WO1992022504A1

WO1992022504A1 - Treatment of waste water

Info

Publication number: WO1992022504A1
Application number: PCT/AU1992/000272
Authority: WO
Inventors: Terence Charles Hughes
Original assignee: Unilever Australia Limited
Priority date: 1991-06-11
Filing date: 1992-06-10
Publication date: 1992-12-23
Also published as: EP0589953A1; BR9206149A; JPH06508060A; ZA924275B; CA2103247A1; EP0589953A4

Abstract

A method of treating waste water to remove contaminants therefrom comprises (a) treating the waste water with calcium carbonate under oxidising conditions and separating a precipitate containing Al and Fe hydroxides, therefrom; and (b) treating the partially neutralised waste water from step (a) with sodium carbonate and separating a precipitate containing further contaminants therefrom including at least one of Zn, Cu, Co, Ni, Cd, Al and Fe; and optionally also comprises (c) treating the liquid phase product of step (b) with one or more fatty acids or salts thereof to precipitate further contaminants therefrom, including at least one of Ca, Mg, and Mn, and (d) treating the liquid phase product of step (c) with one or more fatty acid amines to precipitate anionic contaminants therefrom including at least one of sulphate, phosphate and chloride ions.

Description

TREATMENT OF WASTE WATER

This invention relates to the treatment of waste water in order to reduce the level of contaminants therein and to produce environmentally acceptable products. It is of particular value in the treatment of acidic mine waste water.

In one aspect of the invention, a multi-step process is provided including certain steps which are novel per se. The process may include for example a multi-step process including a first step in which zinc, copper and/or cadmium may be extracted from the mine waste water as saleable products, while iron and aluminium may be extracted to be discarded as chemically neutral slurries or dried solids. Step 2 may include removal of the remaining heavy elements, calcium, magnesium and most of the sodium. Step 3 provides for the removal of sulphate and chloride, and step 4 may prepare or "polish" the water prior to on-site usage or off-site discharge. In preferred aspects of the invention, economic advantages are maximised by recycle of reagents, whereby consumption of reagents is significantly reduced, as well as by production of saleable products.

BACKGROUND OF THE INVENTION

The problems associated with acidic mine waste water in terms of water storage, water treatment and usage and mine rehabilitation are well documented and in this connection reference may be made to the detailed papers and

discussions in the proceedings of the International

Symposium Lisboa 90 "Acid Mine Water in Pyritic

Environments" (September 1990). The ubiquitous generation of acidic base metal mine waste water presents a serious problem in terms of water storage, water treatment and usage and mine rehabilitation. The acidic mine water (AMW) results from the bacterial

catalysed oxidation of base metal sulphides, particularly pyrite (iron sulphide). There is no general solution to the problem of the acid generation, however, there are many options to diminish acidic discharges especially where the oxidisable sulphides are located at or near the

topographical surface (e.g. tailings, dams and dumps, waste rock dumps and shallow mine workings and open pits). These techniques include barrier methods that isolate the

sulphides (mainly pyrite) from oxygen or oxygen containing water, and the use of chemical additives to inhibit the growth or development of iron-oxidising bacteria. Recent procedures have included the incorporation of large amounts of alkalinity and/or phosphate within the sulphides, the use of surface geophysics to identify potential problem areas, the sealing of fractured stream beds using polythene or silicate based grout and the use of anionic surfactants and bacteriocides to inhibit the activity of iron oxidising bacteria. These conventional methods have all attempted to minimise the formation of the acidic mine water but are subject to possible failure due to the vagaries of nature with potentially dangerous results.

The approach detailed below aims to treat the acidic mine water, once formed, in a chemically efficient and cost effective manner to produce high quality dischargeable water and marketable extracted base metals and salts in an environmentally acceptable package.

