WO2021028621A1

WO2021028621A1 - Use of secondary aluminium in precipitation of sulphate from waste water

Info

Publication number: WO2021028621A1
Application number: PCT/FI2020/050528
Authority: WO
Inventors: Tommi KAARTINEN; Pertti Koukkari
Original assignee: Teknologian Tutkimuskeskus Vtt Oy
Priority date: 2019-08-15
Filing date: 2020-08-14
Publication date: 2021-02-18

Abstract

The present invention relates to a method of removing sulphate form waste water in an ettringite precipitation process. Specifically, the present invention relates to a method of removing sulphate form waste water in an ettringite precipitation process, wherein waste aluminium/secondary aluminium is used to precipitate sulphate as ettringite. The present invention also relates to the use of waste aluminium/secondary aluminium in an ettringite process.

Description

Use of secondary aluminium in precipitation of sulphate from waste water

TECHNICAL FIELD

The present invention relates to a method of removing sulphate from waste water in an ettringite precipitation process. Specifically, the present invention relates to a method of removing sulphate from waste water in an ettringite precipitation pro cess, wherein compounds of waste aluminium and/or secondary aluminium are used as a primary aluminium source to precipitate sulphate as ettringite. The pre sent invention also relates to the use of waste aluminium and/or secondary alumin ium in an ettringite process.

BACKGROUND

Sulphate removal from effluents of metal mines and other industrial sites is becoming unavoidable. Tightening environmental regulation requires, among other things, both lower contaminant levels in effluents and increased recycling of process waters. Sulphate accumulation in water hinders water recirculation.

Treatment of sulphate bearing effluents with lime to precipitate gypsum is the traditional method, with which sulphate levels of around 2 000 mg/I can be reached based on gypsum solubility. Significantly lower concentrations are received when aluminium based ettringite precipitation techniques are used. Sulphates are then received as a mixed oxyhydrate precipitate, typically with the chemical formula Ca6AI₂(S0₄)3(0H)i₂-26H₂0.

Document WO98/55405 discloses a process for removing sulphates and cal cium from a water stream which comprises the steps of combining the water stream and an amount of amorphous aluminium trihydroxide (AI(OH)3), allowing the for mation of ettringite as a precipitate and removing the precipitated ettringite from the water stream.

Document WO2014/033361 discloses a method for removing sulphate cal cium and/or soluble metals from waste water which comprises the following steps a) a gypsum precipitation step, b) an ettringite precipitation step, c) a first separation step, d) a neutralisation step, and e ) a second separation step in order to obtain water having reduced sulphate, calcium and/or soluble mineral content.

Known commercial processes for ettringite precipitation include the steps of: 1 ) precipitation of gypsum by addition of lime to reach pH 10-12 as a pre-treatment step followed by separation of gypsum sludge from the water, and 2) AI^3+_addition together with lime in pH 11-12 to remove sulphate as precipitated ettringite followed by separation of ettringite sludge from the water.

These processes use commercial aluminium chemicals as the source of alu minium to ensure sufficient reactivity in the precipitation process.

Aluminum chemical costs are known to form an essential part of the operating costs in the state-of-the-art processes, which challenges the feasibility of the ettring ite precipitation process in sulphate removal.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is to provide a method for removing sulphate from waste waters with the use of recycled waste aluminium compounds as the source of aluminium in the chemical precipitation of sulphate from the waste water. In the present invention, two aluminium compounds containing waste or secondary materials have been proven to be able to replace commercial aluminum chemical as the source of aluminium in the ettringite precipitation.

In one embodiment, the aluminium compounds, such as aluminium hydroxide, containing waste or secondary material to be used as the source of aluminium is a waste product of aluminum refining process, which is rich in aluminum hydroxide as gibbsite. In one embodiment, the aluminium compound, such as aluminium hydrox ide, containing secondary material to be used as the source of aluminium is a pre treated waste product of aluminum refining process.

In one embodiment, the aluminium compounds, such as aluminium hydroxide, containing waste or secondary material is a precipitated aluminum compound, such as aluminium hydroxide, from industrial waste water. In one embodiment, the indus trial waste water originates from a mine. In one embodiment, the industrial waste water originates from forest industry.

Waste aluminum and/or secondary aluminium has been formerly deemed as not applicable as primary source of aluminium in the ettringite process because of its low reactivity.

The objects of the invention are achieved by the method and the use charac terized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows the flow-diagram of the precipitation tests in Example 1 . Figure 2 shows sulphate concentrations as a function of time in ettringite precipita tion tests of Example 1 .

Figure 3 shows the aluminium concentrations as a function of time in two ettringite precipitation experiments of Example 1 .

