The lime-soda sinter process for resource recovery from fly ash: A new look

The lime-soda sinter process is one of the earliest and most thoroughly researched and evaluated of the several methods available for resource recovery from fly ash. The principle product, metallurgical grade alumina, is obtained with yields as high as 90% depending upon how much alumina needs to be left in the residue to form acceptable byproduct cement clinker. The process has the advantages of requiring a relatively low sintering temperature (1100-1200°C), using conventional equipment of carbon steel construction, utilizing a variety of calcium and mineralizer raw materials, and producing only a single byproduct consisting of dicalcium silicate that has been shown to be an attractive raw material for the manufacture of portland cement. An economic feasibility study for a combined facility to produce alumina and cement from the fly ash generated by a 1000 MWe coal-fired power station shows a 10.5% return on average investment. This is increased to 14.2% when a disposal charge of $\$$10/ton of fly ash consumed is credited to the process. Research has shown that the soda ash can be replaced by coal cleaning refuse or that the soda ash and one-fourth of the limestone can be replaced by FGD sludge with a savings in raw material cost in both cases. The return on average investment becomes 14.5% when the refuse is used and 15.2% when the sludge is used. The return could be increased further if an inexpensive fluxing agent were substituted for the alumina deliberately left in the residue.