The nature and impacts of fines in smelter-grade alumina

Abstract

Fines in smelter grade aluminas are recognised as a significant process problem in aluminium smelting. However the understanding of the nature of this fine material and how it impacts on the reduction process are less clearly understood. The combination of new analytical methods such as Variable Pressure Scanning Electron Microscopy and very high field Solid State NMR provide new insights into the spatial distribution of phases within aluminas, and suggest how such fine materials are generated, particularly during calcination in the alumina refinery.

The Pivotal Role of Alumina Pore Structure in HF Capture and Fluoride Return in Aluminum Reduction

“…A high SSA material is an ineffective scrubbing agent if much of the porosity cannot be accessed fully on time scales relevant to dry scrubbing. This consideration has been recently been identified by Perander et al 9,10 who suggest that the finer porosity may be difficult to access. This becomes relevant when it is recognized that the SSA of a material of under-and over-calcined aluminas blended together may have a comparable BET SSA to a more evenly calcined material (so-called bimodal aluminas).…”

“…However, it has been recognized earlier that the smallest pores may not be readily accessible by HF. 9,10 This porosity, the surface area of which is included in the BET method, can vary considerably between different alumina sources and would therefore introduce a significant amount of noise into the model. Pore size distribution (PSD)-based surface areas were therefore explored.…”

“…However, as one moves to increasingly narrow pores, limited access on kinetic grounds has been postulated. 9,10 The transition from unhindered access to kinetically restricted porosity, to which unique surface area loss models are to be applied, is not likely to occur abruptly at some threshold pore diameter. Therefore, our model partitions pores according to a gradually varying function that determines the fraction of the length of a pore of a given diameter that should be treated with the each of the adsorption models.…”

Cracking Behavior and Mechanism of Gibbsite Crystallites during Calcination

“…Inconstant feeding can lead to over‐feeding and under‐feeding in the downstream refinery of aluminum, causing the forming of clusters onside the anode and the reduction of bath component respectively . Second, the content of α‐alumina in ultra‐fine particle is significantly higher than in larger particles, making it difficult to dissolve in the reaction cell . Moreover, ultra‐fine particles can cause health and environmental problems.…”

“…Gibbsite crystallite's cracking is widely considered as a result of the shrinkage of crystallites alongside their c ‐axis during calcination . Various factors, such as the calcination methods, the morphology of gibbsite particles, as well as the transition and storage condition can cast influence on the cracking and breakage of gibbsite particles.…”