WO2021062451A1

WO2021062451A1 - Self-regulating simultaneous control of aluminium replenishment and recovery

Info

Publication number: WO2021062451A1
Application number: PCT/ZA2020/050050
Authority: WO
Inventors: Petrus Johannes Van Staden
Original assignee: Mintek
Priority date: 2019-09-26
Filing date: 2020-09-21
Publication date: 2021-04-01

Abstract

A waste water treatment process wherein ettringite, formed by the addition of an aluminium source to a sulphate-bearing solution, is destroyed in a step wherein Al₂(SO₄)₃ is used for pH control in the absence of H₂SO₄.

Description

SELF-REGULATING SIMULTANEOUS CONTROL OF ALUMINIUM REPLENISHMENT AND RECOVERY

BACKGROUND 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 water streams, in particular acid mine drainage water, typically have pH values as low as 1.5. Such waste water streams also contain high levels of heavy metals, calcium and sulphate and pose a significant environmental challenge. International patent application number PCT/GB98/01610 describes a process, generally referred to as “the SAVMIN process”, which is particularly concerned with the treatment of sulphate-containing mine drainage water as well as sulphate-containing waste/effluent waters.

[0003] In one step of the SAVMIN process, a saturated calcium sulphate water stream (produced in a preliminary process) is combined with a suitable aluminium source such as amorphous aluminium trihydroxide and a neutralising agent e.g. lime, for the removal of sulphate and calcium from solution, to promote the precipitation of ettringite (“ettringite formation step”).

[0004] Sulphuric acid is then added to the ettringite in an ettringite decomposition step to form aluminium trihydroxide and gypsum solids. This reaction proceeds when the pH drops below 8.5. The solids are then separated from each other in a solid/solid separation step. The recovered aluminium trihydroxide can then be recycled to the ettringite formation step as set out above to economise on the consumption of fresh aluminium trihydroxide (or other suitable salts of aluminium that could be used to replenish aluminium losses from the circuit). [0005] During the solid/solid separation step, some of the aluminium trihydroxide is lost with the gypsum discarded stream. This loss must be addressed by adding a fresh source of aluminium trihydroxide to the circuit.

[0006] It has been found that aluminium trihydroxide bought commercially in a solid form is not chemically reactive to sulphate. However, aluminium trihydroxide formed in-situ in the process, either in the ettringite decomposition step or by dissolving aluminium sulphate in water to form aluminium trihydroxide and sulphuric acid, is chemically reactive to sulphate.

[0007] It is generally accepted practice to conduct the ettringite decomposition step with pH control through the addition of sulphuric acid to recover aluminium trihydroxide from the ettringite. At the same time and in the same vessel, aluminium sulphate is dissolved in water, wherein the amount of aluminium sulphate added is dictated by a separate measurement of the amount of aluminium trihydroxide lost from the circuit, to ensure that the amount of aluminium retained in the process remains at a constant level.

[0008] This process does however present challenges. The addition of sulphuric acid presents a safety risk and additional operational requirements must be met to ensure the safety of an operator, thereby increasing operational costs. The addition of aluminium sulphate also increases the levels of aluminium and sulphuric acid in the circuit. It is not possible to separate pH control by sulphuric acid addition from the control of aluminium concentration by the addition of aluminium sulphate, since the addition of aluminium sulphate also affects the pH. A measurement of the amount of aluminium trihydroxide lost from the process cannot be effected in real-time and samples must be taken at regular intervals to be tested. These challenges result in a cumbersome and unstable control strategy. [0009] An object of the present invention is to enhance the regeneration of amorphous aluminium trihydroxide and simultaneously to improve the process control by eliminating the need for the addition of sulphuric acid to the circuit. The latter aspect is important as it minimizes the safety risk to an operator of the process and reduces the costs associated with the process.

SUMMARY OF INVENTION

[0010] The invention provides a method of removing sulphates and calcium from an acid waste water stream, the method including the steps of: a) adding amorphous aluminium trihydroxide to a calcium sulphate-containing solution to form a product water stream containing precipitated ettringite; b) separating the precipitated ettringite from the product water stream; c) decomposing the ettringite through the addition of a predetermined quantity of aluminium sulphate, stoichiometrically calculated to maintain a target pH to recover aluminium trihydroxide in an aluminium trihydroxide and gypsum slurry suspended in a saturated calcium sulphate-containing solution; and d) recovering amorphous aluminium trihydroxide in a solid/solid separation step for use in step (a).

