OA18661A - Improved effluent treatment process for sulphate removal. - Google Patents

Abstract

An acid waste water treatment and method wherein heavy metal hydroxides and gypsum are precipitated in a single operation and wherein amorphous aluminium trihydroxide and gypsum are separated in a single solid-solid separation unit.

Description

IMPRQVED EFFLUENT TREATMENT PROCESS FORSULPHATE REMQVAL

BACKGRQUND OF THE INVENTION

[0001] This invention relates to a process for the removal of heavy metals, calcium and sulphate from contaminated water, typically mine waters.

[0002] Effluent streams, and in particular acid mine drainage water, are generally acidic with pH values as low as 1,5. Another characteristic is the high levels of heavy metals, calcium and sulphate associated with the water. Prior to discharge into the environment these waste streams are normally neutralised with lime, a process which leaves large quantifies of calcium sulphate in solution. The release of such waters into the environment poses a significant environmental challenge.

[0003] International patent application number PCT/GB98/01610 describes a process, generally referred to as the SAVMIN process”, which was developed particularly for the treatment of sulphate-containing mine waters as well as sulphate-containing waste/effluent waters. This process allows for the effective removal of sulphate and calcium from effluent water with the use of amorphous aluminium trihydroxide followed by a subséquent recovery of the latter reagent by decomposing a waste product.

[0004] The SAVMIN process is fully described in the spécification of the aforementioned patent application and the content of that spécification is hereby incorporated fully into this spécification.

[0005] In one stage of the SAVMIN process, a saturated calcium sulphate water stream (produced by preliminary steps) is combined with amorphous aluminium trihydroxide and a neutralising agent, for example hydrated lime, for the removal of sulphate and calcium from solution, to promote the précipitation of ettringite which is removed from the water stream, e.g. by settling, to produce a slurry.

[0006] This is followed by the recovery of amorphous aluminium trihydroxide by decomposing the ettringite slurry at a pH ranging from 4 to 8,5. The pH is lowered by adding sulphuric acid (H₂SO₄), resulting in the formation of a supersaturated calcium sulphate solution.

[0007] The solids resulting from the décomposition step are gypsum and amorphous aluminium trihydroxide. These solids are separated from one another by means of a suitable solid-solid séparation unit, for example, a hydro-cyclone(s).

[0008] The recovered amorphous aluminium trihydroxide is recycled to treat a water stream containing sulphate and calcium. This recovery step ensures that the SAVMIN process is highly cost effective when compared to alternative processes such as ion exchange and membrane séparation techniques.

[0009] The SAVMIN process, however, is characterised by a relatively large number of solid/liquid séparation steps.

[0010] An object of the présent invention is to reduce the number of unit operations which are used in the SAVMIN process (as described in the SAVMIN spécification). This, in turn, results in process simplification and ease of operation, and lowers capital and operating costs.

SUMMARY QF THE INVENTION

[0011] In a (preliminary) step 1 of the SAVMIN process (PCT/GB98/01610) the pH of the acid waste water is raised so that heavy metals precipitate out of solution in the form of hydroxides. The précipitâtes are separated from the waste water by using a solid-liquid separator 10 to generate a first supersaturated calcium sulphate-containing solution. Thereafter, in a step 2, the supersaturated solution is de-supersaturated by using gypsum seed to remove the calcium sulphate as gypsum in a high solid precipitator 12, thereby forming a first saturated calcium sulphate-containing solution which is then treated with amorphous aluminium trihydroxide.

[0012] According to one aspect of the présent invention the heavy métal hydroxides and the gypsum are precipitated in a single unit operation, thereby eliminating a reactor unit and a solid-liquid séparation unit.

[0013] Figure 2 in the SAVMIN patent spécification illustrâtes the recovery of amorphous aluminium trihyroxide from ettringite wherein the ettringite slurry is decomposed by lowering its pH by the addition of sulphuric acid. A second supersaturated solution of calcium sulphate is formed with amorphous aluminium trihydroxide in suspension. The amorphous aluminium trihydroxide is then separated from the second supersaturated solution in a liquid-solid separator 18. Following the removal of the amorphous aluminium trihydroxide, the supersaturated calcium sulphate solution is de-supersaturated by removing calcium sulphate as gypsum using a liquid-solid separator tor 22.

[0014] In the présent invention, the formation of the amorphous aluminium trihydroxide and the gypsum is carried out in one reactor and a single solid-solid séparation unit is used to separate the amorphous aluminium trihydroxide and the gypsum.