PRIOR ART

US patent 4,652,381 discloses a process of treating

industrial waste water contaminated with environmentally unacceptable amounts of sulfuric acid and heavy metals such as lead, copper or zinc which permits lowering of the concentration of the contaminants to a level permitting discharge to the sewer. The water to be treated is

directed to a first reaction and settling vessel where calcium carbonate is added along with an oxidation medium such as air which also functions to stir the stored waste water. Sufficient calcium carbonate is added to bring the pH of the solution to a level of about 5 and at the same time react with the heavy metals present such as lead, copper or zinc. Calcium sulfate and respective heavy metal carbonates precipitate and settle to the bottom of the treatment zone where they may be readily removed. In a second treatment vessel, calcium hydroxide along with enough calcium carbonate to maintain an excess of carbonate ion are added to complete separation of the heavy metals. Final removal of precipitate from the solution is

accomplished through a suitable filter.

US patent 5,013,453 discloses a method which can be used to remove dissolved heavy metals and/or iron from nearly any aqueous stream. The invention is particularly useful in removing the large concentrations of copper, nickel, zinc, gold, silver, cadmium, tin, chromium and lead from pickling acid wastes and other acidic waste streams formed in the metal finishing industries. In the method of the

invention, a selected carrier precipitate is created within an aqueous waste solution which is contaminated with heavy metals and/or iron. The contaminants are thereby caused to coprecipitate with the carrier precipitate and are thus removed from the aqueous solution.

JP 80049555 discloses treatment of waste water containing heavy metals and organic matter involving biological oxidation to decompose the organic matter. Heavy metals are converted to their carbonates and the pH is

subsequently raised to precipitate heavy metal hydroxides, the precipitate also containing heavy metal carbonates.

In JP 59203692, waste water from pulp production is

oxidised and treated with calcium carbonate and a high molecular weight organic flocculant.

US patent 3,725,266 discloses a process for removing one or more metal compound contaminants from contaminated

industrial waste water comprising precipitating the metal compound contaminants from the waste water to form an aqueous metal compound sludge, allowing the sludge to settle, concentrating the sludge by centrifugation and reclaiming the metal compound contaminants from the sludge for reuse as either the metal compounds or the free metals. The preferred precipitating agents as hydroxides and carbonates, such as, sodium hydroxide, calcium hydroxide and sodium carbonate, which produce the insoluble metal hydroxides and carbonates, respectively.

CHEMISTRY OF ACIDIC MINE WATER FORMATION AND CONTROL Prevention and/or control of acidic mine water (AMW) formation depends upon an understanding of the chemical, biological and geological characteristics of base metal sulphides. A series of chemical reactions (given below) describe AMW formation which results from the exposure and weathering of pyritic material (FeS₂) normally present in base metal mine and coal wastes to the combined effects of atmospheric oxygen, water and iron and sulphur oxidising bacteria such as

Thiobacillus ferrooxidans (T. ferrooxidans),

Ferrobacillus ferrooxidans (F. ferrooxidans) and

Thiobacillus thiooxidans (T. thiooxidans).

2FeS₂ + 70₂ + 2H₂O = 2Fe²⁺ + 4S0₄ ^2- +4H⁺ - - - - - (1)

Fe²⁺ + 1/2 O₂ + 2H⁺ = Fe³⁺ + H₂O - - - - - (2 )

(bacterial assistance)

Fe³⁺ +3H₂O = Fe (OH) ₃ (s) + 3H⁺ - - - - - (3 )

The stoichiometry of equation (1) shows that one mole of FeS₂ produces two moles of acid (H⁺). In turn Fe²⁺

generated by reaction (1) is oxidised into Fe³⁺ and

produces an additional three moles of acid (equation 3). The net result is that for every mole of pyrite oxidized, four equivalents of acid (H⁺) are produced.

As the pH in the immediate vicinity of the pyrite falls to less than 3, the increased solubility of iron and the decreased rate of Fe(OH) precipitation affects the overall rate of acid production. At this point, ferrous iron is oxidised by T. ferrooxidans and the ferric iron in turn oxidizes the pyrite : FeS₂ + 14 Fe³⁺ + 8 H₂O = 15 Fe²⁺ + 2SO₄ ^2- + 16H⁺- - - - - (4)

The rate of acid production is high and is limited by the concentration of ferric ions. Fe³⁺ activity becomes significant at a pH of approximately 2.5; a vicious cycle of pyrite oxidation and bacterial oxidation of Fe²⁺ results from the combined effects of reactions 2 and 4. The rate of reaction 2 exerts primary control on. the cycle by limiting the availability of Fe³⁺ which is the major oxidant of pyrite.

