Treatment of acidic water containing dissolved ferrous cations

Details Treatment of acidic water containing dissolved ferrous cations Treatment of acidic water containing dissolved ferrous cations

2000-03-13

2002-07-16

Hoey, Betsey Morrison (Department: 1724)

Liquid purification or separation

Processes

Making an insoluble substance or accreting suspended...

C210S713000, C210S724000, C210S738000, C210S758000

Reexamination Certificate

active

06419834

ABSTRACT:

THIS INVENTION relates to the treatment of water. More particularly, the invention relates to a process for the treatment of water which is acidic and contains dissolved ferrous (Fe
2+
) cations, optionally in association with dissolved sulphate (SO
4
2−
) anions.
The Applicant is aware of relevant prior art constituted by U.S. Pat. Nos. 5,427,691, 3,738,932, CH-A-590 791 and Patent Abstracts of Japan, Volume 009, No. 222 (1985) & Database WPI, AN 85-1501615 & JP-A-60 084196.
In U.S. Pat. No. 5,427,691 processes are disclosed for the treatment of acidic waters containing dissolved Fe
2+
cations by means of aeration to oxygenate the water and by raising the pH of the water in the presence of suspended particulate material, to oxidize the Fe
2+
cations to Fe
3+
cations and precipitate them as Fe(OH)
3
. Lime is employed to raise the pH of the water to above 7 and the aeration takes place with the water at a pH above 7. A prior art process is also discussed in U.S. Pat. No. 5,427,691, in which limestone is used to remove Fe
3+
cations. To remove Fe
2+
cations using limestone, however, the Fe
2+
cations must first be oxidized to Fe
3+
cations, and doing so at acidic pH levels with air (dissolved oxygen) is described as ‘almost impossible’ because of the slow reaction rates.
In U.S. Pat. No. 3,738,932, similarly, a process is disclosed for the treatment of acidic waters containing dissolved Fe
2+
cations by means of aeration to oxygenate the water and by raising the pH in the presence of suspended particulate material, to oxidize the Fe
2+
cations and precipitate them as Fe(OH)
3
. In this case, likewise, lime is used to raise the pH of the water to above 7 and the aeration takes place with the water at a pH above 7. No se of limestone or dolomite is described.
In CH-A-590 791 and Patent Abstracts of Japan supra, it is disclosed that it is known to use the oxidizing action of specific bacteria to oxidize Fe
2+
cations to Fe
3+
cations; and in said Patent Abstracts of Japan a two-stage process is disclosed whereby acidic sulphuric acid-containing waste water containing high concentrations of Fe
2
+ cations is subjected, in a first stage, to a bacterial oxidation wherein the Fe
2+
cations are bacterially converted to Fe
3+
cations. Then, in a second stage, calcium carbonate is added to the water which has been subjected to the bacterial oxidizing treatment, to precipitate Fe(OH)
3
.
According to the invention there is provided a process for the treatment of raw water containing dissolved Fe
2+
cations and dissolved H
+
cations so as to reduce the concentration of Fe
2
+ cations therein, the process comprising the oxidation of dissolved Fe
2+
cations and the formation in the water of solid Fe(OH)
3
from said Fe
3+
cations, the process comprising the steps of:
oxygenating the water to achieve a dissolved oxygen concentration in the water of at least 0.1 mg/l, effective to oxidize the Fe
2+
cations to Fe
3+
cations; and
raising the pH of the water, the oxidation of the Fe
2+
cations and the formation of the Fe(OH)
3
being carried out in the presence of suspended particulate material in the water, the particulate material being present in the water at an effective concentration of at least 5 g/l,
the process being characterized in that, in combination,
the raising of the pH acts partially to neutralize the water;
the raising of the pH is by dissolving limestone or dolomite in the water; and
the dissolving of the limestone or dolomite in the water and the oxidation by dissolved oxygen of the Fe
2+
cations to Fe
3+
cations in the water take place together in the same body of partially neutralized water in the presence of the particulate material.
The process may comprise an agitation step whereby the water is agitated as it undergoes the oxygenating and the raising of its pH. Agitating the water may be by fluidizing it in a fluidized bed, fluidized eg by upward flow of air or oxygen through a body of the water containing the particulate matter, the particulate matter comprising particulate matter, such as calcium carbonate, added to neutralize the water, particulate matter precipitated from the water such as ferric hydroxide or gypsum, and/or slimes added to the water as a microorganism support. Instead, agitating the water may be by upward flow of the water through a fixed bed of particulate matter, or a packed bed of a support medium of the type described hereunder, conveniently at turbulent flow rates. Instead, the water may be mechanically agitated, so as to provide a fully-mixed or completely-mixed body of water, which again may be turbulent. Instead, the process can be carried out in a pipe or tube, along which turbulent flow takes place; and the water may be circulated through said beds or along said pipe or tube by means of a pump.
The water to be treated, i.e. the raw water, will typically have a Fe
2+
cation concentration of at least 100 mg/l, usually 150-5000 mg/l and more usually 200-4000 mg/l; and will typically have a pH of at most 7, usually 1-6 and more usually 2-5. Often, the water to be treated will also contain SO
4
2−
ions at a concentration of at least 200 mg/l, usually 200-25 000 mg/l, and more usually 1000-10 000 mg/l. This water will typically have an Acidity, i.e. an HCO
−
3
acidity, expressed as mg/l of CaCO
3
, of 200-30 000, usually 400-25 000 and more usually 1000-10 000.
Oxygenating the water may be such as to achieve a dissolved oxygen concentration in the water of at least 0.5-8 mg/l, the suspended solid material having a particle size distribution whereby at least 50% by mass thereof has a particle size of less than 500 &mgr;m, the particulate material being present in the water at a concentration of at least 10 g/l, the raw water having a dissolved Fe
2+
cation concentration of more than 100 mg/l, and the process acting to to decrease the dissolved Fe
2+
cation concentration to less than 100 mg/l.
More particularly, the raw water may contain dissolved SO
4
2−
anions, the process including the biological oxidation of dissolved Fe
2+
cations to Fe
3+
cations and the process being carried out at a temperature of 0-90° C., preferably 5-40° C. In this case the raw water may have a dissolved SO
4
2−
anion concentration of at least 200 mg/l, the biological oxidation being carried out by microorganisms selected from
Ferrobacillus ferrooxidans;
Ferrobacillus thiooxidans;
Thiobacillus thiooxidans; and
mixtures of any two or more thereof.
The microorganisms may be supported on a support medium, to increase the concentration of microorganisms in the agitated water. While the support medium may be of metal or synthetic plastics material, such as rings, plates (which may be corrugated) and superimposed corrugated plates such as those available in South Africa under the Trade Mark TERBO PLASTIC, to provide a surface area for microorganism growth of at least 10 m
2
of support medium area/m
3
of agitated water, preferably 100-1000 m
2
/m
3
and more preferably 200-500 m
2
/m
3
, the support medium instead is conveniently a packed or suspended particulate material, in particular a finely divided particulate material, such as a slimes or sludge material or sediment added to the agitated water either continuously, or at the start of the process to be progressively supplanted by solids produced by the process as the process proceeds. The particulate material present in the agitated water may have a particle size of at most 500 &mgr;m, preferably 5-200 &mgr;m, and more preferably 10-100 &mgr;m. This particulate material may be present at a concentration of 10-500 g/l, preferably 50-200 g/l; and the particulate material may provide a particle surface area in the agitated water of at least 100 m
2
/m
3
of agitated water, preferably 100-10 000 000 m
2
/m
3
. Examples of suitable particulate materials for initial employment are waste coal fines and gypsum (when sulp

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