Sensing and modelling for oxygen lead softening
Creators: name: Kapusta, Joël Patrick Thierry
Publisher: name: University of British Columbia
Year: 2009

description: Lead ores generally contain significant amounts of arsenic and antimony. The process of lead softening, corresponding to the removal of these hardening impurities, is therefore of necessity in any pyrometallurgical lead refining circuit. Even in a smelter where electrorefining is used, a partial softening might be required. The treatment of silver bearing concentrates, while providing additional revenues, also introduces additional amounts of arsenic and antimony into the circuit. The efficiency of the Betts electrorefining process depends on the stability of the corroding lead anodes. Anode slime stability is directly related to the antimony and arsenic content of the anodes. Thus, control of bullion quality prior to anode casting is key to optimization of the electrorefining operations, improvement of lead production, and addition of revenues. The (partial) softening stage is the ideal location in a smelter for such a control. When composition spikes are regular occurrences, the batch softening circuit tends to become somewhat complicated to reduce their effects. In this thesis work, the option of continuous single pass softening has been explored. In particular, the two critical components to this revision, i.e. a method of continuously monitoring bullion quality, and a control strategy, were investigated. An oxygen probe for continuous measurements in molten lead has been designed in the laboratory prior to being tested in an industrial environment. A commercially available yttria stabilized zirconia serves as solid electrolyte. The reference system is composed of a Cu-Cu₂O mixture. Both lead wire and conducting lead (probe housing) are made of 316 stainless steel. Sealing is achieved by means of a high temperature magnesia cement. An additional plug of copper powder, isolated from the reference system by a layer of alumina powder, serves as oxygen getter to eliminate oxygen ingress from the atmosphere. The extremity of the lead wire that is inserted into the probe is coated with cement to avoid any short-circuit with the Cu plug. These features were decisive in the success of the probe which provides a continuous measurement for several consecutive days. Once it was established that the probe was giving satisfactory measurements in the laboratory, i.e. quick response, proper response to temperature changes and oxygen potential changes, a testing campaign was carried out in the plant. The campaign was successful and a correlation between measured emf and As+Sb bullion content was established. Since any control decision would be based on the probe readings, it is crucial to regularly ensure the proper functioning of the probe. A method based on the thermal arrest technique has been tested and calibrated to provide for a quick assessment of the probe reliability. A thermodynamic model of the current semi-batch process was developed. An analysis of the process showed that a thermodynamic model can be used to represent process operation, and an equilibrium analysis gave a reasonable fit to operating data. The model developed for semi-batch softening was modified to continuous softening in order to simulate continuous single pass softening. The preliminary calculations showed that the concept of continuous single pass softening will meet the target set for lead softening, assuming the process operates close to thermodynamic equilibrium. A slag high in As+Sb can be produced at bullion compositions in the target range for electrorefining. Based on the data presented in this thesis, a simple feedback control algorithm could be developed to regulate O₂ injections to a continuous softener on the basis of the measured level of As+Sb in output bullion.