Process engineering approach to conversion of alum sludge and waste glass into zeolite LTA for water softening

Abstract

This study addressed the need to better understand process engineering aspects of zeolite LTA synthesis from waste materials. In particular, concepts such as zeolite yield, recycling of mother liquor, softening performance and regeneration of calcium loaded zeolite LTA were evaluated. Zeolite LTA was made via alkaline fusion pre-activation of alum sludge and waste glass at 700 °C for 2 h; followed by hydrothermal treatment at 80 °C for 5 h. Increasing the SiO2/Al2O3 ratio of the feed mixture from 1 to 2.3 doubled the yield of zeolite LTA to 67%. Consequently, a feed mixture of composition 9.2Na2O:Al2O3:2.3SiO2:276H2O was adopted for subsequent tests. Recycling of the alkali and other dissolved species in the mother liquor improved zeolite LTA crystallinity (80 to 86 wt%) and calcium exchange capacity from 73 to 78 mg/g. Implementation of recycling depends upon the cost/benefit analysis regarding energy demand for evaporation of mother liquor relative to savings by more effectively using the excess alkali reagent. Calcium uptake on zeolite LTA was reduced when treating simulated river water (65.5 mg Ca2+/g) due to competitive ion-exchange. Regeneration of calcium loaded zeolite LTA required a NaCl solution with a molarity of at least 2 M. A notable reduction of calcium loading capacity was detected after several loading/regeneration cycles. The outlined technology proposed a feasible and scalable approach to recycling of wastes, producing a commodity, which could be successfully used locally by the water treatment plant as a water softener.