The present invention relates to the treatment of water of relatively low pH. The method is of particular application to the treatment of acidic ground water, or mine void water, having dissolved mineral species contained therein, particularly dissolved iron or sulphates. BACKGROUND TO THE INVENTION

The treatment of acidic waste water, often containing large concentrations of dissolved metal contaminants, is required by mining companies and land and water management authorities around the world. Mining operations may produce acid and metalliferous drainage, often referred to as acid mine drainage (AMD) or acid rock drainage (ARD), from dewatering operations or leaching of water through waste rock, mining overburden, or sulphide-containing mine tailings that are exposed to air. Drainage water from exposed acid sulphate soils has similar chemical properties. Frequently, the water being removed is simply discharged to the local environment, such as local waterways. Such action can be damaging to the environment, killing native flora and fauna, promoting the growth of undesirable bacteria and causing staining and discolouration.

Some ground waters are acidic, and have relatively large quantities of dissolved metals, such as iron, aluminium, copper, lead, zinc and manganese. These metals must be removed from the water before it can be released into the environment or used for agricultural purposes such as irrigation.

Modern environmental regulations generally require the treatment of AMD and some ground waters to meet strict requirements before it is released to the environment, or used for agriculture. Such requirements include an acceptable pH range, suitable alkalinity and dissolved oxygen concentrations and upper limits on the concentration of dissolved mineral species within the water.

Typically low-pH waters with high levels of dissolved metals are treated by dosing with lime (Ca(OH) ₂ ). This acts to raise the pH of the water to the point where the metals of concern are insoluble, and precipitate out at their respective iso-electric points on the pH response curve. In addition, it may be desirable to increase the alkalinity of the water. The iron and other minerals, such as manganese, may then be oxidised by aeration and/or the use of a chemical oxidant such as potassium permanganate. A chemical coagulant such as alum may then be added, and the minerals removed either through a clarifier or by the use of a settling pond (with a chemical flocculant, such as a polyelectrolyte, employed). Before the treated water is discharged, a final pH adjustment may be necessary.

This treatment process is an expensive one, with a high consumption of chemicals. It also raises significant occupational health and safety risks, requiring the careful transportation and supply of caustic and corrosive materials such as lime and potassium permanganate. The treatment processes themselves also have environmental consequences. For example, the manufacture of lime (Ca(OH)2) requires high energy input and produces carbon dioxide, thus demonstrating a significant "carbon footprint".

In an attempt to reduce the costs associated with this process, attempts have been made to use limestone (CaCOa), rather than lime (Ca(OH)2) as the pH-raising agent. In addition to being a cheaper material, limestone is less caustic than lime, its use reduces the hazards of handling and application and its use represents a smaller carbon footprint than use of Ca(OH) ₂ .

These attempts have not always been successful. Limestone has a relatively slow rate of dissolution, and is prone to the development of metal hydroxide and sulphate coatings on the surface of limestone particles, in a process known as 'armouring'. Armouring is particularly problematic in the treatment of water with high ferrous iron (Fe ²⁺ ) concentration or high sulphates. Armouring causes a significant reduction in the rate of limestone dissolution as successive layers of metal hydroxide or sulphates build up on particle surfaces over time. As a consequence, to date the use of limestone has generally been restricted to waters having low concentrations of iron and sulphates.

The present invention seeks to provide an alternative method of treating water which at least partially alleviates many of these concerns. The present invention seeks, for instance, to provide a means by which limestone can be used in the successful treatment of acidic water containing dissolved iron. SUMMARY OF THE INVENTION

in accordance with a first aspect of the present invention there is provided a method of increasing the pH of acidic water, the method including the steps of i) introducing oxygen into the water to raise the dissolved oxygen concentration; and ii) passing the water through a pH-raising media including limestone (calcium carbonate).

Increase of pH in this manner can assist in the removing of dissolved mineral species from the water, particularly ferrous iron and sulphates. Oxygenation of the water prior to its passage through a limestone media bed helps to raise the pH of the water and increase the rate of dissolved mineral species oxidation. Preferably, the method includes the further step of aerating the water after it has been passed through the limestone media to promote oxidation of mineral species contained therein, to remove carbon dioxide which has become dissolved within the water and thus to further increase pH.