[0015] In accordance with this aspect of the invention there is provided a method for the removal of sulphates and calcium from an acidic water stream which includes the steps of:

(1) raising the pH of the water stream to precipitate impurities from the stream and to form a first supersaturated calcium sulphate-containing stream;

(2) removing the impurities and de-supersaturating the first supersaturated calcium sulphate-containing stream in a first liquid-solid séparation step to form a first saturated calcium sulphate-containing solution;

(3) adding amorphous aluminium trihydroxide to the first saturated calcium sulphate-containing solution to form a product water stream containing precipitated ettringite;

(4) removing the precipitated ettringite, in a slurry, from the product water stream in a second liquid-solid séparation step;

(5) lowering the pH of the ettringite slurry to recover amorphous aluminium trihydroxide in a second supersaturated calcium sulphate-containing stream, and (6) removing the amorphous aluminium trihydroxide in a solid/solid séparation step to form a third supersaturated calcium sulphate-containing solution.

[0016] In step (1) of this method, the pH may be increased by adding caicium hydroxide, calcium oxide or hydrated lime to the acidic water stream. The pH is preferably raised to a value of between 10.0 and 12.0.

[0017] The impurities may include iron, aluminium, manganèse, magnésium and other heavy metals. These impurities are precipitated out of solution as hydroxides.

[0018] Following step (4), the pH of the product water stream may be lowered by adding CO₂ to precipitate calcium carbonate. The calcium carbonate may be separated from the product water, in a third liquid-solid séparation step, to form a purified water.

[0019] In step (2) the first supersaturated calcium sulphate-containing streams may each be de-supersaturated by removing calcium sulphate in the form of gypsum.

[0020] In step (5) of this method, the pH of the ettringite may be lowered the addition of an acid such as sulphuric acid, or hydrochloric acid, or CO₂ or SO₂. The pH is lowered to a value between 4 and 8.5. Preferably, the pH is lowered to a value between 8 and 8.5.

[0021] The second and third supersaturated calcium sulphate-containing streams may include calcium sulphate in the form of gypsum. The gypsum may be in a crystallised form.

[0022] In step (6), the solid/solid séparation may be achieved by means of size exclusion, wherein particles of the crystallised gypsum are larger than particles of the amorphous aluminium.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The invention is further described by way of example with reference to the accompanying drawings which, in combination, constitute a flow sheet for the SAVMIN process which incorporâtes modifications according to the présent invention, and wherein, specifically:

Figure 1 shows a first stage which embodies a heavy métal and gypsum précipitation stage,

Figure 2 shows a second stage which embodies an ettringite précipitation stage.

Figure 3 shows a third stage which embodies a carbonation stage, and

Figure 4 shows a fourth stage which embodies an ettringite décomposition stage.

DESCRIPTION OF PREFERRED EMBODIMENT

[0024] Figures 1 to 4 illustrate aspects of four stages of an effluent treatment process based on the SAVMIN process which is modified in accordance with the teachings of the présent invention. These stages involve the removal of metals and sulphate at ambient conditions from contaminated mine waters.

[0025] Figure 1 illustrâtes a modified first stage of the SAVMIN process. In a step 10 waste water 12, typically acidic mine water, is contacted with an alkali 14 such as hydrated lime (Ca(OH)2) to form a first supersaturated calcium sulphatecontaining stream 16 at a pH between 10.0 and 12.0. The supersaturated calcium sulphate-containing stream 16 contains solids 18 in the form of crystallised gypsum and precipitated impurities such as heavy métal hydroxides. The solids 18 are removed from the stream 16 in a liquid-soiid séparation step 20 to form a first saturated calcium sulphate solution 22.

[0026] In the SAVMIN process the precipitated impurities and the gypsum are removed in separate liquid-soiid séparation steps (see Figure 1 - blocks 1 and 2 of the SAVMIN patent spécification).

[0027] In a step 24 in a second stage of the présent invention, shown in Figure 2, amorphous aluminium trihydroxide 26, hydrated lime 28 and a “top-up aluminiumcontaining stream 27 (in the form of aluminium trihydroxide or aluminium sulfate) are added to the saturated calcium sulphate solution 22 to form an ettringitecontaining slurry 30.

[0028] Ettringite 32, in the form of a slurry, is removed from the ettringitecontaining slurry 30 in a liquid-soiid séparation step 34, thereby forming a high pH product water 36 containing low amounts of sulphate.

[0029] In a neutralisation step 38 of a third stage (Figure 3) gaseous carbon dioxide 40 is added to the product water 36 to form a calcium carbonatecontaining stream 42. Calcium carbonate 44 is removed from the stream 42 in a liquid-soiid séparation step 46 to form a purified product water 48.

[0030] In a décomposition step 50 of a fourth stage (Figure 4) acid 52, such as, but not limited to, sulphuric acid or hydrochloric acid, is added to the ettringite 32, causing it to décomposé and form a second supersaturated calcium sulphatecontaining slurry 54 (i.e. containing crystalized gypsum) in which amorphous aluminium trihydroxide is suspended.

