Based on the results of the pilot plant work, the MIM Holdings Board of Directors approved the construction of an A$65 million demonstration plant, capable of producing 60,000 t/y of lead bullion. This plant operated from early 1991 until 1995. It was initially designed to treat 20 t/h of lead concentrate using lance air enriched to 27%. However, the oxygen originally designated for its use was diverted to the more profitable copper smelting operations, and the feed rate to the lead ISASMELT demonstration plant was severely restricted. When there was sufficient oxygen available in 1993 to increase the enrichment level to 33–35%, treatment rates of up to 36 t/h of concentrate were achieved, with residual lead in the final reduction furnace slag being in the range of 2–5%.
The two-stage approach to ISASMELT lead smelting was partly driven by the relatively low lead content of Mount Isa lead concentrates (typically in the range of 47–52% lead during the lead ISASMELT development period). Trying to produce lead bullion in a single furnace with such low concentrate grades would result in excessive fuming of lead oxide with a huge amount of material that would have to be returned to the furnace to recover the lead and, consequently, a higher energy demand as that material had to be reheated to the furnace temperatures.
Concentrates with higher lead contents can be smelted directly into lead metal in a single furnace without excess fuming. This was demonstrated on the large scale in 1994, when 4000 t of concentrate containing 67% lead were treated at rates up to 32 t/h with lance air enriched to 27%. During these trials, 50% of the lead in the concentrate was converted to lead bullion in the smelting furnace, while most of the rest ended up as lead oxide in the smelting furnace slag.
Like the lead ISASMELT pilot plant, the lead ISASMELT demonstration plant suffered from constraints imposed by the waste gas handling system. In the case of the demonstration plant, the problem was caused by sticky fume that formed an insulating layer on the convection tube bundles of the waste heat boilers, significantly reducing the heat transfer rates and thus the ability of the boilers to reduce the waste gas temperature. As the plant used baghouses to filter lead fume from the waste gas, it was necessary to reduce the temperature of the gas below the point at which the bags would be damaged by high temperatures. The problem was solved by allowing cool air to mix with the hot waste gas to lower the temperature to a level at which the baghouse could operate. This reduced the ISASMELT plant's capacity because it was again limited by the volume of gas that could be filtered by the baghouse.
The lead ISASMELT demonstration plant was mothballed in 1995 because there was insufficient concentrate to keep both it and the rest of the lead smelter operating. It was too small to treat all the Mount Isa lead concentrate by itself.