The limestone media used may include particles having a range of sizes. It is considered preferable for the limestone particles to have diameters in the range 0.25-4.0mm, although particles in the range 0.2-10.0mm are considered to have a useful effect. The passing of the water through the limestone is preferably done in an up-flow reactor vessel, in which the water up-flow velocity is sufficient to fluidise a bed of limestone media. The reactor vessel may be arranged such that water velocity is slowed towards the top of the reactor vessel. This may be achieved by shaping the vessel such that there is an increase in its cross sectional area between the entry and exit points of water being passed through the vessel. This has the effect of reducing the size of limestone particle which is lost due to overflow. The limestone media may be placed above a layer of inert, high density particulate matter, such as aggregate material having a particle size of about 10- 20mm. The aggregate material may be basalt. Alternatively a proprietary underdrain system can be utilised that allows passing of influent water and gases into the bottom of the reactor vessel in a substantially uniform manner. It has been found that a particularly advantageous effect can be obtained by supplying gas to the base of the reactor vessel. Such gas, bubbling through the vessel, can help to increase turbulence in the water and to increase the agitation of the limestone particles. The resulting increase in abrasion acts to prevent armouring of the limestone particles, whereby precipitated iron hydroxide and/or metal sulphates such as calcium sulphate build up on the surface of limestone particles.

This gas may be ambient air. Alternatively, it may be CO ₂ enriched air. The latter has the additional effect of further increasing the alkalinity of the water being treated. In addition, the CO ₂ forms carbonic acid and acts to decrease the pH of the water, thus providing a more intense reaction with the calcium carbonate. This assists in preventing the on-set of armouring.

In a preferred embodiment of the invention, recovered ferric iron hydroxide and oxy-hydroxide sludge is recycled into the influent water prior to it entry to the upflow reactor vessel in order to obtain an auto catalytic effect on the oxidation of ferrous iron. The recycling of precipitates also has the benefit of co-precipitation of other metals including manganese, increasing sludge density, and improving lime use efficiency. BRIEF DESCRIPTION OF THE DRAWINGS It will be convenient to further describe the invention with reference to preferred embodiments of apparatus used in conjunction with the method of the present invention. Other embodiments are possible, and consequently the particularity of the following discussion is not to be understood as superseding the generality of the preceding description of the invention. In the drawings: Figure 1 is a schematic view of a water treatment installation for treating water in accordance with a first embodiment of the present invention;

Figure 2 is a schematic view of a water treatment installation for treating water in accordance with a second embodiment of the present invention;

Figure 3 is a schematic view of a water treatment installation for treating water in accordance with a third embodiment of the present invention;

Figure 4 is a schematic view of a water treatment installation for treating water in accordance with a fourth embodiment of the present invention; Figure 5 is a front sectional view of an upflow reactor from within the water treatment installation of Figures 1 to 4;

Figure 6 is a front sectional view of an alternative upflow reactor for use within the water treatment installation of Figures 1 to 4; Figure 7 is a side sectional view of a further alternative upflow reactor for use within the water treatment installation of Figures 1 to 4;

Figure 8 is a plot of water pH against time for two water samples undergoing testing; and

Figure 9 is a plot of water pH, alkalinity and iron content during a testing operation.

DESCRIPTION OF PREFERRED EMBODIMENT

Figure 1 shows a water treatment installation 10 for the treatment of acidic influent water containing dissolved mineral species. The installation includes an air supply in the form of an air compressor or blower 20, an air inclusion means in the form of an in-line diffuser 30, an upflow reactor 50, and a degassing means in the form of an aerator 40.

Acidic influent water is supplied to the water treatment installation 10 via an inflow pipe 12. In an initial treatment step, it undergoes oxygenation so as to raise the dissolved oxygen concentration of the water. In the embodiment shown, the oxygen concentration is raised above 5mg/L. It is considered desirable to raise the oxygen concentration close to, or greater than, oxygen saturation. In testing, an oxygen concentration between 5mg/L and 10.5mg/L has proved efficacious.

In the embodiment of Figure 1 , oxygenation is achieved by the injection of compressed air into the inflow pipe 12, using an in-line diffuser 30. The compressed air is supplied to the in-line diffuser 30 from the air compressor 20.

