Henk Nugteren

Coal fly ash-slag-based geopolymers: microstructure and metal leaching.

This study deals with the use of fly ash as a starting material for geopolymeric matrices. The leachable concentrations of geopolymers were compared with those of the starting fly ash to evaluate the retention of potentially harmful elements within the geopolymer matrix. Geopolymer matrices give rise to a leaching scenario characterised by a highly alkaline environment, which inhibits the leaching of heavy metals but may enhance the mobilization of certain oxyanionic species. Thus, fly ash-based geopolymers were found to immobilize a number of trace pollutants such as Be, Bi, Cd, Co, Cr, Cu, Nb, Ni, Pb, Sn, Th, U, Y, Zr and rare earth elements. However, the leachable levels of elements occurring in their oxyanionic form such as As, B, Mo, Se, V and W were increased after geopolymerization. This suggests that an optimal dosage, synthesis and curing conditions are essential in order to obtain a long-term stable final product that ensures an efficient physical encapsulation.

Immobilisation of lead smelting slag within spent aluminate-fly ash based geopolymers.

This study presents the solidification/stabilisation and immobilisation of lead smelting slag (LSS) by its incorporation in coal fly ash - blast furnace slag based geopolymers. It also explores the use of a spent aluminium etching solution (AES) as geopolymer activator instead of the commonly used silicate solutions. The compressive strength of the geopolymers produced with the AES was lower than when applying a K-silicate solution as activator (100MPa versus 80MPa after 28 days). Compressive strength was not affected when up to 10% of the FA was replaced by LSS. NEN 12457-4, TCLP, SPLP and NEN 7375 leaching tests indicated that mobile Pb from LSS was highly immobilised. The diffusion leaching test NEN 7375 revealed exceeding of the Dutch Soil Quality Regulation threshold limits only for Se and Sb. On the condition that the remaining excess leaching can be reduced by further refinement of the mixture recipes, the proposed process will have the potential of producing waste-based construction materials that may be applied under controlled conditions in specific situations.

Improvement of Environmental Quality of Coal Fly Ash by Applying Forced Leaching

A coal fly ash containing leachable trace elements in amounts higher than allowed by Dutch law for its free application as granular building material was subjected to forced leaching using water, citrate-, oxalate-, EDTA- and carbonate-solutions. With water only, the free lime and substantial amounts of Mo (30%), Se (20-40%) and SO4 2- (40-50%) are removed. Using the extraction agents, these percentages increase to 60-90% and in addition 15-20% Cr, V and Sb are removed. The environmental quality of the washed fly ash was checked with the Dutch standard leaching test (NEN7343). Leachability of Mo, Se and S decreased by a factor of 2-4 after washing with water and a factor 2-18 (including Cr) after washing with the extracting agents. Although leaching values for individual elements complying with Dutch standards have been achieved, none of the conditions proved to produce a residue fully compliant with the law. Improvements by removing components remaining in the moisture after filtration are required. For a feasible process further reduction of reagent consumption and simple methods for processwater recycling are needed.

Fly ash refinement and extraction of useful compounds

Abstract The chemical refining of coal ash can have different objectives. On this basis, we can divide them into several categories. Quality improvement Improving technical quality Improving environmental quality Extraction of valuables Integral treatment technologies