Figure 4 shows the calcium concentrations as a function of time in two ettringite precipitation experiments of Example 1 .

Figure 5 visualises the batch aluminium recovery experiment in Example 2.

Figure 6 visualises the ettringite precipitation experiment in Example 2.

Figure 7 shows the summary of the results from the Al-recovery in Example 2. Figure 8 shows the summary of the results from the sulphate removal in Example 2. Figure 9 shows the kinetics of sulphate removal in the ettringite precipitation with recovered aluminium.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is based on a finding that commercial aluminium chem icals as source of aluminium in ettringite precipitation process can be replaced by waste aluminium compounds and/or secondary aluminium compounds.

The invention relates to a method of removing sulphate form waste water in an ettringite precipitation process, wherein a waste aluminium compound and/or a sec ondary aluminium compound is used as the primary source of aluminium to precip itate sulphate as ettringite. In one embodiment, the waste aluminium compound is aluminium hydroxide AI(OH)3_, aluminium hydroxide oxide AIO(OH) and/or alumin ium chloride AlC . In one embodiment, the waste aluminium compound and/or sec ondary aluminium compound is derived from aluminium refining process as gibbsite. In one embodiment, the waste aluminium compound and/or secondary aluminium compound is derived from industrial waste water as a AI(OH)3-precipitate. In one embodiment, the waste aluminium and/or secondary aluminium is amorphous. The waste aluminium and/or secondary aluminium recovered from the aluminium refin ing process or from the industrial waste water is exploited in the method of the in vention in solid from or in a form of a slurry and/or sludge.

The invention relates also to use of waste aluminium compound and/or sec ondary aluminium compound as the primary source of aluminium in an ettringite precipitation to precipitate sulphate from a waste water. In one embodiment, the waste aluminium compound is aluminium hydroxide AI(OH)3_, aluminium hydroxide oxide AIO(OH) and/or aluminium chloride AICI3. In one embodiment, the waste alu minium and/or secondary aluminium is derived from aluminium refining process as gibbsite. In one embodiment, the waste aluminium and/or secondary aluminium is derived from industrial waste water as a AI(OH)3-precipitate. In one embodiment, the waste aluminium/secondary aluminium is amorphous. The waste aluminium and/or secondary aluminium compound recovered from the aluminium refining pro cess or from the industrial waste water is exploited in the present invention in solid from or in a form of a slurry and/or sludge. In one embodiment, the industrial waste water originates from a mine. In one embodiment, the industrial waste water origi nates from forest industry.

Without being bound by any theory, the waste aluminium hydroxide AI(OH)3_, and/or secondary aluminium hydroxide AI(OH)3 in a crystalline form (as gibbsite) could be heat-treated. However, no heat-treatment is needed when the waste alu minium hydroxide AI(OH)3_, and/or secondary aluminium hydroxide AI(OH)3 is in an amorphous form.

In one embodiment, the secondary aluminium is pre-treated to make it reactive by heating to a temperature of 50°C-80°C in normal pressure/atmospheric pressure.

In one embodiment, the waste aluminium and/or secondary aluminium is de rived from industrial waste water as a AI(OH)3-precipitate. In one embodiment, the Al-content in the precipitate is in the range of 20-35% (of dry mass). In one embod iment, the Al-content in the precipitate is in the range of 20-25% (of dry mass). The dry matter content of the precipitate is typically in the range of 15-30% depending to a great extent on the method of solid-liquid separation. The dry matter content of the precipitate does not have great impact on the applicability of the invention, as the precipitate is typically slurried with water before introducing into sulphate re moval process.

The precipitated waste aluminium is solubilised in alkaline conditions. Once solubilised, pH above 11 and enough calcium available, the aluminium and calcium react together with sulphate contained in the waste water to form ettringite, that then precipitates and thus removes sulphate from waste water according to the equation below:

3CaO + 3Ca²⁺ + 3S0₄ ² + 2AI(OH)₃(s) + 28H₂0 -> [3CaO3CaS0₄ AI₂0₃(s)-31 H₂0]

In one embodiment, the waste aluminium and/or secondary aluminium is de rived from aluminium refining process as a waste aluminium sludge. In one embod iment, the Al-content in the sludge and/or slurry is in the range of 20-35% (of dry mass). In one embodiment, the Al-content in the sludge and/or slurry is in the range of 25-30% (of dry mass). The dry matter content of the sludge is typically in the range of 15-30% depending to a great extent on the method of solid-liquid separa tion. The dry matter content of the sludge does not have significant impact on the applicability of the invention, as the sludge is typically slurried with water before introducing into sulphate removal process. In one embodiment, the aluminium is present as aluminium hydroxide with the crystal form of gibbsite. In one embodiment, the waste aluminium sludge is mixed with water in sludge :water-ratio 1 :2. In one embodiment, the sludge is pre treated by heating to a temperature of 50°C-80°C in normal pressure/atmospheric pressure. In one embodiment, the sludge is pre-treated by heating to a temperature of about 60°C.