In alternative embodiments, such as those shown in Figures 2 to 4, the inline diffuser 30 is replaced with an aerator 32. The aerator 32 can, for example, be a conventional spray-type aerator or a screen-type aerator. The oxygenated water proceeds along a supply pipe 14. The length of the supply pipe 14 is sufficient that the oxygen supplied to the water by the in-line diffuser 30 has time to dissolve in the water before reaching the upflow reactor 50. Alternative embodiments of the upflow reactor 50 are shown in Figures 5 to 7.

Figure 5 shows an upflow reactor 50 having a lower portion 52, a stepped portion 54 and an upper portion 56. The lower portion 52 has a circular base 62, and a cylindrical side wall 64. The upper portion has a cylindrical side wall 66, having a larger diameter than that of the lower portion 52. The stepped portion 54 has a frusto conical side wall 68, connecting an upper end of the lower portion side wall 64 to a lower end of the upper portion side wall 66. It will be appreciated that the vessel 50 may be of other shape, such as square in cross section. A water distribution system 70 is located across the base 62 of the lower portion 52. The water distribution system 70 can take several forms. These include a series of uniformly spaced slotted pipes, a grid having uniformly spaced outlet nozzles, or a perforated plate located across the base 62 to create a water receiving compartment. Other arrangements are possible. In the embodiment of Figure 5 the water distribution system 70 consists of a plurality of water feed pipes 72 extending across the base 62 from a first side to a second side of the reactor 50. The water feed pipes 72 include outlet slots spaced along their length. The reactor 50 includes a water inlet 74 located at the first side of the base 62, connected to the feed pipes 72. Water supplied to the inlet 74 through the supply pipe 14 thus flows into the feed pipes 72 and exits through the outlet slots into the reactor 50.

The reactor 50 contains a layer of inert material 80, which may be an aggregate material such as basalt, about the water distribution system 70. The aggregate material preferably has a particle size of about 10-20mm. The inert material 80 acts to improve the uniformity of water flow from the water distribution system 70 into the reactor 50, and also to prevent blockages of the outlet slots.

The reactor 50 contains a layer of pH-raising media 82 above the inert layer 80. The pH-raising media is preferably calcium carbonate. This may be in the form of calcite pellets, of lime sand, of crushed limestone rock, of some combination of these, or indeed of some other form of calcium carbonate particles. The diameters of the media particles are desirably in the range 0.2- 10.0mm, and preferably in the range 0.25-4.0mm. In the embodiment of the drawings, the depth of the inert aggregate layer 80 is between 100mm and 250mm, and the depth of the pH-raising media 82 is between 2m and 3m when settled. It will be appreciated that these dimensions depend in large part on the height and cross sectional area of the reactor 50, the hydraulic loading rates used, the properties of the influent water and the desired properties of the discharge water. Importantly, the media depth is sufficient to allow water to have sufficient contact time with the media whilst allowing for a steady flow rate through the system. Testing has suggested that a limestone contact time of 2 to 3 minutes, together with a hydraulic retention time of 5 to 7 minutes, has been found to be sufficient to achieve practical completion of Fe ²⁺ oxidation.

The up-flow reactor 50 includes an overflow collecting channel 84 about its upper end. Water spilling over the top of the reactor 50 is caught in the channel 84, and directed to an output pipe 18. The effect of the stepped portion 56 is to slow the velocity of the water as it rises through the reactor 50. Under the operation of Stokes' law, the size of calcium carbonate particle which will be carried with the water out of the vessel decreases along with the velocity of the water. It is therefore desirable to decrease water velocity towards the top of the reactor 50. This can be achieved by having a higher cross-sectional area of the reactor at its top than at its base.

Several different reactor shapes achieve this effect. It can be achieved by means of a single stepped portion 56 as shown in Figure 5, by a sequence of such stepped portions, or by use of continually tapered reactor 51. Such a reactor, which may be frusto-conical or frusto-pyramidal in shape, is shown in Figure 6.

It will be appreciated that the present invention may be applied without taking advantage of this effect. Figure 7 shows a reactor 53 with straight sides, which can be used in place of the reactor 50. The reactor 53 shows an alternative water distribution system 70, with the reactor 53 having at its base a perforated spreader plate 77, having a cavity 78 beneath it. Water is supplied into the cavity 77 through the water inlet 74, and flows from the cavity 78 into the reactor 53 through the spreader plate 77. In use, water to be treated is introduced via the water inlet 74 into the reactor vessel 50. It is necessary to control the flow rate of this water so that it is introduced to the reactor vessel at a hydraulic loading rate which provides an up- flow velocity sufficient to fluidise the inert material 80 and the pH-raising material 82. Although a positive result is expected with up-flow water velocities above about 35m/hr, a velocity of 80-130m/hr has proved successful in fluidizing the largest limestone particles.

