The Bottom-blown Oxygen Converter or BBOC is a smelting furnace developed by the staff at Britannia Refined Metals Limited (“BRM”), a British subsidiary of MIM Holdings Limited (which is now part of the Glencore group of companies). The furnace is currently marketed by Glencore Technology. It is a sealed, flat-bottomed furnace mounted on a tilting frame that is used in the recovery of precious metals. A key feature is the use of a shrouded lance to inject oxygen through the bottom of the furnace, directly into the precious metals contained in the furnace, to oxidize base metals or other impurities as part of their removal as slag.J M Floyd, “Submerged bath smelting applied to the non-ferrous metal industry,” in: The Paul E. Queneau International Symposium, Extractive Metallurgy of Copper, Nickel and Cobalt, Volume I: Fundamental Aspects, Eds R G Reddy and R N Weizenbach (The Minerals, Metals and Materials Society: Warrendale, Pennsylvania, 1993), 473–488.
Introduction Ores mined for their base metal content often contain precious metals, usually gold and silver. These have to be removed from the base metals as part of the refining processes used to purify the metals. In the case of copper electrolytic refining, the gold and silver fall to the bottom of the electrolytic refining cell as “slimes” that are subsequently treated to recover gold and silver as byproducts. In the case of lead refining, silver, gold, and other precious metals are typically removed using the Parkes process, in which zinc is added to the impure lead bullion to collect the silver, gold and other precious metals.R J Sinclair, The Extractive Metallurgy of Lead (The Australasian Institute of Mining and Metallurgy: Melbourne, Victoria, 2009), 211–213.
The BRM lead refinery at Northfleet in England uses the Parkes process followed by liquation and a vacuum induction retort to recover precious metals. The product of this process is a feed for the BBOC consisting of a mixture of lead, silver (60–75%), zinc (2–3%) and copper (2–3%), with trace amounts of gold.R P Knight, “Oxidation refining in the Bottom Blown Oxygen Converter,” Erzmetall, 48 (8), 1995, 530–537. Prior to the development of the BBOC, BRM used cupellation in a 15 tonne (“t”) reverberatory cupellation furnace to recover the precious metals from this mixture. Three of these furnaces were used to produce 450 t of silver per year.
Cupellation works by exposing the mixture at high temperature to air or oxygen.R J Sinclair, The Extractive Metallurgy of Lead (The Australasian Institute of Mining and Metallurgy: Melbourne, Victoria, 2009), 216. The base metals, being less noble than silver and gold, react with the oxygen to form their oxides, which separate from the noble metals to form a slag that floats on the top of the residual metals (or “doré”). At BRM, the doré contains 99.7% silver.
To maximize the oxygen transfer from the blast air in the reverberatory furnace, a shallow bath is used, thus increasing the surface-area-to-volume ratio of the furnace.R P Knight, “Further applications of the bottom blown oxygen converter,” in: International Symposium on Injection in Process Metallurgy, Eds T Lehner, P J Koros and V Ramachandran (The Minerals, Metals and Materials Society: Warrendale, Pennsylvania, 1991), 335–346.
A problem with using reverberatory furnaces for cupellation is that the zinc oxidizes first, forming a crust across the top of the molten material. This crust prevents the penetration of oxygen to the rest of the material, and so it has to be manually broken up and removed using a rabble bar. This is both labor-intensive and also results in the loss of some of the silver. Similarly, the oxidized lead slag has to be removed when it forms to maintain the operation, and its removal also results in loss of silver.
The BBOC was developed by BRM personnel as a way of reducing these and other problems, such as low energy efficiency and low oxygen utilization, associated with the reverberatory cupellation process.
Description The BBOC furnace is a cylindrical steel vessel with a protective internal lining of refractory bricks. It is mounted on a tilting frame that allows it to be held at different angles at different stages of its operating cycle (see Figure 2). A hood is fixed over the top of the furnace, providing a seal that prevents lead and other fumes from escaping during the furnace’s operation (see Figure 1).
The key feature of the BBOC is the shrouded lance that passes through the refractory bricks at the bottom of the furnace. This lance allows oxygen to be injected directly into the molten metal contained in the furnace, away from the refractory lining. Doing so allows the region of high reaction rates to be removed from the vicinity of the lining, thus reducing its wear.
By injecting the oxygen directly into the bath, rather than blowing it on top (as in the case of the reverberatory cupellation furnace or top-blown rotary converters), the oxygen transfer efficiency is not impeded by the presence of the slag layer. It results in an oxygen utilization efficiency approaching 100%.
The lack of interference in the oxygen transfer by the slag layer has a couple of key benefits. The first is that the increased certainty in the estimation of oxygen utilization efficiency means that it is easier to calculate the endpoint of the process, making process control much easier. The second is that a thicker slag layer can be tolerated (because the oxygen does not have to pass through it), and this means that the losses of silver to the slag are reduced (because it is the silver at the interface between the metal and slag that becomes entrained during the removal of the slag and the thicker the slag layer, the smaller the silver content of the removed slag). BRM reported a decrease in the silver content of the BBOC slag compared to the reverberatory furnace slag of 50%.
BRM found that the reaction rate of the BBOC was 10–20 times that of its reverberatory cupellation furnace.
Refractory wear in the BBOC is largely confined to the slag line, at the top of the metal, where attack by litharge (lead oxide) is greatest. This is combated by using fused-grain, direct-bonded magnesite-chrome bricks to line the inside of the furnace shell.
Operation
Figure 2 shows the positions of the BBOC at various stages of the operating cycle.
The BBOC is held in an upright position during the charging stage. A solid or liquid charge is added using an overhead crane. The furnace is then tilted forward so that the lance is above the charge, and the charge is melted using an oil or natural gas burner that is inserted near the top of the furnace. Once the charge has been melted, the furnace is tilted back into the blowing position and oxygen is blown into the bath. Slag formed from the oxidation of lead and zinc is removed periodically by tilting the furnace forward again and pouring it off.
The oxygen flow rate during blowing for a three tonne capacity furnace is 20–30 Nm3/h. Zinc is initially oxidized to form a zinc oxide dross on the surface of the charge, but as lead oxide subsequently forms, a fluid slag of zinc and lead oxides is created. Most of the copper is removed at the same time as the lead. The final removal of copper to a level of 0.04% is undertaken at the end of the process by further additions of lead to collect the copper.
If the lance needs to be replaced at any time during operation, this is done by tilting the furnace forward until the lance is above the surface of the bath, where it can be removed and replaced without the contents of the furnace draining through the hole in the furnace lining.
The cupellation process finishes when the silver is about 99.7% pure. At this point, the silver is poured from the furnace and transferred to another furnace, where a flux is added to upgrade and remove the oxygen from the silver to produce market bullion of 99.9% purity.
History Early development at BRM 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.K R Barrett, “Operation of the bottom blown oxygen cupel at Britannia Refined Metals, Ltd,” in: Today's Technology for the Mining and Metallurgical Industry, MMIJ/IMM Joint Symposium 1989, Kyoto, Japan, 2–4 October 1989, (The Mining and Materials Processing Institute of Japan, and the Institution of Mining and Metallurgy: 1989), 589–595. 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 Nm3/h to 7500 Nm3/h.
Commercialization After the successful operation of the BBOC at BRM, MIM Holdings Limited (“MIM”) decided to license the technology to other smelter and refinery operators. Early adopters included Hindustan Zinc Limited, which by 1995 had two 1 t BBOC plants operating in India, and ASARCO Inc., which was operating a 3 t BBOC furnace at its Omaha, Nebraska, refinery.
Rand Refinery