After a long gap, I have appeared before you to interact on a small but very critical process issue of Bayer Alumina Plant.
In the past, we have always discussed about the negative impact of high silica in bauxite specifically with regard to operational problems, process control issues, higher consumption of input materials and deterioration in quality of product hydrate or alumina. In previous articles, this issue was elaborated at length with measureable / quantifiable approach so as to focus on its importance in Bayer Alumina Refineries.
In present post, we will briefly discuss about the advantages and disadvantages of low silica in processing bauxite for production of calcined alumina. The question mark (?) on the subject itself forces us to think and re-think about the positive and negative aspects of low silica in bauxite.
It’s a well-known fact that all Alumina producers wish to have low silica bauxite for obvious reasons discussed earlier. Now, before coming to the subject directly, following questions arise-
(i) What type of silica is harmful for Bayer’s process?
(ii) What should be the cut-off silica content in bauxite for acceptable quality of alumina?
Though we have discussed these issues in several articles, still it is prudent to freshen up with salient features as outlined below-
We are well aware that predominantly there are two types of silica in bauxite designated with regard to Bayer process of alumina (a) Non-reactive silica and (b) Reactive silica. ROM bauxite mined adopting either open cast or underground mining methodology contains a number of metallic oxides including silica. This mined silica as percentage of total dry bauxite is termed as total silica. The part of total silica which reacts with caustic soda and forms sodalite complex through sodium silicate route is termed as Reactive silica. Majority of Reactive silica is found in form of Kaolinite. And thus, the non-reacting component of total silica with caustic soda is popularly known as Non-reactive silica. Quartz is non-reactive silica. The silica being abrasive in nature causes erosion in components of various process equipment like heat exchangers, pipelines, valves, pump casing and impellers etc. Thus higher silica content affects the capital investment of Alumina refinery because robust material of construction for equipment component in contact with bauxite slurry. Hence higher silica in bauxite causes higher initial investment for the plant and higher repair maintenance cost for routine replacement of critical components of equipment.
Generally, it has been seen from bauxite analysis that Reactive silica in bauxite ranges from 50% to 80% out of Total silica in bauxite. Plant operation is affected because of high silica in bauxite as it has high settling velocity and abnormally high abrasive nature.
Now coming to the topic with raised question mark, it’s true that low reactive silica in bauxite is considered as a serious process problem as it directly affects the quality of product alumina because of higher silica content in alumina. The SiO2 content in calcined alumina produced in all modern alumina a refinery is controlled at around 0.015%. The alumina having higher silica content is considered as inferior quality. For achieving 0.015% SiO2 in alumina, the SiO2 in aluminate liquor feeding to crystallizers / precipitators should be around 0.45 gpl. The ineffective desilication because of delay in achieving super-saturation of silica across desilicators, gpl SiO2 in liquor increases to much higher level which ultimately deteriorates the product alumina quality.
The above elaboration concludes that moderate level of Reactive silica in bauxite at around 1.5% in bauxite is considered to be the best with regard to plant efficiencies and product quality as well.
Trust, the subject has been covered here to your desired level of satisfaction in understanding the criticality of the subject issue. We would welcome your valued comments / remarks to further substantiate the subject.