Liquid purification or separation – Processes – Making an insoluble substance or accreting suspended...
Patent
1991-12-06
1994-01-25
Hruskoci, Peter
Liquid purification or separation
Processes
Making an insoluble substance or accreting suspended...
210704, 210717, 210725, 210728, 210912, 210913, 210914, C02F 124
Patent
active
052813398
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the removal or recovery of contaminants such as metals or organic chemicals from solution or suspension.
With an increased emphasis on environmental safety and pollution control the efficient removal of contaminants from wastewater has became of vital importance to the manufacturing industry.
Metals, particularly heavy metals, are one of the insidious pollutants of our environment. They are not biodegradable and may exist in a number of forms associated with living organisms, water, sediments and suspended matter. The heavy metals accumulate in sediments and are subsequently taken up by organisms. With the process of bio-accumulation through the food chain many edible species of aquatic life are affected to the extent that their consumption may be hazardous.
Also the removal of oils and other organic chemical residues from waste water streams is critical to efficient pollution control.
The most widely used method of treatment of waste water containing heavy metals is by precipitation and sedimentation. Although this treatment method can, when operating at peak efficiency, give an effluent containing less than 1 ppm of each heavy metal, under normal operating conditions it often exceeds the limit (e.g. 10 ppm) for disposal to sewer set by Trade Waste Laws. The reasons for this range from bad maintenance routine to the presence of interfering substances such as oils.
Sedimentation is a slow process requiring large settling volumes.
The conventional process is also subject to number of interferences which can effect the rate of settling--particularly if the system uses NaOH for neutralization. Even well operated clarifiers can have 5 to 50 ppm of suspended solids in the overflow. Contaminants, such as oils and substances which produce gases, are the most common source of problems since they interfere with the sedimentation process by increasing the buoyancy of the particles.
The flocculants used to speed up the sedimentation process in conventional technology are almost exclusively organic polyelectrolytes. These are expensive and require a certain degree of mixing before clarification is attempted. Depending on the size of the clarifier used, a separate mixing zone may be required in the process. The polyelectrolytes also add to the volume of sludge generated.
The polymer flocculants often required pH values of 9 to 10 for peak performance. Although these pH values are acceptable for disposal to sewer (max. pH=10) a considerable expense is incurred in raising the pH to this level.
The conventional technique of precipitation results in gelatinous precipitate that, even after flocculation and sedimentation yields a wet bulky sludge (95% water) that requires disposal. One of the most promising methods of separation and concentration of metal ions and fine particles is adsorbing colloid flotation.
Adsorbing colloid flotation has a number of attractive features (i) low energy requirements (ii) high removal efficiency (iii) reasonable capital requirements and (iv) comparatively low maintenance and operating costs, thereby potentially providing a low cost method of heavy metals recovery from industrial wastewaters.
The process generally involves the production of a hydroxide precipitate of the metal ions via pH adjustment by adsorption and/or co-precipitation with a floc generating material such as Fe(OH).sub.3 or Al(OH).sub.3, rendering the floc hydrophobic by adsorption of a surfactant and its subsequent removal by flotation with air bubbles.
Batch flotation yield high levels of metal removal for most ion combinations but with a time requirement of up to 10 minutes. Similarly laboratory studies of copper recovery using iron hydroxide as the adsorbing colloid and sodium lauryl sulphate as the frother required about 25 minutes but achieved levels in the effluent consistently less than 1 ppm. Most studies of heavy metal removal using these techniques have been carried out on simulated wastewaters made up from pure components.
Two studies on actual industrial effluents were based on copper r
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Harding Ian H.
Mainwaring David E.
Sanciolo Peter
Hruskoci Peter
ICI Australia Operations Proprietary Limited
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