[0031] The ettringite 32 décomposés in the step 50 at a pH of between 4 and 8.5.

For optimum results, however, the pH of the décomposition step 50 should be between 8 and.8.5.

[0032] Gypsum and aluminium trihydroxide are separated from one another in a solid/solid séparation step 58 to form a gypsum-containing slurry 60 and aluminium trihydroxide-containing slurry 62. Slurries 60 and 62 each contain a portion of the supersaturated sulphate-containing slurry 54. The solid/solid séparation step 58 is mainly achieved by means of size exclusion.

[0033] A portion of the gypsum slurry 60 is sent to the ettringite décomposition step 50 for seeding. The remaining portion of the gypsum slurry 60 is removed from the System as by-product or waste.

[0034] The aluminium trihydroxide slurry 62 is recycled to stage 2 for use in the step 24.

[0035] In the SAVMIN process (see Figure 2 of the SAVMIN patent spécification) after décomposition ofthe ettringite (step 5) amorphous aluminium trihydroxide is recovered using a separator 18. Thereafter gypsum, which is precipitated in a reactor 20, is separated using a separator 22.

[0036] The modified process as herein described therefore éliminâtes two reactors from the original process. This leads to a réduction in plant size and i i i i reagent costs, significantly lowering primarily the CAPEX and slightly reducing the OPEX of the process.

[0037] Successful solid-solid séparation of the amorphous aluminium trihydroxide slurry from the gypsum-containing slurry is possible due to the différence in particle size of the gypsum and the amorphous aluminium trihydroxide. The séparation is enhanced by increasing the différence between the particle size of the gypsum and the amorphous aluminium trihydroxide. This is achieved by growing the gypsum particles/crystals by means of seed recycling to form larger particles/crystals. Amorphous aluminium trihydroxide does not readily crystallise nor grow in particle size.

[0038] A further benefit arises by working in the aforementioned pH range of 8 to 8,5 (as is described hereinafter in the examples), a 99.5% recovery of amorphous aluminium trihydroxide precipitate 62 is achieved. This is to be contrasted with the recovery rate of “greater than 95%’’ of amorphous aluminium trihydroxide described in the SAVMIN spécification. Additionally, the co-precipitation of basic aluminium sulphate, in the ettringite décomposition step 50, is minimised. This is important because it prevents the réintroduction of sulphate in the ettringite précipitation step when recycling the amorphous aluminium trihydroxide that is also precipitated. The introduction of additional sulphate, in the form of basic aluminium sulphate, increases the lime and amorphous aluminium trihydroxide requirements in the ettringite précipitation step. Ultimately this would lead to an increase in the acid requirement in the ettringite décomposition step.

[0039] Aspects of the invention are further described in the following examples:

EXAMPLE 1

[0040] This example illustrâtes the effect of pH on the formation of aluminium précipitâtes.

[0041] The précipitation of various aluminium phases, namely aluminium 5 trihydroxide (AI(OH)₃), from sulphate media at pH values of 6.5, 7.0, 7.5, 8.0 and

8.5 was investigated. The effect of variations in pH on the types of solid phases formed was examined. The sulphate medium used consisted of aluminium sulphate solutions (AI₂(SO₄)₃) prepared at 10 g/L. The pH of the medium was controlled with the addition of a caustic soda (NaOH) solution at a concentration of

500 g/L. Results from the précipitation tests revealed that the precipitated phases contained, in addition to aluminium, high amounts of sulphates. This indicated the formation of two phases, namely aluminium trihydroxide (AI(OH)₃) and basic aluminium sulphate with the general formula (AI(OH)_x(SO₄)_y). It was also found that the optimum pH for the formation of AI(OH)₃ is in the range of 8.0 to 8.5. At this pH the amount of aluminium sulphate formed was reduced.

[0042] Table 1: Assay of solids formed

	pH 6.5	pH 7.0	pH7.5	pH 8.0	pH 8.5
Al, %	26	26	28	32	34
SO4 2’, %	18	16	14	12	10

EXAMPLE 2.

[0043] A fully integrated pilot plant operated as per the diagram of the type shown in Figures 1 to 4, capable of Processing 10 L/h of water, was operated for a period of 2 weeks. The combination of the heavy métal précipitation stage and the gypsum de-supersaturation stage was successful and average précipitation efficiencies of 98%, 97%, 96%, 96% and 25% were achieved for magnésium, manganèse, aluminium, iron and sulphate respectively. The results in the ettringite précipitation stage showed that the target sulphate concentration of 400 mg/L (SANS Class I spécification) in the overflow was reached, and potable water was produced after the carbonation stage in Figure 3. The results from the ettringite décomposition stage showed a 99.5% recovery of amorphous aluminium trihydroxide precipitate.