[0011] The method may include a first preliminary step of raising the pH of the water stream to precipitate impurities from the stream to form impurity-precipitates suspended in the calcium sulphate-containing stream.

[0012] In the first preliminary step the pH may be increased by adding calcium hydroxide, calcium oxide or hydrated lime to the acidic water stream. The pH is preferably raised to a value of between 10 and 12. [0013] The stream consisting of the impurity-precipitates suspended in a calcium sulphate solution may proceed to a separation step to form the calcium sulphate-containing solution separate from the solids.

[0014] The impurities may include iron, aluminium, manganese and other heavy metals. [0015] In step (c) of this method, the target pH may be a value between 4 and 8.5.

Preferably the target pH is 6.5.

[0016] The predetermined amount of aluminium sulphate may be determined from the expression, (V_n^).x, wherein: x represents the number of moles of aluminium stoichiometrically required in step (a) to convert all soluble sulphate to ettringite; i\ represents the efficiency according to which aluminium trihydroxide recovered in step (d) is recycled and wherein 0 £h <1.

[0017] If h = 0.5, pH control through the addition of aluminium sulphate alone provides an adequate quantity of aluminium to replenish aluminium losses in step (d) i.e. to take account of the aluminium losses in the gypsum discard stream

[0018] The predetermined quantity of aluminium sulphate calculated in step (c) may be one mole for every mole of ettringite in step (a) thereby yielding 4 moles of aluminium trihydroxide and 6 moles of gypsum in step (d).

[0019] If h is consistently maintained well above 0.5, sulphuric acid may be added in step (b) at a constant rate, which is easily achieved, or at a rate maintained in proportion to the flow rate of an upstream flow (e.g. feed water inflow, also easily achieved), and the predetermined amount of aluminium sulphate may be adjusted such that the simultaneous addition of aluminium sulphate and sulphuric acid maintains the target pH. The last mentioned action requires a control loop in which pH is a controlled variable and the aluminium sulphate flow is a manipulated variable.

[0020] The saturated calcium sulphate-containing aluminium trihydroxide in suspension may include calcium sulphate in the form of gypsum. The gypsum may be in a crystallised form.

[0021] In step (d) amorphous aluminium trihydroxide may be recovered in a solid/solid separation step which may be achieved by means of size exclusion, wherein particles of the crystallised gypsum are larger than the particles of the amorphous aluminium trihydroxide. [0022] A portion of the calcium sulphate-containing solution that enters step (a) may be diverted to dilute the saturated calcium sulphate slurry of aluminium trihydroxide and gypsum prior to carrying out step (d).

DESCRIPTION OF THE DRAWINGS

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

Figure 1 shows a first stage which embodies first and second preliminary processes to effect heavy metal and gypsum precipitation; Figure 2 shows a second stage for precipitating ettringite; and

Figure 3 shows a third stage which embodies a modified ettringite decomposition and recovery step. DESCRIPTION OF PREFERRED EMBODIMENT

[0024] Figures 1 to 3 of the accompanying drawings illustrate aspects of three stages of an effluent treatment process based on the process described in PCT/GB98/01610 (“the original SAVMIN process”) which is modified in accordance with the teachings of the present invention. These stages involve the removal of metals and sulphate at ambient conditions from contaminated mine waters.

[0025] Figure 1 illustrates the first stage which includes first and second preliminary processes of the present invention. In a step 10, waste water 12, typically acid mine water, is contacted with an alkali 14 such as hydrated lime (Ca(OH)2) to form a super-saturated stream from which gypsum and other compounds might precipitate to de-super-saturate it to yield a stream of impurity precipitates which may include gypsum, suspended in a calcium-sulphate solution 16 at a pH between 10 and 12. The calcium-sulphate solution stream 16 contains solids 18 in the form of crystallised gypsum and precipitated impurities such as heavy metal hydroxides and may also include crystallised gypsum. The solids 18 are removed from the stream 16 in a liquid-solid separation step 20 to form a calcium sulphate solution 22.

