A controllable continuous mass-feed system for aluminium smelters

Abstract

A continuous mass flow rate alumina feeding system has been developed that is capable of improving the alumina dissolution in aluminium reduction cells. This system will also enable improved cell control and reduced power consumption, as well as a range of other benefits. The unique feature of this system is its ability to detect operating problems at an early stage, enabling remedial action to be taken. The continuous mass flow rate alumina feeding system comprises a flow control valve, a solids mass flow meter and a computer-controlled feeding strategy. · A series of trials were carried out in order to develop the best possible system. The flow control valve was used to adjust the rate at which alumina was discharged from a feed hopper. A variable and discrete-type valve configuration was tested. The variable flow valve arrangement was preferred because it permitted alumina to be discharged over a range of flow rates, including the balanced feed condition in which the concentration of alumina in the electrolyte remains approximately constant. The solids mass flow meter was required to calibrate the mass flow rate of alumina being discharged at set valve positions. In deriving a suitable operating design, flow meter trials focused on the mass sensing mechanism, flow chamber design, and the calibration sequence. Regular valve calibrations were automated in order to overcome flow rate irregularities arising from material property changes in the feed stream. The calibration sequence took less than two minutes to complete and was accurate to within ±6% of actual flow rate values. This sequence was independent of material property changes. The feed control strategy was responsible for activating valve movements, and hence, setting the discharge rate in line with process requirements. · The feed control strategy also included a sequence for implementing on-line calibration of the flow control valve. Based on this work, two final product continuous mass flow rate alumina feeding systems have been proposed. Each system will require a space that is approximately 1 15 mm wide, 230 mm in length, excluding the total length of the feed control valve, and 200-300 mm deep, excluding the height of the feeding chute. Both units have been designed to be fitted beneath the alumina feed hopper(s) in a Hall-Heroult cell. The final product valve and flow meter arrangements are modular in design 'to minimise installation times. Two possible installation scenarios were proposed, based on the upgrade of a 170 kA P69-type centre-break and feed cell. In each case, the new cell superstructure will need to be modified away from the cell and swapped with the old superstructure during a hot change over sequence. The overall cost of the preferred feeding system is expected to be less than $NZ 500, per feed point, excluding retrofit costs. A simple model was developed in order to evaluate the potential ways in which a continuous mass flow rate alumina feeding system could change cell control. In addition to predicting the alumina concentration in the electrolyte, it was found that the new system should also be capable estimating the rate at which alumina is supplied to the electrolyte by non-direct feeding contributions, such as the back feeding of sludge. Both of these calculations were found to be largely dependent on bath volume changes and the flow rate error at set valve positions. The concentration of alumina in the electrolyte should be able to be calculated to within 25% of actual values in the 1.5 to 3.5 wt% range. It is expected that there will be approximately 60% error associated with calculation of non-direct alumina feeding contributions based on a value equal to 10% of the faradic requirement of the cell. In summary, this work presents two very simple continuous mass flow rate alumina feeding systems that are capable of leading to major improvements in cell performance.