In one embodiment, the waste aluminium/secondary aluminium contains sul phur residues 4 weight-%, at the maximum. In one embodiment, the waste alumin ium is essentially free of sulphur acid residues.

The following examples are given to illustrate the invention without, however, restricting the invention thereto.

EXAMPLE 1 - Waste from aluminium refining

The aluminium hydroxide containing secondary material, a waste product of aluminum refining process, had the major constituents shown in Table 1.

Table 1. Major elements present in waste aluminium sludge in concentrations greater than 0,01 % (fluorine and heavier elements included).

Mine process water used in the experiments had main characteristics shown in Table 2. Sulphate concentration was measured before these experiments and other parameters represent long-term average data.

Table 2. Characteristics of mine process water

Mine process

Component water pH* 8,2

Sulphate, mg/I 9 000

Calcium, mg/I* 430

Sodium, mg/I* 143

Potassium, mg/I* 132

Magnesium, mg/I* 1 610

* long term average

Figure 1 shows the flow diagram of the precipitation tests. The experiments were conducted at room temperature as batch tests in titanium reactors with contin uous mixing. The first stage was pre-treatment of the water with lime in order to remove sulphate to gypsum saturation level as well as raise the pH to the level that is favourable for subsequent ettringite precipitation in the second stage.

In gypsum precipitation 10% lime solution (100 g analytical grade Ca(OH)2 (>96%) in 1 000 ml) was added to 10 litres mine process water to raise and maintain the pH at approximately 12. At the end of the test at 30 minutes, the solution was filtered (0.45 pm), and sulphate concentration was measured.

In the ettringite precipitation tests, the lime-treated water was further treated by adding aluminium and precipitating sulphate as ettringite. In the first experiment the waste aluminium hydroxide sludge was added as one-time dose in the beginning of the test preceded by slightly crumbling the pieces of the sludge before weighing and adding to the solution. For the second experiment, in order to improve the water solubility and thus the reactivity of the aluminium sludge, the sludge was mixed with water in sludge to water ratio of 1 to 2 and simultaneously heating the suspension to 60^°C. The obtained suspension was added as one-time dose in the beginning of the test. For comparison with proven aluminium chemical, extra-pure anhydrous AlC (>99%) was used as the aluminium source in the third experiment. AICI3 was first solubilized, and added as 25% solution as one-time dose in the beginning of the ettringite precipitation stage.

Aluminium dosing was calculated based on molar ratios of aluminium and sulphate (sulphate concentration after lime precipitation). Molar ratio AI:S0₄ of 2 was chosen for the tests with waste aluminium resulting in three times the theoreti cally needed amount to remove sulphate according to the equation below:

3CaO + 3Ca²⁺ + 3S0₄ ² + 2AI(OH)₃(s) + 28H₂0 ->[3Ca0-3CaS0₄-AI₂0₃(s)-31 H₂0]

Molar ratio (AI:S0₄) of 1 was used for AICI₃ to make 1 ,5 times the theoreti cally needed amount. Also lime was added to the ettringite precipitation stage to maintain pH at 11 -12 and to ensure that enough calcium was present for the for mation of ettringite according to the above equation.

Samples of 30 ml were taken from the solution during the ettringite precipita tion tests, which were filtered (0.45 pm) immediately and measured for sulphate concentration. Based on this, different test durations were achieved as the test was terminated when 1 ) sulphate concentration was below the detection limit of the used analytical method (40 mg/I and 150 mg/I, Hach Lange DR3900 spectrophotometer and LCK 353 and LCK 653 kits), or 2) no changes in sulphate concentration was observed between two measurements.

Two batches of gypsum precipitated water was prepared. In batch 1 the re sidual sulphate concentration after gypsum precipitation was 1835 mg/I and pH 12.2. Batch 1 was used for ettringite precipitation with non-pre-treated waste alu minium hydroxide. In batch 2 residual sulphate concentration was 2010 mg/I and pH 12.1 . Batch 2 was used for all other experiments.

Figure 2 shows sulphate concentration as a function of time in the three ettringite precipitation experiments.