The presence of the acidic environment together with the oxidised Fe (and probably Mn) readily attacks other base metal sulphides present allowing the dissolution of Cu, Zn, Cd, Co and Ni and other trace metals present in the

sulphides (e.g. Mo, Sn, Ag, Hg, Sb, Bi). The high sulphate levels will result in Pb being precipitated as an insoluble sulphate following the decomposition of Pb ore (galena).

Some occlusion of Ag in the Pb sulphate will probably occur and less soluble basic salts of other metals may also precipitate (e.g. Sn, Mo). The products of oxidation of the listed metal sulphide ores by T. ferrooxidans are shown in the following table:

METAL SULPHIDE ORES OXIDISED BY T. FERROOXIDANS

Mineral ore Formula Soluble Ions Arsenopyrite Fe S₂ Fe As₂ Fe, some As³⁺ at very low pH values

Bornite Cu₅ Fe s₄ Cu Fe

Chalcocite Cu₂ S Cu

Chalcopyrite Cu Fe S₂ Cu Fe

Covellite Cu S Cu

Enargite 3 Cu₂S As₂ S₅ Cu (As)

Galena Pb S Insoluble

sulphate

Marcasite Fe S₂ Fe

Millerite Ni S Ni

Molybdenite MoZ₂ Mo

Orpiment As₂S₃ (As)

Pyrite Fe S₂ Fe

Sphalerite Zn S Zn

Tetrahedrite Cu₈ Sb₂ S₇ Cu (Sb)

Further appreciation of the nature of the problem may be gained by comparison of a waste water composition set out in the following Table 1, with the impurity levels allowed by the New South Wales (Australia) State Pollution Control Commission (SPCC) for Schedule 2 Discharge Water together with U.S. Drinking Water Standard Requirements in Table 2. The data in Table 1 are from Woodlawn Mine, Tarago, N.S.W. Australia.

TABLE 1. - WASTE WATER COMPOSITION (ppm)

Element North Dam South Dam Lake Rex or ion Water Water Water

Cu 100 77 41

Pb 1.5 1.4 0.2

Zn 3000 1860 737

Fe 200 92 41

Cd 12 9.3 5.0

Mn 130 84 31

Na 300 370 82

K 3 2 1

Al 610 355 170

Ca 400 390 140

Mg 1700 920 330

SO 16900 9420 3880

Cl 200 120 550 pH 3.1 3.2 3.3

TABLE 2. WATER QUALITY STANDARDS

Parameter SPCC Sch -2. (ppm) U8 EPA Drinking

Water (ppm)

Sb -

As III 0.05 As V 0.05 Ba 1.0

Be - Cd 0.01 0.01 Cr III 0.05 Cr IV 0.05 Cu 1.0 1.0

CN 0.22* Fe 0.3 0.3

Pb 0.05 0.05 Mn 0.05 0.002

Hg 0.002

Ni -

NO₃ 10

NO₂ -

Se 0.01

Ag 0.05

S -

Tl -

Zn 5.0 5.0

SO₄ 250

Cl 250

pH 6.8 - 8.5 5 - 9

* total CN - present as complex ion e.q. with Fe, Cu, Ni DEVELOPMENTAL EXPERIMENTATION

Experimental measurements showed that good cation and anion removal from the Woodlawn AMW could be achieved using ion exchange techniques but capital costs and reagent costs were high for a purification plant operating solely on ion exchange (IX) processes. Likewise Reverse Osmosis (RO) or Electrodialysis (EDR) is capable of providing high quality water when used as a stand-alone-technique but is

inefficient in the presence of high levels of dissolved solids and membrane fouling may occur due to gypsum (CaSO₄) precipitation or the presence of organic contaminants.

Both these techniques (RO and EDR) and probably IX are ideal for "final polishing" of the water prior to discharge but a relatively cheap and efficient chemical pretreatment operation is required to remove the bulk of the dissolved solids to decrease the operational cost of IX, RO or EDR.

