Development of value-added products from sugarcane boiler ash : final report 2011/903

Abstract

Bagasse fly ash from co-generation plants is often recycled into the sugarcane fields. However, bagasse fly ash contains quartz (naturally accumulated by the cane) that may be re-suspended into respirable-sized particulate by wind erosion and cultivation, where it can cause silicosis. Therefore, the transformation of the bagasse fly ash waste stream into value-added products such as zeolites and or concretes has been investigated. Factorial Design Analysis (neural network development) has been utilised to define the optimal conditions for zeolite manufacture and addition rates to concretes.

Zeolite development shows that the extremely versatile zeolite-X can be manufactured at temperatures that allow the utilisation of waste steam from the sugar mill, under alkaline hydrothermal conditions. The zeolite produced has a very high cation exchange capacity, making it desirable for water treatment, soil addition, and other industrial uses. Moreover, the zeolites developed are self supported providing very high surface areas on relatively large particles, which facilitate the treatments of waste waters by increasing flow rates. Adsorption kinetics of Cu and Zn to zeolite indicates that these are rapid second order kinetics, which coupled with the high exchange capacity, increases the usefulness of the product in water treatment systems.

Concretes developed showed improved strengths, and chemical resistance, particularly to chemicals (sulphate and chloride) and acids. Acid resistance in concrete improves service life of the concrete, particularly to organic acids found in silage and spilled cane juice. Hence the concretes developed have immediate application with sugar mills, where cane-juice often leads to the rapid exposure of concrete aggregate and concrete deterioration.

Economic assessments based on the test results are promising, with a modest capital investment in plant and equipment to process the bagasse fly ash to zeolites. Similarly there is a modest investment to process the ash for use in concrete works close to the mills. This modelling suggests that there could be a return on investment of about 23% for zeolite manufacturing, and 21% on reuse of the bagasse fly ash into concretes. This does not include the cost recovery from dealing with the bagasse fly ash as a product rather than a waste for disposal.

Both of the successful outcomes (zeolites and concretes) provide alternatives to the issue of bagasse ash disposal, which is becoming increasingly restricted. Both outcomes provide new products with increased and improved performance that provide a value added pathway for a sugar mill waste stream. Furthermore, the zeolite production is achievable at temperatures which can utilise the residual steam developed in the cogeneration boilers, to generate further efficiencies from the calorific value of the sugar cane bagasse.