[0026] In a step 24 of the second stage, shown in Figure 2, hydrated lime 28 and an aluminium trihydroxide-containing stream 30 are mixed with at least part of the calcium sulphate containing solution 22 to form a dilute ettringite-containing slurry 32. The aluminium trihydroxide-containing stream 30 is recovered from stage 3 of the present invention, as is described more fully herein. The reaction in the step 24 proceeds as follows: 2AI(0H)₃ + 3CaS0₄₍aq) + 3Ca(OH)₂₍aq) + 26H₂0 = AI (S0₄)3.6Ca(0H)₂.26H₂0(_S) [1] 10027] The dilute ettringite-containing slurry 32 is then subjected to a liquid-solid separation step 34 to yield a thickened ettringite containing-slurry 36 and a high pH product water 38. In a neutralisation step 40, gaseous carbon dioxide 42 is added to the product water 38 to form a calcium carbonate stream 44. Calcium carbonate 46 is removed from the stream 44 in a solid/liquid separation step 48 to form a purified product water 50.

[0028] Referring to Figure 3, the thickened ettringite slurry 36 is subjected to a decomposition step 52 wherein the thickened ettringite-containing slurry 36 is decomposed to yield amorphous aluminium trihydroxide (AI(OH)3) in suspension in a saturated calcium sulphate-containing slurry 58 (also containing gypsum). The step 52 proceeds according to the following equations consisting of steps [2a] and [2b] yielding the combined result [2], wherein gypsum is represented by CaSC for simplicity, although di-hydrated-gypsum also includes 2 moles of H2O for every mole of CaS04:

AI₂(S04)₃ + 6 H2O = 2AI(0H)3 + 3H2SO4 [2a]

Al2(S04)3.6Ca(0H)2.26H₂0(s) + 3H S04 = 2AI(OH)₃ + 6CaS0₄₍s) + 32H₂0 [2b] AI₂(S04)3.6Ca(0H)2.26H20(s) + AI₂(S04)₃ = 4AI(OH)₃ + 6CaS0 ₍s) +26H₂0 [2]

[0029] In order for the reaction [2] to proceed, the pH should be below 8.5. In the original SAVMIN process, this is achieved through the addition of sulphuric acid to the ettringite- containing slurry. In the present invention, this acid is derived from the addition of a predetermined amount of aluminium sulphate Al2(SC>4)3 54 (Figure 3) to the thickened ettringite-containing slurry 36 according to reaction [2a]. This illustrates why aluminium sulphate is a preferred reagent as, when it is dissolved in water, it hydrolyses to produce both a reactive form of aluminium trihydroxide and sulphuric acid according to equation [2a].

[0030] The saturated calcium sulphate-containing slurry 58 is then subjected to a dilution step 60 wherein the calcium sulphate solution 22 (formed in stage 1) is added to the saturated calcium sulphate-containing slurry 58 to form a diluted calcium sulphate- containing slurry 62.

[0031] The use of the calcium sulphate solution 22 in the dilution step 60 eliminates the need of adding a dilution water stream as well as the use of a settler to dewater and concentrate the aluminium trihydroxide 30. The use of a flocculant is also eliminated.

[0032] The diluted calcium sulphate-containing slurry 62 is then subjected to a solid/solid separation step 64, wherein gypsum 66 and aluminium trihydroxide 30 are separated from one another. The solid/solid separation step 64 is mainly achieved by means of size exclusion. The separation is enhanced by increasing the difference between the particle size of the gypsum particles/crystals by means of seed recycling to form larger particles/crystals. Amorphous aluminium trihydroxide does not readily crystallise or grow in particle size. A portion of the gypsum slurry 66 is sent to the ettringite decomposition step 52 for seeding. The remaining portion of the gypsum slurry 66 is removed from the system as by-product or waste 68. As stated, the aluminium trihydroxide 30 is recovered and then recycled for use in the step 24. In order to optimise the process, the amount of aluminium in the aluminium sulphate-containing stream 54 should equal the amount of aluminium lost in the solid/solid separation step 64 to waste 68 according to the expression

).x, wherein: a) x represents the number of moles of aluminium stoichiometrically required in the step 24 to convert all soluble sulphate to the ettringite slurry 32 shown in the reaction [1] and the step 24; and b) q represents the efficiency according to which aluminium recovered in the step 64 is recycled to the step 24; and c) 0£q <1. [0033] In the original SAVMIN process (as per PCT/GB98/01610) the aforementioned requirement is achieved by adding sulphuric acid 56 in the ettringite decomposition step 52 and a “top-up” stream of aluminium sulphate 30 in the ettringite formation process 24. However, as stated hereinbefore, the addition of aluminium sulphate 54 contributes both aluminium and sulphuric acid to the process. It is therefore not possible to separate pH control through the addition of sulphuric acid 56 in the ettringite decomposition step 52 from the addition of aluminium sulphate 54 to replenish the aluminium lost in the solid/solid separation step 64.