Research conducted by Maree and du Plessis, reported in Water Science and Technology in 1994, suggests that the required water velocity for fluidization, (V, in m/hr) was a function of particle size (p _s , in mm) according to the formula:

V=-3.7 +66.4p _s -8.3p 2

¹ S

This reveals that fluidization of 4mm diameter particles requires an upflow velocity of 129m/hr.

As a corollary, it suggests that dropping the flow rate at the top of the reactor vessel 50 to 35m/hr will mean that particles will have to be smaller than about 0.6mm in diameter in order to be lost to the reactor vessel. Based on this, it can be calculated that use of a cross sectional area at the top of the vessel 50 twice that at the base of the vessel will limit losses from the top of the vessel to about 0.3% of calcium carbonate by volume. A further mechanism is proposed to assist in the fluidization and agitation of the calcium carbonate, being the supply of air or gas alongside the water distribution system 70. The presence of such a system, producing gas bubbles, adds turbulence to the water at the base 52 of the reactor 50, and significantly lowers the water velocity required to achieve fluidization. In testing, a water velocity reduction of 50% was found not to significantly diminish reactor performance when gas was added in this way.

In the embodiments of Figures 1 and 2, air is supplied to the base of the reactor 50 from the air compressor 20. In the embodiment of Figure 3, air is captured from the degassing aerator 40 for supply into the reactor 50. Tests have suggested that the volume of air supplied to the base of the reactor 50 should be in the order of 0.2-0.4m ³ /minute for each square metre of cross-sectional surface area of the water at the top of the reactor vessel. It is preferred that the gas is delivered to the reactor 50 using a separate but complementary system to the water distribution system. Proposed gas delivery systems include air pipes with appropriately sized and spaced orifices along their length, and a grid of uniformly spaced outlet nozzles. As water passes up through the calcium carbonate, CaCθ ₃ dissolves into the water, raising the pH and the alkalinity:

(a) In the case of acidity due to reaction with iron sulphide minerals and the presence of sulphuric acid;

H ₂ SO ₄ + CaCO ₃ → CaSO ₄ + H ₂ CO ₃ (b) In the case of the presence of dissolved carbon dioxide from bores or as a consequence of the reaction in (a) above

CaCO ₃ + H ₂ CO ₃ → Ca(HCO ₃ ) ₂

Tests have demonstrated pH rises from pH of about 3 to pH of about 7-8 following aeration after water passes from the reactor vessel. (c) With pre-oxygenated water, as the water passes up through the bed of calcium carbonate media, a significant proportion of the dissolved iron is oxidised within the media bed:

4Fe(HCO ₃ ) ₂ + O ₂ → 4Fe(OH) ₃ + 8CO ₂

From the outlet pipe 18, water is directed to the aerator 40, where the water is further oxygenated, thus driving the iron oxidation reaction in (c) above to completion and removing CO ₂ produced during the oxidation reaction. The rate of oxidation of iron is highly dependent on the pH of the solution. For pH values greater than 5.5 the rate of oxidation of iron (III) increases in the order of 100-fold per pH unit. The pH increases as water passes up through the bed of calcium carbonate media, but a further pH increase is achieved by directing the water through a post-reactor aerator 40 so as to remove the CO ₂ produced during the oxidation of iron, as otherwise this will form carbonic acid.

It will be understood that the solubility of iron (3+) is related to the pH. Solubility drops from about 1.0mg/L at a pH of 4 to about 1.O x 10 ^~6 at a pH of close to 8. Oxidised iron thus precipitates from the water which has passed through the reactor vessel 50. Removal of iron preciptates is generally done within a clarifier 86, into which the water is passed following aeration in the aerator 40. Different embodiments of the invention feature enhancements to the process.