EXAMPLE 3

[0044] This example illustrâtes heavy métal and gypsum précipitation, ettringite précipitation and ettringite décomposition steps of the invention.

[0045] A mini pilot plant capable of Processing 100 L/h of acid mine water using the Consolidated process of Figures 1 to 4 was operated continuously for a period of four weeks. The feed to the plant consisted of a synthetic solution containing bivalent cations such as Mg²⁺, Ca²⁺, Mn²⁺, as well as SO₄ ²' and Fe²⁺. The average feed composition is presented in Table 2.

[0046] Table 2: Feed water composition (expressed in mg/L)

Mg	Al	Si	Ca	Ti	Cr	Mn
67	42	6	295	2	2	39
Co	Ni	Cu	Zn	Pb	Fe	soZ
<2	<2	<2	<2	<2	4	1308

[0047] The results of the pilot campaign showed that the process was effective at removing heavy metals from contaminated water. The treated water produced was nearly free of heavy métal ions, namely iron, aluminium, manganèse and magnésium. Removal efficiences of 97% and 93% were obtained for magnésium and manganèse, respectively. Lime consumption was averaged at 1.4 kg/m³ of feed water.

[0048] The removal of sulphate and calcium ions from contaminated water via ettringite précipitation produced SANS Class I water in terms of sulphate (< 400 mg/L) with sulphate removal efficiencies ranging from 80% to 91%, and calcium removal efficiencies as high as 74%. The corresponding aluminium trihydroxide consumption rate was in the range of 0.9 to 1.1 kg/m³ of feed water at an aluminium trihydroxide feed ratio of approximately 1.1 to 1.3 times the stoichiometric amount required. The consumption of lime ranged between 1.0 and \

1.8 kg/m³ of feed water. Aluminium trihydroxide was regenerated in the ettringite | i ί i i I i décomposition step with the addition of sulphuric acid at a rate of around 0.4 kg/m³ of feed water.

Claims (13)

1. (1) raising the pH of the acidic waste water stream to precipitate impurities from the stream and form a first supersaturated calcium sulphate-containing stream;

   1. A method for the removal of sulphates and calcium from an acidic waste water stream which includes the steps of:
2. 2. A method according to claim 1 wherein, in step (1), calcium hydroxide or calcium oxide is added to the acidic waste water stream.

   (2) removing the impurities and de-supersaturating the first supersaturated calcium sulphate-containing stream in a first solid/liquid séparation step to form a first saturated calcium sulphate-containing solution;
3. 3. A method according to claim 1 or 2 wherein, in step (1 ), the pH is raised to a value of between 10.0 and 12.0.

   (3) adding amorphous aluminium trihydroxide to the first saturated calcium sulphate solution to precipitate ettringite in a product water stream;
4. 4. A method according to claim 1,2 or 3 wherein the impurities include iron, aluminium, manganèse, magnésium and other heavy metals.

   (4) removing the precipitated ettringite, in the form of an ettringitecontaining slurry from the product water stream using a second liquid-solid séparation step;
5. 5. A method according to any one of daims 1 to 4 wherein, following step 4, the pH of the product water stream is lowered by adding CO2 to precipitate calcium carbonate.

   (5) lowering the pH of the ettringite-containing slurry to décomposé the ettringite and form amorphous aluminium trihydroxide (recovered) and gypsum contained in a second supersaturated calcium sulphatecontaining stream, and (6) separating the recovered amorphous aluminium trihydroxide and gypsum in a solid-solid séparation step to form an aluminium trihydroxide containing slurry and a second saturated calcium sulphate-containing solution.
6. 6. A method according to claim 5 wherein the calcium carbonate is separated from the product water in a third liquid-solid séparation step to form a purified water.
7. 7. A method according to any one of daims 1 to 4 wherein the first, and the second, supersaturated calcium sulphate-containing streams are desupersaturated by removing calcium sulphate as gypsum.
8. 8. A method according to any one of daims 1 to 7 wherein, in step (5), the pH of the ettringite is lowered by the addition of sulphuric acid, hydrochloric acid, CO2 or SO2.
9. 9. A method according to any one of daims 1 to 7 wherein, in step (5), the pH of the ettringite is lowered to a value between 4 and 8.5.
10. 10. A method according to claim 9 wherein, in step (5), the pH of the ettringite is lowered to a value between 8 and 8.5.
11. 11. A method according to any one of daims 1 to 10 wherein the second and third supersaturated calcium sulphate-containing streams include calcium sulphate in the form of gypsum.
12. 12. A method according to claim 11 wherein the gypsum is in a crystallised form.
13. 13. A method according to claim 12 wherein, in step (6), the solid-solid séparation is achieved by means of size exclusion, wherein particles of the crystallised gypsum are larger than particles of the amorphous aluminium.