In the Paragomina’s region, as well as the whole Brazilian north, gibbisitic bauxite deposits commonly shows high amount of kaolinite. The processing of this bauxites (called as High Silica Bauxites - BASR) became a challenge because in the conventional conditions of the Bayer process, the kaolinite is undesirably leached by NaOH solution, and then precipitated as sodalite. The formation of this phase brings a significant increase in process costs, both by increasing the processing time as the irreversible loss of NaOH robbed from the system to form sodalite, which is then discarded in the red mud. Given this metallurgical problem this study aimed to investigate the crystal chemistry of sodalite formed in conventional conditions of the Bayer process, so that, from these results, studies to reduce these losses of the process and facilitate the processing of BARS can be developed in the future. The materials investigated were kaolinitic gangues from 4 lithologies of an exploration well in Miltonia 3 mine (BN, BNC, BC and BCBA), as well as a kaolin from IMERYS S.A. The digestions were carried out in Teflon-lined, stainless steel autoclaves, using 1 g of solid material, 25 mL of NaOH solution and at a temperature of 150ºC in an oven. The NaOH concentration and the reaction time ranged from 2,5 to 5,0 M and 60 to 420 min, respectively. Then the solid material was characterized by XRD, DTA/TG, FTIR, SEM and ICP-OES. The results of the starting materials showed that the kaolin sample is essentially constituted by kaolinite, which has a high structural ordering degree. All the samples of kaolinitic gangue showed the same minerals: gibbsite, kaolinite, hematite, goethite and anatase. By the observation of the XRD patterns and DTA curves can be noted that the BN and BNC samples are more reactive than the others, possibly due a lower structural ordering degree and particle size. In the experiments with kaolin, it can be observed that are formed not one, but two sodalite phases, which coexist practically throughout the whole process and tending to an equilibrium phase. These two phases differs themselves by the amount and behavior of the NaOH and H2O molecules within the framework. The results of the refinments showed that these phases were: basic sodalita with cell parameter (ao) ~ 8,96 Å, which is predominant in the initial stages of the transformation, and hydrosodalite with ao ~ 8,85 Å dominant in the secondary stage (mainly in 180 min). The XRD results from kaolinitic gangues showed that in 60 min there was no full kaolinite/sodalite conversion, and the sodalite patterns in BN and BNC were more intense and well defined than BC and BCBA, confirming that these samples have more reactive kaolinites. The increase of the reaction time and NaOH concentration provided a slight increase of the structural sodalite order. It may be noted that in almost all experiments the only phase formed was basic sodalite. The exceptions were the lithologies: BCBA and BC, which hydrosodalite was formed in the highest time of reaction and NaOH concentration, thus showing that this phase is directly associated with a higher time, concentration and crystallinity of kaolinite available in the reaction medium. It can be observed that throughout the reaction, in shorter time and concentration it is not possible to achieve a balance, which leads to a constant interchange of predominant phase in the system. In the higher time and concentration experiments, a balance is virtually reached, in order that it cannot be observed separated diffraction peaks. However there is no significant increase of the structural ordering even for extreme times as 3 days of reaction. The ammonium chloride experiments showed that this reaction medium allows the formation of more crystalline phases than all others achieved in this research. However, it did not result in the diminished of the sodium consumption.
|
