ISASMELT


The ISASMELT process is an energy-efficient smelting process that was jointly developed from the 1970s to the 1990s by Mount Isa Mines and the Government of Australia's CSIRO. It has relatively low capital and operating costs for a smelting process.
ISASMELT technology has been applied to lead, copper, and nickel smelting. As of 2021, 22 plants were in operation in eleven countries, along with three demonstration plants located at Mt Isa. The installed capacity of copper/nickel operating plants in 2020 was 9.76 million tonnes per year of feed materials and 750 thousand tonnes per year across lead operating plants.
Smelters based on the copper ISASMELT process are among the lowest-cost copper smelters in the world.

The ISASMELT furnace

An ISASMELT furnace is an upright-cylindrical shaped steel vessel that is lined with refractory bricks. There is a molten bath of slag, matte or metal at the bottom of the furnace. A steel lance is lowered into the bath through a hole in the roof of the furnace, and air is injected through the lance, causing vigorous agitation of the bath.
Mineral concentrates or materials for recycling are dropped into the bath through another hole in the furnace roof or, in some cases, injected down the lance. These feed materials react with the oxygen in the injected gas, resulting in an intensive reaction in a small volume.
ISASMELT lances contain one or more devices called "swirlers" that cause the injected gas to spin within the lance, forcing it against the lance wall, cooling it. The swirler consists of curved vanes around a central pipe forming an annular flow. They are designed to minimize pressure losses changing the angle from axial to tangential thus creating a strong vortex. The vortex helps mix liquids and solids with oxygen in the bath. The cooling effect results in a layer of slag "freezing" on the outside of the lance. This layer of solid slag protects the lance from the high temperatures inside the furnace. The tip of the lance that is submerged in the bath eventually wears out, and the worn lance is easily replaced with a new one when necessary. The worn tips are subsequently cut off and a new tip welded onto the lance body before it is returned to the furnace.
ISASMELT furnaces typically operate in the range of 1000–1200 °C, depending on the application. The refractory bricks that form the internal lining of the furnace protect the steel shell from the heat inside the furnace.
The products are removed from the furnace through one or more "tap holes" in a process called "tapping". This can be either continuous removal or in batches, with the tap holes being blocked with clay at the end of a tap, and then reopened by drilling or with a thermic lance when it is time for the next tap.
The products are allowed to separate in a settling vessel, such as a rotary holding furnace or an electric furnace.
While smelting sulfide concentrates, most of the energy needed to heat and melt the feed materials is derived from the reaction of oxygen with the sulfur and iron in the concentrate. However, a small amount of supplemental energy is required. ISASMELT furnaces can use a variety of fuels, including coal, coke, petroleum coke, oil and natural gas. The solid fuel can be added through the top of the furnace with the other feed materials, or it can be injected down the lance. Liquid and gaseous fuels are injected down the lance.

Advantages of the ISASMELT process

The advantages of the ISASMELT process include:
  • High productivity with a small footprint: Glencore's copper smelter in Mount Isa treats over 1 million t/y of copper concentrate through a single furnace 3.75 m in diameter. The small footprint makes the process well suited to retrofitting to existing smelters where there are significant space constraints
  • Simple operation: the ISASMELT furnace does not require extensive feed preparation as the feed can be discharged from a belt conveyor directly into the furnace
  • high energy efficiency: installing an ISASMELT furnace in the Mount Isa copper smelter reduced energy consumption by over 80% compared with the roaster and reverberatory furnaces previously used there
  • Flexibility in feed types: ISASMELT furnaces have been used to smelt copper, lead and nickel concentrates with a wide range of compositions, including high levels of magnetite, and secondary materials, such as copper scrap and lead-acid battery paste
  • Flexibility in fuel types: ISASMELT furnaces can operate with a variety of fuels, including lump coal of varying ranks, coke, petroleum coke, oil, natural gas, and liquid petroleum gas, depending on which is the most economic at the smelter's location
  • High turn-down ratio: the feed rate to a single ISASMELT installation can easily be scaled up or down, depending on the availability of concentrate and the needs of the smelter
  • Low feed carry over: ISASMELT furnaces typically lose about 1% of the feed as carry-over with the waste gas, meaning less material needs to be returned to the furnace for retreatment
  • Effective containment of fugitive emissions: because the furnace has only two openings at the top, any fugitive emissions can easily be captured
  • High elimination of deleterious minor elements: due to the flushing action of the gases injected into the ISASMELT furnace slags, copper ISASMELT furnaces have a high elimination of minor elements, such as bismuth and arsenic, that can have deleterious effects on the properties of the product copper
  • High sulfur dioxide concentration in the waste gas: the use of oxygen enrichment gives the ISASMELT plants high sulfur dioxide concentrations in the waste gas stream, making acid plants cheaper to build and operate
  • Relatively low operating cost: the energy efficiency of the process, the simple feed preparation, the relative lack of moving parts, low feed carry-over rates, low labour requirements and the ease of replacing lances and refractory linings when they are worn give the ISASMELT process relatively low operating costs
  • Relatively low capital cost: the simplicity of the construction of the ISASMELT furnaces and the ability to treat concentrate without drying make it cheaper than other smelting processes.