Development of the process of the invention evolved through several stages employing various techniques, as follows.

1. Ion exchange processes Cation and anion ion exchangers were used to investigate the removal of soluble ions from the Woodlawn AMW. The cation ion exchanger removed all cations other than the alkali metals (sodium and potassium) very efficiently. The capacity for the alkali metals was low but was probably acceptable and the AMW could be fed directly to the resin. Regeneration was achieved with hydrochloric acid followed by conversion to the sulphate form for use.

The removal of sulphate was more difficult and both strong and weak base anion exchangers were used. Again stripping and regeneration was achieved.

The overall costing for a large AMW treatment plant was uneconomic in terms of both capital costs and the

regeneration reagent costs. A further disadvantage was the inability to handle organic non-ionic contamination and the difficulty of obtaining elemental separations without very careful reagent elution control.

2. Chemical precipitation with RO and sodium sulphate crystallisation.

A series of experiments were carried out aiming at

producing a zinc carbonate/zinc hydroxy carbonate

precipitate directly from the Woodlawn AMW. A stepwise addition process was developed using a range of pH

adjusting chemicals to change the pH of the AMW such that the precipitation of the component cations occurred in a controlled manner. Neutralisation was carried out

successively with CaCO₃ in stage 1 to a pH about 5, and a residue separated containing Fe and Al. In stage 2, treatment with Na₂CO₃ raised the pH to 8 to 8.2, and a residue separated containing Zn as a basic carbonate, as well as Cu, Cd, Co and Ni. In stage 3, lime was added to increase tήe pH to 10 to 10.5 to precipitate Mg and Mn, and in stage 4 NaOH was added to increase the pH to 11 to 12, thus precipitating Ca and more Mn. After pH adjustment with sulphuric acid, the liquid phase product of stage 4 was treated by reverse osmosis to produce high quality clean discharge water and the concentrate was crystallised to produce a marketable sodium sulphate. Other products were zinc carbonate, zinc oxide and zinc metal and a Cu, Cd, Co, Ni fraction which was readily converted to a Cu cementate.

Extensive testing showed the process to be technically viable but expensive. 3. Chemical precipitation with RO

The process is similar to 2. (above) but the concentrate from the RO is directed to an evaporation dam. The cost is reduced, but with a potential rehabilitation problem occuring in terms of the long term operation of the evaporation dam.

4. Chemical precipitation with an evaporator This process replaced the RO with a vapour recompression evaporator and the residue again was directed to an

evaporation dam. The cost was slightly lower than 3.

(above) but with the same long term rehabilitation problems with the dam. Further Process Development

A review of the AMW technology showed that although

discharge quality water could be produced using the

combination of pH adjustment/precipitation and RO, EDR or Evaporators, there were a number of problem areas. These were:

1. Cost of treatment - both capital and running costs were too high.

2. Increasing levels of calcium and sodium.

The slow but continuous precipitation of gypsum (CaSO₄) on membrane and evaporator surfaces causes a rapid loss of performance.

- Sodium salts are very soluble and are expensive to crystallise.

3. High reagent consumption

- The addition of pH or Eh adjusting reagents to

change the acidity or alkalinity of the total volume of water is often an inefficient way of removing minor components.

4. Potential build up of problem contaminants

- The progressive build up of magnesium (difficult to precipitate at a pH of less than 10 - 11) and manganese (difficult to precipitate quantitatively with pH adjustment alone) causes purity problems in the proposed sodium sulphate product.

5. Slow processing speeds

- The use of settling tanks or thickeners and bulk

filtration are often slow processes if the solids are bulky or gelatinous. Hydroxides in particular are often slow to filter.

6. Contamination from particulate or organic components

A pretreatment filtration or settling system may be required.

A cost effective study of the AMW purification process showed that the first two stages were justified in terms of efficiency and peformance. e.g. Stage 1

AMW (pH 2.5 - 3.3) is treated with crushed limestone in an agitated tank. A slight excess of limestone is required. The pH of the water rises to 4.9 - 5.0 and results in the precipitation of 90 - 95% of the Al and Fe. Oxidising conditions are maintained using air sparging. This precipitate settles quickly and can be removed by thickening, it also acts as a feed water particulate removal stage with a significant reduction in organic content due to adsorption on the bulky Fe/Al hydroxides.