Particularly where the supplied water is high in dissolved O ₂ , or the water is pre-oxygenated, there is a tendency for salts within the water to precipitate in the reactor vessel 10, forming an 'armour', for instance of FeIII oxyhydroxide coatings, Fe-Al hydroxysulphate or gypsum (calcium sulphate), over the surface of the calcium carbonate particles and preventing further dissolution of the calcium carbonate. The supply of air or gas into the base of the reactor so that air bubbles percolate up through the bed of calcium carbonate media significantly increases the level of turbulence and agitation within the fluidised bed, and helps to prevent armouring. As a further enhancement, CO ₂ can be introduced into the water. The CO ₂ attacks the limestone:

CaCO ₃ + H ₂ O + CO ₂ → Ca ²⁺ + 2HCO ₃ ^" and further helps in preventing the occurrence of armouring. CO ₂ can be introduced through the introduction of a carbon dioxide source into the water, or more efficiently the CO ₂ produced during oxidation of the iron can be recycled into the source water as carbon dioxide enriched air. CO ₂ can be captured (for instance by stripping) and supplied back into the influent water stream at the base of the reactor 10. This is shown in the flow cycle of Figure 3. In this embodiment, the reactor vessel 50 includes a cover 88. The cover 88, which is removable to allow for the addition of calcium carbonate, traps the CO ₂ within the water passing into the outlet pipe. This CO ₂ is then stripped from the effluent water in an aerator or stripper 40. A further enhancement is shown in Figure 4. In the system of Figure 4, sludge 92 containing precipitated ferric iron (as ferric hydroxide) is obtained from the clarifier 90. This sludge 92 is then recycled into the supply pipe 14. The recycling of the sludge 92 produces an auto catalytic effect on the rate of Fe ²⁺ oxidation. Research suggests that the time required to oxidize Fe ²⁺ can be decreased by 80% when Fe(OH) ₃ is supplied in sufficient quantities. In addition, hydrous oxides can provide catalytic sites for manganese oxidation, and iron hydroxides are known to co-precipitate manganese and other metals. The recycling of sludge thus can also assist in reducing manganese concentrations in the treated water. The recycling of sludge has a further benefit in increasing the density of solids produced. Essentially, each recycled particle grows in size as additional metal hydroxide precipitates on its surface. This results in a faster, more efficient clarification step, in addition to other benefits.

Examples

Pilot scale testing has been conducted on water obtained from a ground water source.

Test i The water tested had the following properties on supply to the system: pH: 5.92

Oxygen: 0.36mg/L dissolved O ₂ (4.4% saturation)

Iron: 27mg/L ferrous iron (Fe ²⁺ )

Alkalinity: 49mg/L (expressed as CaCOs) In a first test, two water samples were passed through a reactor vessel 50. The first of these samples, marked as 94, was not subject to an initial oxygenation step before being passed through the reactor vessel. The second sample, marked as 96, was pre-oxygenated to approximately 86% saturation (7.2mg/L).

Figure 8 shows a plot of the water pH against time after the water samples had passed through the reactor vessel and then continuously aerated using a diffuser- type aerator. It is apparent that oxygenation rapidly increases the pH of treated water. Indeed, pre-oxygenation increased the pH of water immediately after its passage through the reactor vessel by about one pH unit.

Test 2 Further testing was conducted on another ground water sample, having the following properties: pH: 5.9

Oxygen: 0.51 mg/L dissolved O ₂ (4.4% saturation)

Iron: 29mg/L ferrous iron (Fe ²⁺ ) Alkalinity: 50mg/L (expressed as CaCOa)

In these tests, compressed ambient air was injected into the inflow pipe 12 at varying distances from the reactor vessel 50, so as to vary the concentration of dissolved oxygen in the water entering the reactor vessel 50. In this test, the flow rate was 65m/hr at the base of the reactor vessel 50 and 29m/hr at the top of the reactor vessel, resulting in a hydraulic residence time of 6.9 minutes. An air injection rate equivalent to 0.283m ³ /minute for each square metre of cross sectional area of water at the top of the reactor was used to provide bubbling of air through the reactor.

