Long Way of Energy Conservation and Pollutant Reduction in Inorganic Silicides   

By Ge Yongsheng, Qingdao Dongyue Sodium Silicate Co., Ltd.    

Inorganic silicides mainly include sodium silicate, silica white, silica gel and 4A zeolite.

  1. Sodium silicate

China produces at least 1.50 million tons of sodium silicate a year. The energy consumed is mainly upon coal. Based on 350 kilogram of standard coal per ton of product on average, the consumption of coal in the sector is around 530 000 tons a year. Major raw materials for the sodium silicate production include quartz sand and soda ash. Producing one ton of sodium silicate needs to consume 400 kilogram of soda ash and producing one ton of soda ash consumes 390 kilogram of coal on average. If the coal consumption in soda ash is added, producing one ton of sodium silicate needs to consume 506 kilogram of standard coal. The total emission of CO2 (carbon dioxide) is 2.19 million tons and the emission of SO2 (sulfur dioxide) is 15 300 tons a year in the sector.

    (1) Energy conservation
Great attention is already paid to energy conservation in the sodium silicate production in the world. Many producers have strengthened management and actively adopted technical renovation measures such as development of large-scale kilns, quality improvement and full insulation of kiln materials, increase of waste heat recovery and application of wet-process production technology. The consumption of coal per ton of sodium silicate in these producers is mostly controlled at around 280 kilogram and even 230 kilogram. China has more than 200 sodium silicate producers. They are mostly small enterprises with simple equipment and outdated process. Most of them are distributed in the vicinity of coal producing areas and rely on the low coal price. The consumption of coal per ton of product is usually around 500 kilogram and even as high as 1000 kilogram in some outdated producers.

    (2) Emission reduction
The target of emission reduction in the sodium silicate production mainly focuses on the control of SO2 and waste water emission. The sulfur content in coal is mostly more than 1% and sometimes reaches 3% in China. Flue gas generated in the sodium silicate production can hardly reach the emission standard. Desulfurization units have a high operating cost. It is reported that in domestic sodium silicate producers only Jiangsu Debang Chemical Group Co., Ltd. owns a desulfurization unit and keeps it in operation.
   Most of small sodium silicate producers use low-priced anhydrous sodium sulfate as raw material to replace soda ash. In such process the SO2 content in flue gas is more than 4% and pollution is extremely serious. Such enterprises are mostly located in townships and villages. SO2-containing flue gas has rendered surrounding crops into yellow and withered. Many experts strongly claim for the prohibition to the use of this process. Due to its low production cost, however, this sodium silicate process is protected by local governments pursuing after profits.
   In normal cases, the process water generated in the sodium silicate production gets into final products with no emission of waste water. Today, however, most producers sell glass state sodium silicate to other places for melting. The waste water in melting can not be digested internally and has mostly to be discharged without any treatment. Moreover, some township and village enterprises use the water quenching process. The waste water generated has a high concentration and alkalinity. Its direct discharge causes serious damages to farmlands. Some enterprises located in cities do not treat with waste water either and the direct discharge causes pollution to rivers.

  2. Silica white

    (1) Energy conservation
Silica white is a downstream product of sodium silicate. The output of silica white in China already reaches around 600 000 tons a year today, holding the first place in the world. Rhodia has constructed a silica white factory in Qingdao, Shandong province. Energy conservation and emission reduction in the factory is done well and worth of being a demonstration for domestic enterprises. First of all, in water scrubbing water contained in filter cakes is removed as much as possible to reduce the energy consumption in drying. The energy consumption per ton of product is around 370 kilogram of standard coal. Fuel used in drying is town gas. There are very few pollutants in its discharge and production waste water can reach the discharge standard in the factory. Many silica white enterprises in China, however, directly burn raw coal in the production. One ton even two tons of coal is consumed in producing one ton of silica white. Producing one ton of silica white needs to consume 1.4 tons of sodium silicate (the energy consumption equivalent to 708 kilogram of standard coal). Along with the coal consumption in silica white, the total energy consumption per ton of silica white is around 2.2 tons of standard coal. If the consumption of water, power and steam is added, the total energy consumption per ton of silica white is around 2.5 tons of standard coal in those domestic factories.
   The consumption of water in the silica white production is huge. Producing one ton of silica white usually needs to consume around 40 tons of water. In the project proposal prepared by Huber Engineered Materials of the United States that is launching a silica white project in China, the consumption of water per ton of product is designated at 22 tons.

    (2) Emission reduction
Scrubbing water used in the silica white production contains great quantities of sulfate ion, producing serious pollution. How to properly handle scrubbing water is a big problem in the silica white production. Zhuzhou Xinglong Chemical Industry Co., Ltd. controlled advanced water treatment technology that has already been applied for patent.
   The production process of silica gel and 4A zeolite is similar to silica white, but their capacity is much lower and the development trend is not as robust as silica white.