Stage 2.

The partially neutralised water from Stage 1. (pH 4.9 - 5.0) is then treated with sodium carbonate (Na₂CO₃) in an agitated tank. The pH of the water rises to 8.0 to 8.2 and results in the precipitation of Zn, Cu, Co, Ni and Cd and the remaining Al and Fe. The precipitate is processed to produce marketable products. Zn is converted to zinc oxide or zinc metal and Cu is

produced as a cementate containing the Co, Ni and Cd. The Al from Stage 1. together with Al from the Stage 2. precipitate can be extracted to market a chemically purified Al product such as aluminium sulphate.

The following stages using pH adjustment over a wide range (to remove Ca and Mg) and expensive plant (RO., EDR,

Evaporators) were reviewed and further development showed the possibility of a more effective technology based on fatty acid derivative chemistry, as well as a further more effective technology based on the use of organic bases to remove soluble anions.

THE APPLICATION OF FATTY ACIDS TO THE TREATMENT OF ACIDIC MINE WASTE WATER Following the cost effective removal of Fe, Al, Zn and Cu from the acid mine water by the stepwise pH adjustment using limestone and sodium carbonate, a novel approach to the removal of other cations has been developed. Long chain fatty acids are well known for their formation of insoluble precipitates with Ca and Mg (e.g. use of soap in hard water). Experimental work at Woodlawn Mine on AMW has demonstrated that Oleic (C₁₈H₃₄O₂) and Stearic

(C₁₈H₃₆O₂) acids effectively precipitate Ca, Mg plus other heavy metals from the partially treated Woodlawn waste water. These C-18 acids were most conveniently added as their soluble Na salts and were shown to be more efficient for cation removal than the shorter chain length WSR-C reagents. ppm Cu Zn Fe Mn Ca Mg Al Cd SO

Feed Water <0.1 2.5 <0.1 32 495 600 0.24 .16 9440 pH 8.3

After Oleic <.01 0.15 <.01 0.01 23 55 0.05 <.01 9240 Treatment

After <.01 0.02 <.01 0.01 5 15 <.01 <.01 9100

Stearic

Treatment

Conditions 5 gms Fatty Acid as Na salt to 11 L

pH 8.3 Feed Water stirred in flotation cell for 20 mins. Temperature used = 20-22°C.

The metal fatty acid salts can be removed by filtration or flotation and on acidification (pH1-2) with hydrochloric acid the fatty acids are regenerated and the cations solubilised for treatment as a concentrated solution.

Some removal of Na is also observed but the efficiency of alkali metal removal is very dependant on fatty acid composition and temperature.

The use of the fatty acids allowed the simple removal of Ca and Mg and also reduced the remaining heavy metals to levels suitable for discharge. Also the application of ultra filtration (UF) and microfiltration (MF) techniques using ceramic filters or filter pads is promising for the rapid, continuous and efficient removal of the fatty acid metal salts.

THE USE OF ORGANIC BASES TO REMOVE SOLUBLE IONS FROM ACID MINE WATER

Extensive research and development has demonstrated the ability to remove the cations from AMW in a cost effective manner. However, prior to the present invention, the removal of the corresponding anions (essentially sulphate, phosphate and chloride) has only been achieved by

crystallising sodium salts which is expensive in terms of both capital and running costs. We have now developed an alternative approach using organic bases, such as

nitrogen-containing fatty acid derivatives, to form insoluble or partially soluble salts or sulphate micelles which can be removed from the AMW.

Organic bases such as amines (primary, secondary and tertiary), amides, diamines and quaternary ammonium

compounds can form insoluble or partially soluble salts with sulphate anions in acidic aqueous solutions. Chloride salts formed with the organic bases tend to be more soluble but some co-removal of chloride with the sulphate

precipitates is often observed.

The following considerations need to be taken into account for anion precipitation:

(a) The solubility of sulphate or chloride salts or

compounds in acid mine water or processed feed solutions and the effect of other cations or anions.

