially reducing the costs of metal extraction from ore.

[00109] It will be appreciated by those skilled in the art that the invention is not restricted in its use to the particular application described. Neither is the present invention restricted in its preferred embodiment with regard to the particular elements and/or features described or depicted herein. It will be appreciated that the invention is not limited to the embodiment or embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope of the invention as set forth and defined by the following claims.

[001 10 ] Throughout the specification and the claims that follow, unless the context requires otherwise, the words "comprise" and "include" and variations such as "comprising" and "including" 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.

[001 1 1] The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement of any form of suggestion that such prior art forms part of the common general knowledge.

Claims

1. A system for processing acid mine drainage (AMD) water to reduce the content of metal ions in said water, said system comprising a plurality of cavitation units connected in series with each cavitation unit comprising a water inlet and a water outlet and at least one venturi oxidiser between each water inlet and water outlet and wherein at least a first upstream cavitation unit in the series of cavitation units comprises a feed solution inlet configured to mix a carbonate and/or hydroxide metal salt feed solution with the AMD water in said cavitation unit.

2. The system of claim 1 , wherein the carbonate and/or hydroxide metal salt feed solution comprises an alkali and/or alkaline earth metal salt.

3. The system of claim 2, wherein the carbonate and/or hydroxide metal salt feed solution comprises a hydroxide salt of an alkali metal or an alkaline earth metal.

4. The system of claim 2, wherein the carbonate and/or hydroxide metal salt feed solution comprises a carbonate salt of an alkali metal or an alkaline earth metal.

5. The system of any one of claims 2 to 4, wherein the alkali metal is selected from one or more of the group consisting of lithium, sodium and potassium.

6. The system of any one of claims 2 to 4, wherein the alkaline earth metal is selected from one or more of the group consisting of magnesium and calcium.

7. The system of any one of claims 1 to 6, wherein the carbonate and/or hydroxide metal salt feed solution comprises one or more of the group consisting of calcium hydroxide, potassium hydroxide, magnesium carbonate, calcium carbonate, potassium carbonate, sodium carbonate, sodium hydroxide, and magnesium hydroxide.

8. The system of claim 7, wherein the carbonate and/or hydroxide metal salt feed solution comprises sodium hydroxide.

9. The system of claim 7, wherein the carbonate and/or hydroxide metal salt feed solution comprises magnesium hydroxide.

10. The system of any one of claims 8 to 9, further comprising a sodium hydroxide and/or magnesium hydroxide feed solution production unit for forming a sodium hydroxide and/or magnesium hydroxide feed solution from AMD water, the sodium hydroxide and/or magnesium hydroxide feed solution production unit comprising: a sulphate reduction station for reducing sulphates present in the AMD water to produce sulphate reduced water, the sulphate reduction station comprising a first bioreactor containing sulphate reducing bacteria; a calcium carbonate precipitation station for precipitating calcium carbonate from the sulphate reduced water and for separating precipitated calcium carbonate therefrom to produce a calcium depleted solution; a sodium carbonate and/or magnesium carbonate conversion station for converting sodium carbonate and/or magnesium carbonate in the calcium depleted solution into sodium hydroxide and magnesium hydroxide respectively.

1 1. The system of claim 9, wherein the sodium carbonate and/or magnesium carbonate conversion station comprises a unit having at least one venturi oxidiser and configured so that calcium depleted solution passing through the cavitation unit is converted into sodium hydroxide and/or magnesium hydroxide feed solution.

12. The system of claim 9, wherein the sodium carbonate and/or magnesium carbonate conversion station is configured to convert sodium carbonate and/or magnesium carbonate in the calcium depleted solution into sodium hydroxide and magnesium hydroxide by degrading the carbonate to oxide using heat, thereby forming hydroxide when mixed with water.

13. The system of any one of claims 10 to 12, wherein the system and/or sodium hydroxide and/or magnesium hydroxide feed solution production unit are configured so that at least some of the sodium hydroxide and/or magnesium hydroxide feed solution and/or water from any one of bioreactor and/or calcium carbonate precipitation station is removed from the system to be used and/or stored as treated AMD water.

14. A method for processing acid mine drainage (AMD) water to reduce the content of metal ions in said water, the method comprising: passing AMD water to be treated and a carbonate and/or hydroxide metal salt feed solution through a first cavitation unit comprising at least one venturi oxidiser under conditions to oxidise the solution and reduce the content of metal ions in the water to produce a first reduced metal ion content solution; passing the first reduced metal ion content solution and, optionally a carbonate and/or hydroxide metal salt feed solution through a second cavitation unit comprising at least one venturi oxidiser under conditions to oxidise the solution and reduce the content of metal ions in the water to produce a second reduced metal ion content solution; and, optionally, passing the second reduced metal ion content solution and, optionally, a carbonate and/or hydroxide metal salt feed solution through at least one further cavitation unit under conditions to oxidise the solution and reduce the content of metal ions in the water to produce treated water.

15. The method of claim 14, wherein the carbonate and/or hydroxide metal salt feed solution comprises an alkali and/or alkaline earth metal salt.

16. The method of claim 15, wherein the carbonate and/or hydroxide metal salt feed solution comprises a hydroxide salt of an alkali metal or an alkaline earth metal.

17. The method of claim 16, wherein the carbonate and/or hydroxide metal salt feed solution comprises a carbonate salt of an alkali metal or an alkaline earth metal.

18. The method of any one of claims 15 to 17, wherein the alkali metal is selected from one or more of the group consisting of lithium, sodium and potassium.

19. The method of any one of claims 15 to 17, wherein the alkaline earth metal is selected from one or more of the group consisting of magnesium and calcium.

20. The method of any one of claims 15 to 19, wherein the carbonate and/or hydroxide metal salt feed solution comprises one or more of the group consisting of calcium hydroxide, potassium hydroxide, magnesium carbonate, calcium carbonate, potassium carbonate, sodium carbonate, sodium hydroxide, and magnesium hydroxide.

21. The method of claim 20, wherein the carbonate and/or hydroxide metal salt feed solution comprises sodium hydroxide.

22. The method of claim 20, wherein the carbonate and/or hydroxide metal salt feed solution comprises magnesium hydroxide.

23. The method of any one of claims 21 to 22, further comprising a step of producing the sodium hydroxide and/or magnesium hydroxide feed solution from AMD water, the method comprising: reducing sulphates present in the AMD water sulphate reducing bacteria to produce sulphate reduced water; precipitating calcium carbonate from the sulphate reduced water; separating precipitated calcium carbonate from the sulphate reduced water to produce a calcium depleted solution; converting sodium carbonate and/or magnesium carbonate in the calcium depleted solution into sodium hydroxide and magnesium hydroxide respectively.

24. The method of claim 23, wherein the sodium carbonate and/or magnesium carbonate in the calcium depleted solution is converted into sodium hydroxide and magnesium hydroxide respectively by passing the calcium depleted solution thorough a cavitation unit having at least one venturi oxidiser and configured so that calcium depleted solution passing through the cavitation unit is converted into sodium hydroxide and/or magnesium hydroxide feed solution.

25. The method of claim 23, wherein the sodium carbonate and/or magnesium carbonate in the calcium depleted solution is converted into sodium hydroxide and magnesium hydroxide by degrading the carbonate to oxide using heat, thereby forming hydroxide when mixed with water.

26. The method of any one of claims 23 to 25, wherein not all of the sodium hydroxide and/or magnesium hydroxide feed solution generated in the cavitation unit is used as the hydroxide and/or magnesium hydroxide feed solution and some is used and/or stored as treated AMD water.