In the context of creating solidification products which are suitable for placement within a responsibly operated landfill, there are two fundamental quality criteria.

1. the creation of a mixture which will set to form a solid product with low leachability and which will remain physically stable over the long term even when exposed to normal ground water, occasional surface waters and/or occasional rain water.
2. the creation of a mixture which will set to form a solid product with relatively low leachability but which needs to be kept clear of air, water or moisture ingress over the long term, in order either to prevent degradation (e.g. physical destruction of its structure as a result of swelling) or to prevent biological processes from causing excessive degradation.

Products which achieve quality criteria 1 will have low long term liability even if the landfill lining or other containment features around the deposit are ruptured.

Products which achieve quality criteria 2 rely on the containment system around them in order to limit their long term liabilities. The typical mode of failure is for the product to either expand into the form of a compacted powder or soft aggregate or to form a stiff sludge or even a mobile slurry. Control of the initial water input is usually sufficient to prevent the degraded products from becoming a mobile slurry. However, leachate extrusion from the degraded product is a frequent occurrence, accompanied by an associated bulk settlement within the deposit.

Any product which does not reach quality criteria 2 should not be considered other than as a means of making the waste suitable for safe transport to an off site specialist landfill or treatment process.

As part of the BPEO and subsequent work which was carried out for the world’s largest integrated production facility for ferrochrome and stainless steel (for more details see the Integrated Industrial Waste Management Section) RPA developed and designed a solidification process to stabilise the site’s hazardous solid waste residues. This was a particular challenge in view of the total load principles which apply in South Africa and the chromium, nickel, zinc and cadmium content of the residues.

The products from the process were able to demonstrate TCLP and Acid Rain Leach extract concentrations for each of these metals which consistently averaged below the following.

* This was the limit of detection in this work.

In conjunction with this work, RPA designed a 100 hectare landfill adjacent to the site to receive the stabilised wastes in a manner which exploits the physical characteristics of the inerted waste so as to maximise the environmental protection whilst minimising the construction and operating costs.

RPA has recently been commissioned to carry out a similar programme of development and design work for the large iron and steel production site which is also referred to in the Integrated Industrial Waste Management section.

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