(b) The composition of the feed water e.g. pH, Eh, ionic content, organic content.

(c) Feed water temperature - all sulphate and chloride salts appear to be much more soluble in hot water. (d) The ability to recycle the reagents e.g. treat anion precipitate with Na₂CO₃/NaOH at pH 10-11 to reform the amine.

(e) The use of other reagents to bring about the hydrogen substitution to reform the amine and release the anion as an acid. (f) The availability of a water soluble (pH 6-8) reagent which precipitates sulphates would have a distinct advantage in terms of removing anions after the cation precipitation stages. Possibly the formation of sulphate micelles can be considered e.g. with amine oxides and surfactants.

(g) For ease of reagent handling amine gel formation must be avoided.

Anion Precipitation Experiments Bench scale experiments were carried out using brucine, benzidine and dodecylamine.

(a) To precipitate sulphate from dilute hydrochloric acid - only benzidene was successful, however, this is an established analytical procedure; the use of

benzidine is not to be recommended, from a safety viewpoint.

(b) Experiments on Woodlawn AMW pH 3.1, SO^2- 9500ppm, Cl- 120ppm. Note: SPCC discharge limits are:

SO₄ ^2- less than 250ppm

Cl- less than 250ppm

Procedure - The reagent was dispersed/dissolved in warm dilute hydrochloric acid (2% v/v) and

approximately 10gms of each amine were added with stirring to 1L of AMW. The precipitate was allowed to settle and was removed by decantation and

filtration. The filtrate was assayed for sulphate.

The precipitate obtained from the AMW was brown (not white as expected for a pure amine sulphate) which indicated the coprecipitation of other species with the sulphate. Results :

Run No. NDW 1 NDW 1 NDW 3

Temperature 40°C 25°C 22°C

Reaction Time 10 mins 10 mins 30 mins

SO_42- levels in ppm

Benzidine 120 50 30

Brucine 300 200 100

Dodecylamine 350 200 100

After filtration the precipitate was treated with

Na₂CO₃/NaOH at pH 10-11 to recover the amine and to

solublise the anions. Recoveries of 85-90% were obtained.

The precipitation of sulphate from Woodlawn Acid Mine Water (WAMW) feed (pH 3.1) with amines has promise but in

practice the addition of the amine as a soluble chloride would result in exchanging sulphate ions for chloride ions.

To investigate the removal of chloride, the reagents were dispersed in dilute formic acid (1% v/v) and this resulted in a reduction of chloride ion content with brucine and dodecylamine (from 120 ppm to 80 ppm). The removal of anions by precipitation as described above can be applied to the AMW feed directly, however it is best applied following the removal of cations with fatty acid salts, which is discussed in more detail below.

Further Experiments on Sulphate Removal Test solutions were prepared from mixtures of Sulphuric

Acid, Sodium Sulphate and Sodium Hydroxide to give a 3,000 ppm and 10,000 ppm sulphate solution at pH values of 3, 5, 7 and 8. A temperature of 30°C was maintained during the

experiments.

The surfactants used were -

Sodium lauryl sulphate (SLS)

Alkyl benzene sulphonate (ABS)

Ligoin sulphonate (LS)

Dodecylamine (in about 2% solution) was dispersed in 1% (v/v) HCl and added in slight excess to the sulphate test solutions. These were then stirred for 30 minutes. the dispersions were treated with 0.2 ml 0.1% surfactant and stirred for a further 2 minutes.

Solid/liquid separation was achieved using an 0.45 micron micro filtration assembly.

Results

Feed-3,000 ppm Sulphate Solution

Filtrate SO₄ content (ppm)

Surfactant SLS ABS LS

pH of feed solution

3 550 300 160

5 350 200 200

7 150 200 150

8 250 450 450

Feed-10,000 ppm Sulphate Solution

Surfactant SLS ABS LS

pH of feed solution

3 600 250 150

5 300 200 200

7 100 300 130

8 250 500 400 The maximum permitted discharge levels for sulphate is 250 ppm. The results obtained indicate that low sulphate levels can be achieved in aqueous effluents (below 250 ppm) using the technique of surfactant induced micelle formation with separation by micro-filtration following the formation of sparingly soluble amine sulphate salts.