[0034] The value of the parameter q, needed to perform the calculation of the amount of aluminium to be replenished according to the expression {V_n- 1).x is achieved by frequent sampling of the products of the solid/solid separation step, an exercise which cannot be performed in real-time. The control strategy in the original SAVMIN process therefore becomes difficult and still requires the addition of sulphuric acid in the ettringite decomposition process. [0035] In the present invention, from the reaction [2a] it can be seen that one mole of aluminium sulphate (which represents 2 moles of aluminium) yields 3 moles of sulphuric acid which, from the reaction [2b], is seen to be the exact amount of sulphuric acid required for the reaction [2b] which produces 2 moles of aluminium. From a mass balance over the flowsheet it follows that if one mole of aluminium sulphate (54) is added to replenish the aluminium lost in the solid/solid separation step 64 to waste 68 (represented by (¹ 1).x), for every mole of aluminium required in the ettringite formation step 24 (and reporting to the stream 32) which is represented by x, the amount of acid required in the decomposition step 52 will match the acid requirement for the reaction [2] This condition is satisfied if: (¹/„-1).c = x, from which it follows that q = 0.5 [3]

[0036] According to the equation [3], by using the hydrolysis of aluminium sulphate 54 added in the step 52 to satisfy the acid demand shown in the reaction [2], a recovery efficiency of 50% of the aluminium trihydroxide 30 in the solid/solid separation step 64 is implied and the process control can be achieved by manipulating the amount of aluminium sulphate 54 added to maintain a target pH, with pH being a relatively simple single parameter to measure and control in real-time.

[0037] The applicant has found by experimentation that the addition of aluminium sulphate 54 in the step 52 in the proportion of one mole of aluminium sulphate for each mole of ettringite contained in the slurry 36, results in a terminal pH of 6.5.

[0038] If q exceeds 0.5, such that the aluminium trihydroxide 30 recovered in the step 64 exceeds 50%, the method can be expected to provide a degree of oversupply of aluminium trihydroxide 30 in the step 24 which will self-correct by a temporary accumulation of unreacted aluminium trihydroxide until the rate of loss of the aluminium trihydroxide with the gypsum 66 to waste 68 equals the rate of fresh supply.

[0039] Certain parameters could lead to a deviation in the value of q from 0.5 used in the implementation of the invention. One such parameter is the amount of the gypsum 66 which is entrained together with the aluminium trihydroxide 30 which is recycled from the step 64 to the step 24. As gypsum is slightly soluble, it yields CaS04(aq) in solution, which, according to reaction [1], will consume aluminium trihydroxide. In that case a larger amount of aluminium trihydroxide 30 would be required in the step 24 i.e. above the quantity of aluminium trihydroxide 30 which is required in the step 24 to react with the sulphate in the feed water 22 (saturated calcium-containing stream) and the sulphate contributed by the entrained gypsum (not precipitated with the solids 18 during the solid/liquid separation step 20). However, a self-regulating steady-state control process is still obtained by maintaining a target pH through the manipulation of a single variable, namely the addition of aluminium sulphate 54 in the step 52. [0040] Should r\ consistently be well above 0.5, forcing 50% efficiency by pH control through the addition of the aluminium sulphate 54 alone, as described, would be unnecessarily wasteful. In that event, sulphuric acid 56 is added to the step 52 at a constant rate, or at a rate maintained in proportion to the flow rate of the upstream flows 16 or 22 (e.g. the feed water inflow). The rate should be maintained at a value less than the stoichiometrically required quantity to complete the reaction [2] so that the sulphuric acid addition on its own would be insufficient to maintain the pH below 8.5. The target pH would then be achieved through the addition of the aluminium sulphate 54.

