Alumina Feeding and Raft Formation: Raft Collection and Process Parameters

Abstract

Alumina, alongside with electricity and carbon, is the raw material used for production of aluminium in the Hall-Héroult process. An efficient dissolution process is important to acquire stable conditions for the cell, resulting in lower energy consumption. Under certain conditions, the alumina will not dissolve upon addition but remains afloat on the bath surface as a so called raft. The conditions under which the rafts form are still not fully understood, although it is likely that their behaviour is influenced by operational conditions which in turn depend upon bath and alumina properties. In order to obtain more knowledge on the conditions for raft formation, an industrial measurement campaign was performed at Alcoa Mosjøen in which raft behaviour was recorded alongside collection of bath and alumina samples as well as the rafts themselves. The current paper describes the procedure utilized for data collection together with an analysis of bath and alumina properties, aiming to correlate these with raft flotation times. Raft floating times were found to vary between 5 and 140 seconds during normal operating conditions.

The Micro- and Macrostructure of Alumina Rafts

“…For reference, sample images of raft 10 following collection (as of Gylver et al [14]) is given in figure 2. As indicated, the intact part of the raft had a diameter of approximately 15 cm and height of 2 cm.…”

“…secondary alumina (SGA) properties, bath temperature and chemistry. In order to study the influence of these conditions, alumina and bath samples were collected in parallel to the rafts, as described in Gylver et al [14]. Of the 12 rafts collected during the measurement campaign, 5 selected rafts have been analyzed further as described in table 1, together with key SGA properties and STARprobe TM [15] data.…”

“…Table 1 Overview of rafts selected for further analysis along with key operational parameters from [14]. F, BJH and MOI denote fluor concentrations (absorbed from scrubbing), porosity and moisture respectively, cf.…”

“…F, BJH and MOI denote fluor concentrations (absorbed from scrubbing), porosity and moisture respectively, cf. [14] for further details, while Al 2 O 3 , xsAlF 3 , T b and T s denote alumina concentrations, excess AlF 3 , bath temperature and superheat, all of which obtained by STARprobe TM measurements.…”

A heat and mass transfer problem for the dissolution of an alumina particle in a cryolite bath

“…Feeding is typically carried out by breaking the crust that forms at the top of the cell and then pouring alumina as a granular material into the bath, where ideally it sinks and dissolves. However, experimental research [3][4][5][6][7] on both the laboratory and industrial scale indicate that, in some circumstances, the alumina particles may form a floating structure of undissolved alumina at the surface of the bath. These rafts may remain afloat for much longer times than the feeding cycle [7], ultimately disrupting the process.…”