The Discharge of Treated Water

Local EPA requirements usually define water quality

characteristics which must be achieved prior to discharge. Practical approaches have included RO, EDR and

Evaporation as final stage purifiers or "water polishing" techniques. However these processes are expensive to install and run and the use of "biobed" systems is

preferred. The "biobed" system uses living biological systems to absorb the remaining heavy metal pollutants. The water is allowed to pass through a series of weir structures, constructed to allow adequate contact with the biological absorbers. Systems using bacteria, algae, moss and simple plants have been used with considerable success.

Another technique utilises the percolation of water through columns of mineral absorbers such as marble chips, clays or zeolites and soils. These materials act in both an

absorption and ion exchange capacity as well as filtering out any remaining particulate matter.

THE INTEGRATED PROCESS

A preferred embodiment of this invention is illustrated in the accompanying Figure 1, wherein:

Acid mine water A, at a pH of 2.5 to 3.3, is treated with crushed limestone 1.1 in an agitated tank 1. The pH of the water rises to 5.0 and results in the precipitation of 90 to 95% of the Al and Fe. Oxidising conditions are

maintained using air sparging.

The product of this stage is separated into a solid phase 1.2 and a liquid phase 1.3. The solid phase containing the Al, Fe residue is removed at 1.4.

The. partially neutralized water from stage 1 (liquid phase) at pH about 5 is then treated with Na₂CO₃ 2.1 in an

agitated tank 2. The pH of the water rises to 8 to 8.2 and results in the precipitation of Zn, Cu, Co, Ni, Cd and the remaining Al and Fe. The product of this stage is

separated into a solid phase 2.2 and a liquid phase 2.3.

The precipitate (solid phase) is treated via 2.4 to remove for marketing Zn as metal and/or oxide and Cu with Co, Ni, Cd as a cementate. It may also be possible to extract the Al and market a chemically purified Al product. In stage 3, soluble fatty acid salts 3.1 are added to the treated water 2.3 at pH 8 to 8.3 to precipitate Ca, Mg, Mn, some of the Na and to scavenge the remaining heavy metals. The precipitate 3.2 is removed either by flotation or filtering via 3.4 and treated to recover the fatty acids for

recycling, and the Ca, Mg, Mn for marketing as chemically purified salts (e.g. CaCO₃, CaSO₄, CaF₂, MgSO₄, MgF₂, MgO, MnO₂) as described in more detail below. The liquid phase 3.3 is treated in stage 4 with fatty acid amines 4.1 to precipitate the anions, sulphate and

chloride. The aromatic amines are relatively stable and can also be recycled. The product of stage 4 is separated into a solid phase 4.2 and a liquid phase 4.3 by

ultra-microfiltration.

Removed amine sulphate 4.4 is subjected to alkali treatment in 4.5 to separate the amine, with production of sulphate salts or sulphuric acid removed via 4.8. The amine is purified in 4.6 and solubilized in 4.7 and recycled to 4.1.

Only minor "polishing" of the liquid phase 4.3 is required. A reverse osmosis (RO) stage was originally contemplated as a final "polishing" step prior to discharge, however, it is believed that in most cases this will not be necessary to meet the SPCC Schedule 2 specifications.

An algal/plant "biobed" system 5 or a marble/clay column is now preferred as the final stage, because only minor

"polishing" is required. Clean water is discharged via 5.1 and an algal-plant residue removed at 5.2.

The solid phase 3.2 recovered from stage 3 via 3.4 contains Ca, Mg, Mn, (Na) and traces of other heavy metals as fatty acid salts, and is treated in the following manner. The said solid phase is acidified in 3.5, producing a solution of cations in acid 3.6, from which products 3.7 are

obtained, comprising MgSO₄, MgO, MgF₂, CaSO₄, CaCO₃, CaF₂. The fatty acids separated from 3.5 are subjected to

saponication in 3.8 to produce sodium fatty acid salts 3.9 which are recycled to 3.1.

