Conference Agenda

Findings are sometimes beyond the objectives of a research and development (R&D) project. This was the case with the Improved Low Temperature Digestion (ILTD) Process. The Updated ILTD Process will shortly be introduced as a state-of-the-art technology for processing gibbsitic bauxites. The Updated ILTD Process is most viable for bauxites or aluminous laterites with high reactive silica content, although this process is associated with several other benefits beyond the saving in the NaOH consumption. A method that makes possible to determine the amounts of important minerals (gibbsite and kaolinite) well below the detection limits of the X-ray diffractometry (XRD) is presented in the paper. A more accurate method for the calculation of the theoretical extraction yield for gibbsite in comparison with the conventional approach is proposed. A novel method for the calculation of the savings in primary energy consumption is explained and proposed. The breakdown of the additional investment costs to the ILTD Process and sintering-leaching step is also provided. The use of the sintering and leaching of DSP (desilication product) by-product for the elimination of the organic impurities at the same time is also explored.

This paper presents information on the pre-processing of bauxite using two different flowsheets for the removal of iron and kaolinite components at the grinding stage at the Pavlodar Aluminium Plant (PAP) of Aluminium of Kazakhstan JSC. These solutions reduce input of impurities into the Bayer circuit: Fe2O3 - by 35-40%; CO2 - by 40-60%; SO3 - by 50-70% and SiO2 - by 12-15%.

The removal of iron sand reduces the mass of Fe2O3 entering the sintering stage by ~20%, partially debottlenecking the sintering furnaces, increasing the throughput capacity of both the Bayer and sintering circuits.

This in-house development of PAP is the main technological solution to increase the capacity of the “Bayer-Sintering in series” plant and was achieved while maintaining alumina product quality. Implementation of this flowsheet also solved the constraint faced from declining ore reserves, allowing lower quality ore to be processed and extend the life of operation.

In the Bayer Process, the dissolution of alumina happens under various digestion conditions. While low temperature digestion dissolves trihydrate alumina phase (THA), high temperature digestion process is required to dissolve mono-hydrate alumina (MHA). In high temperature digestion process, in addition to MHA, the amount of quartz present in bauxite also plays a vital role in the estimation of digestion efficiency. The quartz content is estimated by the difference between the total silica (SiO2) and kaolinitic silica (k.SiO2) in bauxite. Unlike in the low temperature digestion, wherein the amount of k.SiO2 attacked by caustic liquor is solely considered to estimate the desilication efficiency across pre-desilication tanks and low temperature digestors, in the high temperature digestion, the quartz attack is also considered as a factor in the estimation of overall desilication reaction and chemical extraction. Even though 100% attack of k.SiO2 occurs for both double digestion as well as high temperature refineries, the quartz attack is low and is dependent on the quartz content, mineralogy, and conditions prevalent in high temperature digestion. Studies conducted earlier with Indian bauxites from eastern region, indicated that the quartz attack can be maximum of 30%. However, considering the change in bauxite mineralogy over the years and the upgrade of the digestion technology to higher digestion temperatures (~280 degC), it is imperative to study the kinetics of quartz dissolution in detail.

The impact of bauxite quality in terms of varying MHA and quartz content have been studied under high temperature digestion conditions to estimate the extent of quartz dissolution and subsequently its impact on digestion extraction efficiency. This paper presents details of the test work done to establish the relationship between kinetics of quartz dissolution and bauxite quality viz MHA and quartz content under high temperature digestion conditions.

The production of alumina is a crucial process in the aluminum industry since primary metallic aluminum is obtained from alumina. Among the various methods for producing alumina, the Bayer process is the most commonly used. Bauxite, the main ore for alumina production, typically contains gibbsite as the main mineral in Amazonian bauxite in Brazil. In general, Amazonian bauxite has iron and titanium oxides, as well as clay minerals like kaolinite (source of reactive silica). The presence of silica is a common and significant problem in the bauxite and alumina industry due to kaolinite, quartz, and other silicates present in various types of bauxite. In the Bayer process, reactive silica reacts with sodium hydroxide (NaOH), forming the sodalite increasing the alumina production costs and waste generation at the refinery. The purpose of this work was to evaluate the influence of sodium hexametaphosphate as a dispersant to enhance flotation selectivity in the reverse flotation of an Amazonian bauxite, aiming to improve its chemical quality. Bench flotation tests were performed at natural pH (~ 7) with a constant collector dose of 200 g/t of Flotinor 16939 and a variable sodium hexametaphosphate dosage as pulp dispersant (0, 250, 500, 750 and 1000 g/t). The presence of 250 g/t of sodium hexametaphosphate reduced the reactive silica content in flotation concentrate approximately 6 percentage points compared to baseline (without dispersant), from 38% to 32%. The best flotation scenario tested (200 g/t of collector and 250 g/t of dispersant) had alumina recovery of 73% eliminating 68% of reactive silica content. It has potential to reduce approximately 60 kg of caustic soda consumption per ton of bauxite processed in the Bayer Process. Two adsorption mechanisms can explain the research results. At a dosage of 250 g/t of dispersant (best effect of silica reduction), greater dispersion of the flotation pulp may have occurred, increasing the selectivity of flotation (better adsorption of the collector on the kaolinite surface). At dosages higher than 250 g/t, there was a decrease in the potential for reducing the reactive silica content. This may be related to the depressive effect of sodium hexametaphosphate on kaolinite, reducing its flotation. Sodium hexametaphosphate can improve the selectivity of bauxite flotation.

Bauxite is the main ore for alumina and then primary aluminium production, consisting mainly of gibbsite, as aluminium source, iron oxides such as goethite and hematite and kaolinite, a clay mineral commonly found in Amazonian bauxites, as the main carrier of reactive silica. In the process, due to the small particle size, kaolinite is usually removed by attrition and washing of coarse material followed by desliming using hydrocyclones. Kaolinite has a special relevance in this context as in the Bayer process, it reacts with sodium hydroxide, thus increasing caustic consumption in the process. Beneficiation process at Hydro Paragominas is based on the separation of coarser fractions with higher gibbsite content from the clay minerals, where kaolinite is more concentrated. The separation takes place in two-stage hydrocyclone circuits, one for mid-size particles classification and another for fine particles classification. Pilot tests and process simulations have been carried out seeking the optimization of cyclone geometry, by testing different apex and vortex diameters. A total of 36 different conditions were evaluated with optimized condition bringing reactive silica reduction potential of 1.3 percentual points.