Fabrication of water glass adhesive silicate from amorphous silica of rice husk ASH

Abstract

Producing water glass or sodium silicate adhesive from locally available rice husk ash (RHA) is an innovative thinking to create a new industrial sector in Bangladesh. New understandings and advances in water-glass adhesive production technologies and modification of water glass as adhesive are highlighted in this research paper. The purpose of this research is to produce high performance water glass adhesive having outstanding binding properties and fast drying time for corrugated carton packaging industries from amorphous silica rich agricultural biomass waste- RHA through hydrothermal treatment. Water glass is basically a compound containing silica (silicon dioxide, SiO2) and sodium oxide (Na2O) combined in various ratios (1.6 to 3.9 weight ratio of SiO2/Na2O) to show very useful properties of being soluble in water. Soluble silicates with higher weight ratios of silica and alkali (above 3.5 weight ratio of SiO2/Na2O) are comparatively better adhesives than low ratio silicates due to their higher content of polymeric silica. When used as binders and adhesives, which depend on the presence of polysilicate ions, sodium silicates having a SiO2:Na2O weight ratio of 2.5 to 3.8 is required. For maximum adhesive strength the lower ratio (2.5-2.8) types are used because they can be obtained with a higher concentration of solids. For more water-resistant bonds, the higher weight ratios (3.2-3.8) are preferred. High ratio water glass has high binding strength and faster drying properties. Producing higher mole ratio (above 3.5) water glass at lower temperature and pressure is still a challenging issue. In reviewing the literature, no data was found on fabrication of water glass adhesive having higher mole ratio (above 4.0) from amorphous silica of RHA using lower temperature and lower consumption of sodium hydroxide. Based on the new understandings and experimental results reported in this thesis, thus an economically viable route has been proposed to fabricate higher mole ratio water glass adhesive from rice husk instead of quartz sand. Ratio modification was made by increasing polymeric silica content of water glass to increase the binding strength and faster drying properties. The reaction parameters such as strength of alkaline solution, reaction period, agitation and temperature were investigated to establish optimum conditions under which maximum silica conversion could be possible at 88%. RHA was characterized by XRF for its chemical analysis and purity. Other properties of water glass were also characterized. A method has been proposed in this research for producing a stable water glass solution having a stabilized density in the range from 38o Bé° to 40o Bé° and a SiO2:Na2O ratio in the range from 1.8:1 to 4.2:1 with the proper proportion of silica (SiO2) content in the range from 23.7% to 29.9% by weight and alkali (Na2O) content in the range from 7.1% to 13.3% by weight. Aqueous water glass solution is produced by reacting the amorphous silica rich biomass ash preferably RHA (which is produced in the generation of heat and electricity by firing solid biomass fuel at 600-750ºC) with 0.6-1.1 M aqueous alkali metal hydroxide solutions (sodium hydroxide), in a high pressurized closed vessel under elevated pressure (2-8 bar) and at temperatures above 120-170°C; while the ratio by weight of sodium hydroxide to rice husk ash were being maintained by 3:1 to 9:1, and the aqueous water glass solution obtained is subsequently filtered and concentrated at or above 120ºC temperature and under 2-3 bar pressure in an evaporator to get specific density and viscosity which possess proper adhesive properties along with quick drying time. The reaction is carried out at a temperature in the range from 120 to 170°C and under a pressure in the range from 2 to 8 bar. The ratio of biomass ash having greater than 90% amorphous silica to sodium hydroxide by weight between 3:1 and 9:1. The diluted aqueous water glass solution is concentrated at a temperature in the range from 110ºC to 130ºC and under pressure in the range from 1.5 bars to 2.8 bar in order to stabilize the solution against gelation and particle growth. Physical analyses such as viscosity, pH, specific gravity, were carried out on the extracted sodium silicate which was compared with the reference sodium silicate sample. The results of the XRF analysis carried out on the RHA shows that the inorganic content of this ash contains a good percentage amount of SiO2 (>96%). The high silica content therefore justifies the use of the RHA as a silica source for this research work. Under this research, water glass binder having unique adhesive bonding and fast-curing characteristics with high ratio above 3.5 was successfully produced from rice husk ash at lower temperature compare to that of conventional method.