[0041] It should be ensured that the addition rate of the aluminium sulphate 54 is sufficient to replenish the maximum rate of aluminium loss observed by mass balancing. The combination of sulphuric acid 56 and aluminium trihydroxide 30 that would simultaneously reach the target pH and replenish the lost aluminium trihydroxide can be determined through experimentation or through mass balance calculations. In this embodiment, the control strategy is simplified by maintaining the rate of sulphuric acid 56 addition either constant or in proportion to the flow rate of one of the upstream flows (16, 22), which are relatively simple parameters to control, and by maintaining a target pH in the step 52 through the addition of the aluminium sulphate 54. This technique requires only a single control loop, namely the manipulation of the supply of aluminium sulphate 54 to control the pH in the ettringite decomposition stage 52. [0042] The method of the invention satisfies the requirement of producing reactive aluminium trihydroxide 30, in-situ. The method also simplifies the process control as only a single variable is manipulated to maintain a target pH, the measurement of which can be conducted on-line. Furthermore, the method eliminates or minimises the need for the addition of sulphuric acid 56 in the process. This considerably reduces the OPEX of the process. Additionally, the use of sulphuric acid 56 requires extensive safety considerations such as a dedicated acid area, acid-resistant overalls and gloves as well as face shields, which are eliminated in the case where h is maintained at 0.5.

Claims

1. A method of removing sulphates and calcium from an acid waste water stream which includes the steps of: a) adding amorphous aluminium trihydroxide to a calcium sulphate-containing solution to form a product water stream containing precipitated ettringite; b) separating the precipitated ettringite from the product water stream; c) decomposing the ettringite through the addition of a predetermined quantity of aluminium sulphate, stoichiometrically calculated to maintain a target pH to recover aluminium trihydroxide in a saturated calcium sulphate-containing solution bearing a suspension of aluminium trihydroxide and gypsum; and d) recovering amorphous aluminium trihydroxide from the suspension of aluminium trihydroxide and gypsum in a solid/solid separation step for use in step (a).

2. A method according to claim 1 which includes a first preliminary step of raising the pH of the water stream to precipitate impurities from the stream to form a calcium sulphate- containing stream bearing impurity-precipitates suspended in it.

3. A method according to claim 2 wherein the pH is raised to a value of between 10 and 12.

4. A method according to claim 2 or 3 which includes a second preliminary step of removing the impurities in a separation step to form the calcium sulphate-containing solution.

5. A method according to any one of claims 2 to 4 wherein the impurities include iron, aluminium, manganese and other heavy metals.

6. A method according to any one of claims 1 to 5 wherein, in step (c) the target pH is a value between 4 and 8.5.

7. A method according to claim 6 wherein the target pH is 6.5.

8. A method according to any one of claims 1 to 7 wherein the predetermined amount of aluminium sulphate is determined from the expression,

).x, wherein: a) x represents the number of moles of aluminium stoichiometrically required in step (a) to convert all soluble sulphate to ettringite; and b) q represents the efficiency according to which aluminium trihydroxide recovered in step (d) is recycled according to step (d), and wherein

9. A method according to claim 8 wherein the predetermined quantity of aluminium sulphate calculated in step (c) is one mole for every mole of ettringite in step (a) thereby yielding 4 moles of aluminium trihydroxide and 6 moles of gypsum in step (d).

10. A method according to claim 8 or 9 wherein, if q is above 0.5, sulphuric acid is added in step (b) at a constant rate, or at a rate maintained in proportion to the flow rate of an upstream flow and the predetermined amount of aluminium sulphate is adjusted such that the simultaneous addition of aluminium sulphate and sulphuric acid maintains the target pH.

11. A method according to any one of claims 1 to 10 wherein the saturated calcium sulphate-containing solution bearing aluminium trihydroxide in suspension includes calcium sulphate in the form of gypsum in suspension.

12. A method according to claim 11 wherein the gypsum is in a crystallised form.

13. A method according to claim 1 wherein in step (d) the solid/solid separation step is achieved by means of size exclusion, wherein particles of the crystallised gypsum are larger than the particles of the amorphous aluminium trihydroxide.

14. A method according to any one of claims 1 to 13 wherein the calcium sulphate- containing solution derived from the steps described in claim 4 is used to dilute the aluminium trihydroxide and gypsum suspended in saturated calcium sulphate solution prior to carrying out step (d).

15. A waste water treatment process wherein ettringite, formed by the addition of an aluminium source to sulphate-bearing solution, is destroyed in a step wherein Al2(S04)3 is used for pH control in the absence of H2SO4.