Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...
Reexamination Certificate
2000-03-08
2001-08-28
Hruskoci, Peter A. (Department: 1724)
Liquid purification or separation
Processes
Making an insoluble substance or accreting suspended...
C210S713000, C210S724000, C210S726000, C210S912000, C423S122000, C423S128000, C423S129000, C423S158000, C423S165000, C423S166000
Reexamination Certificate
active
06280630
ABSTRACT:
BACKGROUND OF THE INVENTION
THIS invention relates to a process for the treatment of water streams.
Water streams, typically waste waters, and in particular acid mine drainage waters, can have pH values as low as 1.5 or 2. They also typically contain high levels of dissolved calcium, magnesium, heavy metals and sulphate. These water streams are traditionally neutralised with lime prior to discharge, and as a result they contain large quantities of calcium sulphate in solution. The discharge of these water streams into natural water courses presents a growing environmental problem.
The solubility of calcium sulphate dihydrate (gypsum) in water at 20° C. is 2.35 g/l, which means that one cubic meter of water saturated with calcium sulphate would contain 0.55 kg of calcium and 1.31 kg of sulphate. In other words, a saturated acidic mine water stream flowing at 500 m
3
/h, would contaminate natural water courses with 2390 t calcium and 5746 t sulphate annually.
SUMMARY OF THE INVENTION
According to the invention a process for the removal of sulphates and calcium from a water stream comprises the steps of:
combining the water stream and an amount of amorphous aluminium trihydroxide (Al(OH
3
));
allowing the formation of ettringite (3CaO.Al
2
O
3
.3CaSO
4
.31/32H
2
O) as a precipitate; and
recovering the precipitated ettringite from the water stream.
The water stream may be combined with both an amount of amorphous aluminium trihydroxide and an amount of lime (CaO) to raise the pH of the water stream, typically to 11.0.
The mass ratio of amorphous aluminium trihydroxide which is combined with the water stream is preferably such that the ratio of amorphous aluminium trihydroxide to sulphate present in the water stream is up to 1:1.
When the water stream contains calcium-associated sulphate and is saturated with respect to calcium sulphate and a high degree of calcium sulphate removal is required, this ratio may be from 0.1 to 1:1, such as from 0.5 to 0.8:1 or 0.65 to 1:1.
The ettringite may be removed from the water stream by settling and/or filtration. Amorphous aluminium trihydroxide may be recovered from the ettringite removed from the water stream by decomposing an ettringite suspension or slurry as described below. The recovered amorphous aluminium trihydroxide may be recycled to treat a water stream containing sulphates and calcium.
The water stream may be effluent mine water with a pH ranging from highly acidic to highly alkaline.
The pH may be in the range 2 to 8 or greater.
The process may include a first preliminary step of raising the pH of the water stream. The pH may be raised to a value of between 11.0 and 12.6 such as between 11.7 and 12.3 or between 11.7 and 12.0.
The pH of the water stream may be raised above 12.0 to remove sulphates associated with potassium and/or sodium.
The pH of the water stream may be raised by adding lime (CaO) and/or another alkali to it.
The first preliminary step may include the removal of impurities that are insoluble at the raised pH value as precipitates.
The precipitates may be hydroxides.
The impurities may comprise magnesium and/or heavy metals, such as iron, aluminium and manganese.
The process may include a second preliminary step of desupersaturating the water stream by reducing its calcium sulphate content to a saturation level.
The desupersaturating step may involve passing the water stream through a high solids precipitator to cause precipitation of calcium sulphate on gypsum seed in the form of calcium sulphate dihydrate (gypsum).
The gypsum seed may be freshly introduced or recycled from one of the solid/liquid separation steps of the process of the invention.
The process may include a post treatment step of adding carbon dioxide to the water stream to precipitate dissolved lime as calcium carbonate and to reduce the pH of the water stream to a value of between 7.0 and 10.0, preferably to between 7.5 and 9.0.
The precipitated calcium carbonate is preferably recovered by settling and/or filtration.
Alternatively, the post treatment step may involve the addition of sufficient carbon dioxide to from soluble calcium bicarbonate to produce a less corrosive, more stable waste water.
According to another aspect of the invention a process for separating amorphous aluminium trihydroxide from a suspension containing solid amorphous aluminium trihydroxide and solid or dissolved calcium sulphate comprises the steps of:
adding sulphate and/or calcium and a sufficient volume of water to the suspension to ensure that the calcium sulphate is substantially dissolved to form a supersaturated solution of calcium sulphate with amorphous aluminium trihydroxide in suspension; and
recovering the suspended amorphous aluminium trihydroxide.
The suspension containing amorphous aluminium trihydroxide and calcium sulphate may be a decomposed ettringite slurry decomposed in a decomposition step.
The pH in the ettringite decomposition step may have been lowered to decompose the ettringite.
The pH in the ettringite decomposition step is preferably lowered to a pH in the range of 4.0 to 8.5.
The pH in the ettringite decomposition step may be lowered by adding an acid or by sparging with CO
2
or SO
2
. Preferably, sulphuric acid is added to lower the pH and optionally to supersaturate the resulting calcium sulphate solution.
A sufficient volume of saturated calcium sulphate solution may be added to the decomposed ettringite to ensure that the calcium sulphate resulting from the breakdown of the ettringite is substantially dissolved to form the supersaturated solution of calcium sulphate, leaving the amorphous aluminium trihydroxide in suspension.
The process may also comprise the step of desupersaturating the supersaturated calcium sulphate solution by precipitating calcium sulphate on gypsum seed to form gypsum and a saturated calcium sulphate solution.
The saturated calcium sulphate solution may be recycled and added to the ettringite slurry decomposition step.
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patent: 0250626 (1988-01-01), None
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English Translation Of Claims of EP 0584502 Dated Mar. 2, 1994.
English Translation Of EP 0250626 Dated Jan. 7, 1988.
Taylor, R.M. et al. “Remediation of AMD: The Green Precipitate Process.” EPD Congress, (1998) pp. 367-379.
Brown, P W et al. “Kinetics of the Early Hydration of Tricalcium Aluminate in Solutions . . . ” Journal of the American Ceramic Society, vol. 67, (Dec. 1984) pp. 793-795.
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Bensted, J: A discussion of the paper“Solid Solution of Ettringites:Part 1:Incorporation of OH and CO32-in 3CaO.Al2O3.3CaSO4.32H2O”. Cement and Concrete Research.vol. 21,(1991)p679.
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Heil, G et al. “Reduction of Su
Hruskoci Peter A.
Ladas & Parry
Mintek
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