The success of this treatment is illustrated by the process liquor composition set out in the following table 3.

TABLE 3 .

PROCESS LIQUOR COMPOSITION (PPM) REFER FIG. 1)

LIQUOR FROM ELEMENT 1 2 3 4 5

Zn 3000 50 0.3 <0.1 <0.1

Fe 10 0.1 0.1 <0.1 <0.1

Cu 90 0.1 0.1 <0.1 <0.1

Al 50 0.1 0.1 <0.1 <0.1

Cd 12 1 0.1 <0.1 <0.1

Co 7 1 0.1 <0.1 <0.1

Ni 11 1 0.1 <0.1 <0.1

Ca 600 500 20 10 10

Mn 130 50 1 <0.1 <0.1

Mg 1750 1750 20 10 10

Na 300 2700 800 400 400

SO 14000 14000 12700 250 200

Cl 200 200 200 200 200

APPLICATION TO OTHER FIELDS OF THE ACID MINE WATER TREATMENT TECHNOLOGY The process of the present invention can be applied to a wide range of waste waters, in addition to acid mine waste water.

The process of the present invention utilizing the stepwise removal of components from the waste water is a flexible approach which can be applied to a wide range of waste waters. Other waters derived from the mining industry are obvious areas of application but potential exists in other industries for a cost effective procedure which produces high quality discharge water and marketable products which were previously thought to be waste materials. Also in many cases the quality of plant or agricultural feed water is inadequate and potential exists for water purification. Major Industrial Users of Water

Pulping and paper making

Steel production

Power generation

Alumina refining

Meat industry

Coal washing

Dairy industry

Coal to oil conversion

Potato processing

Iron ore mining and benefication

Food canning

Brewing

Wool yarn production

Wool securing

Petroleum refining

Extractive metallurgical industries

Major Industrial Producers of Aqueous Effluents Textile industry

Plastic Materials and Synthetics

Pharmaceuticals

Paint and Coatings

Organic Chemicals and Pesticides

Explosives

Petroleum Refining

Rubber Products

Leather Tanning and Finishing

Ore upgrading and metal smelting

Electroplating and metal finishing

Electronic components

Storage and Primary Batteries

Machinery and Instrument Manufacture

It will be clearly understood that the invention in its general aspects is not limited to the specific details referred to hereinabove.

Claims

CLAIMS :

1. A method of treating waste water to remove

contaminants therefrom, which comprises the following steps:-

(a) treating the waste water with calcium carbonate under oxidising conditions to partially neutralise the waste water and form a precipitate containing Al and Fe hydroxides, and separating the precipitate from the

partially neutralised waste water, and

(b) treating the partially neutralised waste water from step (a) with sodium carbonate whereby further contaminants including at least one of Zn, Cu, Co, Ni, Cd, Al, Mn, Fe and other heavy metals are precipitated, and separating the precipitate formed in this step from the liquid phase.

2. A method according to claim 1, further comprising the following step (c):-

(c) treating the liquid phase product of step (b) with one or more saturated or unsaturated fatty acids or salts thereof to precipitate further contaminants therefrom, including at least one of Ca or Mg, and the remaining proportion of the heavy metal contaminants, and separating the precipitate formed in this step from the liquid phase.

3. Method according to claim 2 further comprising the following step (d):-

(d) treating the liquid phase product of step (c) with one or more amines to precipitate anionic contaminants therefrom including at least one of sulphate or phosphate and other soluble anions, and separating the precipitate formed in this step from the liquid phase.

4. Method according to claim 2 in which the waste water also contains organic contamination, and organic matter is adsorbed onto and removed with the precipitate formed in step (c).

5. Method according to claim 2 in which the liquid phase product of step (b) is treated with sodium oleate.

6. Method according to claim 2 in which the liquid phase product of step (b) is treated with sodium stearate.

7. Method according to claim 3 in which the amine comprises at least one of brucine, benzidine, dodecylamine or fatty acid amines.

8. Method according to claim 3 in which comprises treating the liquid phase product of step (d) with

dodecylamine and subsequently with a surfactant to induce micelle formation, and removing sparingly soluble amine salts thus formed by micro-filtration.