In the experiment with waste aluminium hydroxide the sulphate concentration decreased somewhat slowly but steadily through the test duration of six hours, after which the residual sulphate concentration was below 150 mg/I. The results suggest, that non-pre-treated waste aluminium hydroxide reacts (dissolves) slowly in these conditions and thus becomes the limiting parameter for sulphate removal. Sulphate removal kinetics were much faster in the early stages of the experiment with the pre treated waste aluminium hydroxide slurry. After the fast initial drop in sulphate con centration, only minor decrease was observed during the later stages of the experi ment. This test was terminated after two hours. In the experiment with aluminium chloride the sulphate removal kinetics were also fast, and sulphate concentration continued to decrease until being <40 mg/I at the time of 30 minutes.

The additional results for aluminium and calcium are shown in Figure 3 and Figure 4, respectively. As can be seen from Figure 3, surplus of dissolved aluminium was present in both experiments, mainly due to stoichiometric overdosing. The amount of dissolved aluminium increased slightly in the experiment with pre-treated aluminium hydroxide suggesting further dissolving of the solid aluminium from the hydroxide. From Figure 4 it is evident that the amount of dissolved calcium present, also needed for ettringite formation, was 6 to 7 times lower throughout the test with AICI3. The relatively high residual sulphate concentration in the experiment with pre treated waste aluminium hydroxide is thus explained by the low concentration of dissolved calcium as the limiting factor for ettringite formation.

Waste aluminium hydroxide sludge was successfully used as aluminium source in ettringite precipitation for sulphate removal from mine process water.

When used as received in solid form, the reaction kinetics with waste alumin ium hydroxide in sulphate removal were slow in comparison to highly soluble AICI3, and thus at least suspending the material in water should precede the use.

Pre-treatment of the waste Al-hydroxide by suspending in water and by warming the suspension up to 60°C resulted with fast kinetics in sulphate precipita tion.

Additional analyses of dissolved aluminium species verified that aluminium from pre-treated waste aluminium hydroxide was solubilized enabling ettringite for mation.

EXAMPLE 2 - In situ precipitation of Aluminium hydroxide and use in ettringite pre cipitation

Main characteristics of the pilot overflow from Nickel & Cobalt precipitation are shown in Table 3.

Table 3. Characteristics of pilot-overflow from Ni & Co precipitation

Pilot-overflow from Ni &Co precipitation (Table 3) was used to produce a sulphate bearing waste water for sulphate removal experiments. This was done by raising the pH of the effluent to around 1.5 with 10% lime milk, and subsequently separating the liquid and generated solids with 0.45 pm membrane filtration. Main characteristics of the sulphate bearing effluent are shown in Table 4. Table 4. Characteristics of sulphate bearing waste water

Aluminium recovery

Visualisation of Al-recovery experiments is shown in Figure 5. 500 ml of Ni&Co-overflow (characteristics in Table 3) was mixed in a glass vessel, and 4 M NaoH was added to the solution until pH 4.5 was reached. Nitrogen gas was purged into the mixture during the whole test period to prevent excessive oxi dation of Fe²⁺ to Fe³⁺ and subsequent precipitation, and thus enable more selective precipitation of aluminium. The mixture was mixed for three hours, after which the test was terminated, and generated solids were separated from the liquid (overflow) with 0.45 pm membrane filter. The precipitate was washed twice with 200 ml deion ised water. After the test all the liquid samples and the solid precipitate (after wash ing) were sent to analyses. pH of the filtrate was also measured.

Sulphate removal In the sulphate removal experiment the generated solid from the aluminium recovery experiment was used as aluminium source in ettringite precipitation (Fig ure 6). The solid was kept moist after aluminium recovery experiment in closed ves sel in order to avoid further crystallisation of the generated precipitate and thus lower the reactivity in ettringite precipitation. Before ettringite precipitation experiment, the solid was slurried by adding deionised water in ratio 1 (solid) to 2 (water) and mixing overnight.

In the sulphate removal experiment 4.4 g (dry mass) of the precipitate from Al-recovery was added as slurry to 233 ml of sulphate bearing water (

Table 4). Dose of the Al-containing precipitate was calculated based on Equation 1 , Al-content in the precipitate and sulphate concentration in the sulphate bearing waste water. Lime milk was also added in order to ensure pH to be in the favourable range for ettringite precipitation (>11) and also to ensure enough Ca to be present for ettringite formation. The mixture was mixed for four hours. During the test small samples were taken from the mixture to follow sulphate removal kinetics. After the test generated solids were separated from the liquid with 0.45 pm mem brane filter. The overflow (treated water) was sent to chemical analyses. pH of the filtrate was also measured.

