Alumina Assignment Joseph Griggs – S3436440 Q2. a) The presence of silicates in the Alumina refining process has two major disadvantages for the process overall. If there is no attempt to remove the silicates, or they are improperly removed, the entire process can come to a standstill while the scale formed on piping is treated. Not only does this slow down the process, causing delays and a decrease in productivity for the plant, but money must also be spent on removing the scale. There are two main ways to prevent scale build up: limit the ability for the scale to form on the pipe surface, or remove the silicates early in the process to prevent them from affecting the later stages. Silicates are removed by reacting the active silicate compounds with alkaline solutions containing Cano (lime). This is done at a relatively high temperature (100°C) and takes 8~9 hours. During this process, aluminium in the form of Al2O3 and NaOH, a reactant used in the next processing steps, is consumed. The alternative is the prevent scale forming by adding chemicals which inhibits the scale formation. One example of a scale inhibitor is Sodalite, which prevents scale by adhering to the surface of aluminosilicate crystals which attach to the surface of piping. Once attached, the sodalite prevents further crystal growth, preventing any further build up. This method of prevention involves the introduction of a new chemical into the process costs, and any unreacted sodalite is a concern for later stages in the process. The Bayer process creates large amounts of ‘red mud’, a solution of iron and titanium oxides (giving it the red colour) in sodium hydroxide. Much of this waste is simply stockpiled, as there are very few viable uses for the by-product. The red mud can be stored in various ways depending on the concentration. For low concentrations of 25-30% w/w mud lakes are used to stockpile the red mud, as the solution is mostly liquid. This storage uses a large amount of land and so destroys local environment. Higher concentrations (> 30% w/w) are usually stored as dry solids and take up much less space, but still require some land use for storage, especially at lower percentages. The two significant methods for dealing with red mud are carbonation and seawater neutralization. Carbonation reduces the alkalinity of the red mud by forcing carbon dioxide through the red mud. This reacts with the sodium hydroxide, reducing the pH from 13.5 down to 10.5 (as pH is a log scale this is 1000x reduction in alkalinity), reducing the potential harm the red mud can cause. The other process, which can be quite expensive to due high energy and infrastructure required, is seawater neutralization. Seawater is mixed with the red mud, allowing for naturally occurring calcium and magnesium ions to react with the sodium hydroxide, reducing the pH. The reduction in pH is proportional to the amount of seawater used to treat the red mud, but with enough seawater the red mud can be completely neutralized, which is potentially viable for refineries near the coast. b) An alternative to liquor burning to remove organics midway through the Bayer process, is thermal pretreatment. This method submits the bauxite to thermal pre-treatment, producing a low temperature digestible feed, with low concentrations of organics (Hollitt, Kisler, & Raahauge, 2002). To achieve carbon removal, without loss of aluminium that is to be extracted, the bauxite must be stripped of any water content. Thermal decomposition can also break down a portion of Boehmite in the bauxite into more readily digestible alternatives, making this alternative pathway especially good for areas which have high concentrations of Boehmite. To process the bauxite, initially partial thermal dehydration occurs, which extracts carbon from the bauxite and eliminates it (Hollitt, Kisler, & Raahauge, 2002). This is done in a highly controlled way to keep digestibility at a maximum, while achieving a good reduction in organics. Keeping a high content of digestible aluminium requires avoidance of thermal deactivation of dehydrated Gibbsite (Hollitt, Kisler, & Raahauge, 2002). The extent of deactivation is not measurable, even by NMR, and is seen only in a decrease in digestion performance (Hollitt, Kisler, & Raahauge, 2002). Q3. Pre-Desilication The pre-desilication step in the refining process forms insoluble compounds from the reactive silica in the bauxite. To form these compounds, both NaOH and Al2O3 is consumed, with larger amounts being consumed for greater amounts of reactive silica. The NaOH is a valuable reagent and is very important for the digestion stage. The sample from Location A has double the amount of silica when compared to Location B, so the bauxite from Location B is more suitable for this step of the process. The reactivity of the samples is not specified however, and it is possible that the ratio of reactive silica may be different from the ratio of the overall amount. Further wet chemistry methods could help to further describe the reactivity of the silica in the samples. Digestion The digestion stage breaks down both the Gibbsite (Al(OH)3) and Boehmite-diaspore mix (AlO(OH)) into Bayer liquor. Gibbsite is much easier to break down than Boehmite-diaspore, so the sample form Location A will be much easier to process. The higher ratio of Boehmite-diaspore in the sample from Location B will require more NaOH reagent, increasing costs of the process. The total percentage of aluminium found in the sample from Location B is larger however, which may decrease the extra costs in the digestion stage by producing more aluminium at the end of the process. Several side reactions also take place in this stage. As Location B has a greater concentration of organics, it will produce a greater concentration of VOCs released during the process. This may increase the process cost of this stage, but this is probably negligible as the systems used to treat these are relatively cheap to scale. The higher organic concentration in Location B will also increase the consumption of the oxygen as they react to produce hydrogen and other gasses, which results in reducing conditions. Another major pollutant released in this stage is mercury, and an assessment of the mercury concentrations in the two samples could help to correctly asses the best bauxite source to use. Clarification The clarification stage separates iron oxides (Fe2O3), titanium oxides (TiO2) and silicates formed from pre-desilication from the Bayer liquor. Location A contains a higher concentration of Fe 2O3, and a higher concentration of silicates (assuming the reactivity is similar in both samples). The larger concentrations of these particulates will mean larger clarification vessels, an increase in the amounts of flocculants required to help precipitate the compounds, larger filtration and drying vessels, and larger equipment to deal with the dried cake by-product. The amount of waste (red mud) will also be greater for the sample from Location A. Remaining Stages and Conclusion The remaining stages in the Bayer process are practically the same for the bauxite from both sources, as the silicates, Gibbsite and Boehmite-diaspore, iron oxide and titanium oxide, and organics have all been removed from the process. After the first stages of processing the only difference in the product at this stage is the concentration of aluminium in the feed, which should be greater for the bauxite from Location B due to the higher initial concentration. The source of Bauxite which is recommended is Location A, as the high ratio of Gibbsite to Boehmitediaspore will greatly decrease the cost of the process in the digestion stage. Although the bauxite from this stage will result in a greater amount of red mud produced, the concentration of VOCs and the reducing effect from other organics will be much lower (less than 50% of Location B). However, if a higher yield of aluminium was required, and the increase to the cost of the process was acceptable to achieve this, then the bauxite from Location B may be the only suitable choice. References Hollitt, M., Kisler, J., & Raahauge, B. (2002). THE COMALCO BAUXITE ACTIVATION PROCESS. Proceedings of the 6th International Alumina Quality Workshop. Brisbane: Rio Tinto Limited.