Sludge 92 was recycled for these tests into the base of the reactor vessel 50 at a recycling ratio of 8 (that is, assuming a sludge density of 2.5%, sludge was continuously injected into the base of the upflow reactor to give a sludge concentration of approximately 440mg/L). The water was tested immediately after passing though the aerator 40, and then again after a further 30 minutes. The results were as follows:

Dissolved O ₂ Concentration in Fe "(mg/L) pH

Influent Water mg/L % Immediately 30 minutes after Immediately 30 minutes after post treatment treatment post treatment treatment

10.22 109.3 0 0 7.71 7.73

8.98 100.4 0.27 0 7.76 7.77

7.89 88.1 0.57 0 7.38 7.38

6.6 72.9 1.47 0 7.39 7.45

6.52 72.1 1.5 0.10 7.30 7.34

5.77 63.4 2.82 0.12 7.40 7.45

5.41 59.1 3.37 0.22 7.22 7.27

0.51 5.6 3.35 0.60 7.20 7.26

This data suggests that an influent water oxygen concentration in excess of 6mg/L assists in achieving complete Fe ²⁺ oxidation in a timely manner. Test 3

Testing of the effect of air bubbling in the reactor vessel 50 was conducted on mine void water having the following properties: pH: 2.92

Oxygen: 8mg/L dissolved O ₂ (>98% saturation) after aeration Iron: 12.7mg/L ferrous iron (Fe ²⁺ ) Sulphate: 276mg/L as SO ₄ Acidity: 106mg/L (expressed as CaCOa)

The reactor vessel was run for a period of 48 hours, with the limestone bed being fluidized by maintaining an upflow rate of 88m/hr at the base and 39m/hr at the top, without any air bubbles being supplied to the base of the reactor vessel. The properties of water exiting the reactor vessel (following aeration) were measured after one hour of operation, 24 hours of operation and 48 hours of operation. The results for pH, alkalinity and iron content are shown in Figure 9. After 48 hours limestone particles from within the reactor were examined, and showed a significant build up of armouring on their surfaces. Additionally, a layer of iron oxide/hydroxide was evident on the internal surfaces of the reactor vessel. Figure 9c shows that the total iron content of the effluent water during this period was significantly less than the influent water. This demonstrates that iron was accumulating within the reactor. The reactor vessel was becoming increasingly inefficient, as can be seen in the decrease in resulting pH and alkalinity over this time.

After 48 hours' operation, bubbles of ambient air were introduced at the base of the reactor vessel. This resulted in the immediate 'blowing off' of a plume of precipitated iron. Within two hours, the alkalinity increased from 51mg/L to 110 mg/L, and the pH increased from 7.19 to 7.48. (When a similar trial was conducted using CO2 enriched air recovered by stripping, the alkalinity increased further to 123mg/L)

Upon settling to a steady state condition in a further 24 hours, the high pH and alkalinity were maintained. Additionally, the total iron content being recovered from the vessel was equal to the inlet iron content, indicating that iron was no longer accumulating within the vessel. Importantly, all the ferrous iron had been oxidized.

Re-examination of the limestone particles showed that armouring had been removed.

Test 4

A further test was done, using ground water having the following properties: pH: 5.92

Oxygen: >7.9mg/L dissolved O ₂ after aeration Iron: 26.9mg/L ferrous iron (Fe ²⁺ )

Alkalinity: 49mg/L (expressed as CaCOs)

In this test, water was run through the reactor vessel for a trial period of 14 days.

Compressed ambient air was supplied to the base of the reactor to create gas bubbles during this time. The total hydraulic retention time between the water inflow into the reactor vessel

50 and sampling of effluent water following aeration was about 5 minutes.

Limestone contact time was about 2 minutes. The resulting properties of the effluent water were measured as follows:

This trial demonstrated that full oxidization of Fe ²⁺ continued to be achieved within the 5 minute retention time. This is much faster than has been achieved in other limestone media based systems. It also showed that there was some initial accumulation of iron within the media bed, suggesting that some armouring occurred. It was conjectured that such armouring was likely to have occurred on surfaces which were not subject to abrasion. This was confirmed by microscopic examination of the particles, which revealed pores and crevices in each particle which were filled with iron precipitates, with no armouring apparent on any exposed surfaces. Once these crevices were filled, the system reached an equilibrium position (after eight days) in which the total iron in the effluent water was equal to that of the influent water.

Measurements of water pH were also taken between the reactor vessel 50 and the aerator 40. It was found that pH at this location was consistently between 7.00 and 7.06, about 0.7 pH units lower than the levels recorded after aeration. This demonstrates the importance of CO ₂ removal to increase final effluent water pH.

Modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention. For instance, suitable coagulants and/or flocculants may be employed. The invention also envisages situations where the addition of small quantities of a chemical oxidant such as potassium permanganate may be applicable to enhance the oxidation of minerals that are more difficult to oxidise, such as manganese.