Analytical methods

Solid precipitate generated in Al-recovery experiment was characterised for elemental composition. Semi-Quantitative X-ray fluorescence analysis (XRF) was performed by using a Panalytical Axios mAX 3 kW-X-ray fluorescence spectrometer with semi-quantitative Omnian-programme. Fluorine (F) and heavier elements, were determined from the sample, except for the noble gases. Quantification limit of the method is typically around 0.01 %.

Sulphate concentrations in the liquid samples were determined according to SFS-EN ISO 10304-1 by ion chromatography. Ca, Mg, Na, Al, Mn and Fe in the liquid samples were determined according to SFS-EN ISO 11885 by inductively cou pled plasma optical emission spectrometry (ICP-OES). Ni in the liquid samples was determined according to SFS-EN ISO 17294-2 by inductively coupled plasma mass spectrometry (ICP-MS). Sulphate concentrations for assessing sulphate removal ki netics in ettringite precipitation were determined with a Hach Lange DR3900 spec trophotometer and LCK 353 kits.

Results

Aluminium recovery

Figure 7 summarises the results of Al-recovery. It can be seen from the figure, that Al-precipitation appeared to be almost complete and at least somewhat selec tive. Concentrations of the major substances expect for aluminium remained on the same level in the overflow than in the feed. However, some impurities, mainly sul phur and iron were contained in the precipitate even after the two washing steps. Assuming, that aluminium precipitated as AI(OH)3, the concentration of it in the pre cipitate is around 66 %.

In addition to the chemical analyses also practical observations could be made. Perhaps most essential of these was, that the filtration of aluminium contain ing precipitate took a long time. It is likely that the formed aluminium hydroxide mostly had small particle size which made the filtration challenging. Sulphate removal

Figure 8 summarises the findings from ettringite precipitation. In addition, Fig ure 9 shows the sulphate removal kinetics in the experiment. The recovered alumin ium hydroxide seemed to function very well in the ettringite precipitation resulting in quite low residual sulphate concentration in the overflow. Also the sulphate removal kinetics were quite rapid, and after 60 minutes no further decrease in the sulphate concentration could be observed.

Claims

1. A method of removing sulphate from waste water in an ettringite precipitation process, wherein a waste aluminium compound and/or secondary aluminium com pound is used as aluminium source to precipitate sulphate as ettringite.

2. The method of claim 1, wherein the waste aluminium compound and/or the secondary aluminium compound is derived from aluminium refining process.

3. The method of claim 1, wherein the waste aluminium compound and/or the secondary aluminium compound is precipitated from industrial waste water.

4. The method of any one of claims 1-3, wherein the waste aluminium com pound and/or the secondary aluminium compound is aluminium hydroxide AI(OH)3_, aluminium hydroxide oxide AIO(OH) and/or aluminium chloride AICI3.

5. The method of any one of claims 1-4, wherein the waste aluminium com pound and/or the secondary aluminium compound is aluminium hydroxide AI(OH)3_.

6. The method of claim 5, wherein the waste aluminium AI(OH)3 and/or the sec ondary aluminium AI(OH)3 has the crystal from of gibbsite.

7. The method of claim 5, wherein waste aluminium AI(OH)3 and/or the second ary aluminium AI(OH)3 is amorphous.

8. The method of any one of claims 1-7, wherein the waste aluminium and/or the secondary aluminium is pre-treated by heating to a temperature of 50°C-80°C in normal pressure/atmospheric pressure.

9. Use of waste aluminium compound and/or secondary aluminium compound as aluminium source in an ettringite precipitation to precipitate sulphate from a waste water.

10. The use of claim 9, wherein the waste aluminium compound and/or the sec ondary aluminium compound is derived from aluminium refining process.

11. The use of claim 9, wherein the waste aluminium compound and/or the sec ondary aluminium compound is precipitated from industrial waste water.

12. The use of any one of claims 9-11 , wherein the waste aluminium compound and/or the secondary aluminium compound is aluminium hydroxide AI(OH)3_, alumin ium hydroxide oxide AIO(OH) and/or aluminium chloride AICI3.

13. The use of any one of claims 9-12, wherein the waste aluminium compound and/or the secondary aluminium compound is aluminium hydroxide AI(OH)3_.

14. The use of claim 13, wherein the waste aluminium AI(OH)3 and/or the sec ondary aluminium AI(OH)3 has the crystal form of gibbsite.

15. The use of claim 13, wherein the waste aluminium AI(OH)3 and/or the sec ondary aluminium AI(OH)3 is amorphous.

16. The use of any one of claims 9-15, wherein the waste aluminium and/or the secondary aluminium is pre-treated by heating to a temperature of 50°C-80°C in normal pressure/atmospheric pressure.