1. Field of the Invention
The invention concerns a method of fixing aqueous sludge by means of a pozzolanic reaction.
2. Description of the Prior Art
Waste sludge containing toxic products in the solid phase can be harmful to the environment into which it is discharged because the solid phase is very finely dispersed and therefore has a very large surface area interface with external surroundings. This inevitably favors to a very considerable degree the passing into solution of toxic pollutants as a result of physical dissolving, hydrolysis, biodegradation and other phenomena. Additionally, such sludge includes an interstitial aqueous liquid phase which is itself contaminated to a greater or lesser degree and which constitutes a more mobile fraction.
The environment pollution problem is solved by fixing such sludge using various methods including methods utilizing pozzolanic reactions.
The pozzolanic capacity of a mineral is its capacity for fixing chalk at ordinary temperature to produce a hydraulic binder, by which is meant a substance able to set and therefore to harden and solidify in the presence of water.
This effect is extremely slow and it begins some two to three days after the mixing is done. The material changes very slowly with time and several years may be required to achieve the maximum mechanical strength.
The most commonly used pozzolanic material is fly ash with a particle size in the order of 1/30 micron. A distinction is drawn between silica-alumina ash resulting from the combustion of coal and sulfo-calcic ash produced by burning lignite.
In practice the pozzolanic effect of fly ash can be employed by simultaneously adding such ash and chalk to the sludge to be treated.
The chalk may already be present (in the form of Ca(OH).sub.2) in the sludge (neutralization or precipitation sludge containing excess chalk).
Commercial processes using pozzolanic reactions are already known from such general works as R. B. POJASEK "Toxic and hazardous waste disposal" volume 1. Ann Arbor Science 1979 and U.S. E.P.A. "Survey of solidification/stabilization technology for hazardous industrial wastes" E.P.A. - 600/2-79-056 July 1979.
The POZ-O-TEC process developed by the American company I.U.C.S. uses fly ash and other additives. Two reactions take place simultaneously: a fast reaction between the soluble salts and the chalk and alumina present in the fly ash, and a much slower pozzolanic reaction between the silica and the chalk.
Immediately after treatment the sludge is in the form of a viscous fluid that can be pumped and that is allowed to harden for several weeks at its storage site.
This process was developed for treating sludge resulting from SO washing of thermal power plant smoke and was subsequently used to solidify liquid waste produced by chemical and metal surface treatment industries.
Variants of this process are used in the Research-Cottrell process (see R. B. POJASEK reference above). The treatment system includes a gravity thickener, a centrifuge for dehydrating the sludge, a fly ash conveyor, a chalk conveyor and a dehydrated sludge/fly ash/chalk mixer.
In the case of waste sludge from metal surface treatment, a centrifuge or a conveyor belt press produces a residue containing approximately 80% by weight water. A filter press produces residue containing approximately 70% water.
This high water content has disadvantages because large quantities of chalk and pozzolan are needed to make it set and this results in large weights and volumes of solid waste to be stored.
What is more, the Research-Cottrell mixer must be a powerful and rugged unit, as its function is to intimately mix with the fly ash and chalk a thick medium consisting of the centrifugally concentrated sludge.
An object of the invention is to propose a method of treating relatively low concentration sludge (containing, for example, approximately 3 to 20% by weight of dry material relative to the weight of the sludge) that is simpler in that it does not require a powerful mixer, but which secures improved separation between the solid phase and the liquid phase of the sludge. In other words, the process according to the present invention, recovers 35 to 40% of the aqueous phase of the initial sludge. This aqueous phase can be recycled or purified, for example by means of ion exchanger resins.