    History of the process

Early development (1973–1980)

The ISASMELT process began with the invention in 1973 of the Sirosmelt lance by Drs Bill Denholm and John Floyd at the CSIRO. The lance was developed as a result of investigations into improved tin-smelting processes, in which it was found that the use of a top-entry submerged lance would result in greater heat transfer and mass transfer efficiencies.
The idea of top-entry submerged lances goes back to at least 1902, when such a system was attempted in Clichy, France. However, early attempts failed because of the short lives of the lances on submersion in the bath. The Mitsubishi copper smelting process is one alternative approach, wherein lances are used in a furnace, but they are not submerged into the bath. Instead, they blow oxygen-enriched air onto the surface of the slag. Similarly, a water-cooled, top-jetting lance was the basis of the LD steelmaking process. This does not produce the same intensity of mixing in the bath as a submerged lance.
The CSIRO scientists first tried developing a submerged lance system using a water-cooled lance, but moved to an air-cooled system because "scale up of the water-cooled lance would have been problematic". Introducing any water to a system involving molten metals and slags can result in catastrophic explosions, such as that in the Scunthorpe Steelworks in November 1975 in which 11 men lost their lives.
The inclusion of the swirlers in the Sirosmelt lance and forming a splash coating of slag on the lance were the major innovations that led to the successful development of submerged lance smelting.
From 1973, the CSIRO scientists began a series of trials using the Sirosmelt lance to recover metals from industrial slags in Australia, including lead softener slag at the Broken Hill Associated Smelters in Port Pirie, tin slag from Associated Tin Smelters in Sydney, copper converter slag at the Electrolytic Refining and Smelting Port Kembla plant and copper anode furnace slag at Copper Refineries Limited in Townsville and of copper converter slag in Mount Isa. The work then proceeded to smelting tin concentrates and then sulfidic tin concentrates.
MIM and ER&S jointly funded the 1975 Port Kembla converter slag treatment trials and MIM's involvement continued with the slag treatment work in Townsville and Mount Isa.
In parallel with the copper slag treatment work, the CSIRO was continuing to work in tin smelting. Projects included a five tonne plant for recovering tin from slag being installed at Associated Tin Smelters in 1978, and the first sulfidic smelting test work being done in collaboration with Aberfoyle Limited, in which tin was fumed from pyritic tin ore and from mixed tin and copper concentrates. Aberfoyle was investigating the possibility of using the Sirosmelt lance approach to improve the recovery of tin from complex ores, such as its mine at Cleveland, Tasmania, and the Queen Hill ore zone near Zeehan in Tasmania.
The Aberfoyle work led to the construction and operation in late 1980 of a four t/h tin matte fuming pilot plant at the Western Mining Corporation's Kalgoorlie Nickel Smelter, located to the south of Kalgoorlie, Western Australia.

Lead ISASMELT development

Small-scale work (1978–1983)

In the early 1970s, the traditional blast furnace and sinter plant technology that was the mainstay of the lead smelting industry was coming under sustained pressure from more stringent environmental requirements, increased energy costs, decreasing metal prices and rising capital and operating costs.
Many smelting companies were seeking new processes to replace sinter plants and blast furnaces. Possibilities included the QSL lead smelting process, the Kivcet process, the Kaldo top-blown rotary converter, and adapting Outokumpu's successful copper and nickel flash furnace to lead smelting.
MIM was seeking ways to safeguard the future of its Mount Isa lead smelting operations. It did this in two ways:
  1. working to improve the environmental and operational performance of its existing operations
  2. investigating new technologies.
MIM investigated new technologies by arranging plant testing of large parcels of Mount Isa lead concentrates for all the then process options except for the Kivcet process. At the same time, it had been aware of the use of top-jetting lances in the Mitsubishi and Kaldo processes, and of top-entry submerged combustion lance investigations undertaken by Asarco in the 1960s. This stimulated MIM's interest in the Sirosmelt lance, which was seen as a way to produce a robust submerged lance.
Following the copper slag trials of 1976–1978, MIM initiated a joint project with the CSIRO in 1978 to investigate the possibility of applying Sirosmelt lances to lead smelting.
The work began with computer modelling the equilibrium thermodynamics and was followed by laboratory bench-scale test work using large alumina silicate crucibles. The results were sufficiently encouraging that MIM built a 120 kg/h test rig in Mount Isa. It began operation in September 1980. This was used to develop a two-stage process to produce lead bullion from Mount Isa lead concentrate. The first stage was an oxidation step that removed virtually all the sulfur from the feed, oxidising the contained lead to lead oxide that was largely collected in the slag. The second stage was a reduction step in which the oxygen was removed from the lead to form lead metal.