Staff at BRM began work on an alternative to the conventional reverberatory cupellation furnace in the early 1980s. This included a review of the available technology, including the top-blown rotary converter ("TBRC"), on which test work was undertaken.
One of the first areas investigated was the use of oxygen-enriched blast air in the reverberatory furnace. This was “found to be of marginal benefit and not economically viable."
The BRM staff subsequently tried to increase the oxygen transfer rate by using lances submerged in the bath of the reverberatory furnace and found that there was some benefit in doing this. However, the wear rate of the lances was excessive and it was realized that the basic design of the furnace, with its shallow bath, was not conducive to the development of a high-intensity reactor.
The concept then evolved into a new furnace design, one that had a deep bath, in contrast to the reverberatory furnace design.
Initial tests of the bottom injection of oxygen were carried out on a small scale at Imperial College, London, using a nitrogen-shrouded tuyere. These showed that under certain conditions a protective accretion would form at the tip of the injector, and that oxygen utilization was high, with the oxidation reactions generating sufficient heat to keep the furnace hot until the final stages of refining when the impurity levels were low.
Additionally, the test work on the TBRC had shown that it had a high rate of refractory wear, due to the washing action of the slag caused by the rotation of the furnace, which provided additional pressure to develop an alternate process. The TBRC test work also resulted in low oxygen utilization (about 60%).
Based on the success of the small-scale tests, and with calculations indicating that the new design would have significant energy savings over the reverberatory furnace, the BRM staff built a 1.5 t pilot plant with a working volume of 150 liters (“L”). The oxygen injector was a fixed tuyere, located at corner of the base with the side wall, with an annular nitrogen shroud.
The initial pilot plant tests showed that it was difficult to maintain the protective accretion that had been generated in the small-scale tests, due to the variation in temperature and bullion composition that occurred throughout the cupelling cycle. Without the accretion, the nitrogen shroud could not provide sufficient protection to the injector, and it burned back to the level of the refractory lining, which resulted in damage to the lining.
The solution eventually developed was the concept of the moveable lance system in place of the fixed tuyere that had been used initially. The lance was pushed further into the furnace as its tip was worn away.
The initial lance advancing system was manual, but the current automated system was subsequently developed.
Once a sustainable system had been developed in the pilot plant, and after three years of pilot plant development, a commercial, 3 t-scale BBOC was commissioned at BRM in 1986. Its use reduced the fuel consumption per tonne of silver by 85%, from 30 gigajoules per tonne (“GJ/t”) to 4.5 GJ/t and the exhaust gas volume from 32 000 Nm/h to 7500 Nm/h.