New water atomisation plant to benefit platinum industry

7th September 2012 By: Gia Costella

State-owned minerals researcher Mintek has completed the construction of, and is now commissioning, a new R40-million water atomisation plant at its Johannesburg facility, in Randburg, which it says will benefit the platinum industry in particular.

Commissioning of the plant is expected to be completed within the next few weeks. The new plant will initially be used for a two-year project for an international platinum giant.

“After that, it should open up opportunities in various new areas, where intermediate metal alloys are required in powder form. However, the platinum industry will be the first to use it,” says Mintek pyrometallurgy division specialist consultant Rodney Jones.

The combination of a direct current (dc) arc furnace and the atomisation plant provides a more effective way of recovering valuable metals from a by-product stream, he says.

“It is able to recover greater amounts of valuable metals than current competing processes and produces the metal in the form required for subsequent processing, which is the main reason why the technology was developed,” Jones explains.

“Previously, we were restricted by the size of the alloy ingots produced by our dc arc furnace. The ingots are usually about 700 kg and, as these are typically too large to be used directly for subsequent processing, their size had to be reduced.

“Unfortunately, the typical nickel-containing iron-alloy product is difficult to break or crush for subsequent handling, so an approach was required that is novel for the platinum industry,” he says.

Jones adds that Mintek built the plant to demonstrate the technology, with the intention of having the platinum industry, in particular, employ the new process, which will improve the recovery rate and, in turn, the profitability of existing smelting operations.

The atomisation technology was sourced from UK-based metals processing company Atomising Systems, says Jones.

He notes that the facility is the largest of its kind in the world, where a dc arc furnace is coupled with an atomisation plant.

The atomisation plant currently treats about 6 t/d of molten metal and the atomiser is intended to treat hundreds of kilograms of metal a minute, says Jones.

The 3 MW furnace is in continuous operation, with the metal alloy being tapped from it once a day. When the plant reaches full production, however, Jones says it will treat two batches of 6 t each of metal alloy a day.

The molten alloy is tapped into a ladle furnace where its temperature is controlled before it is discharged by using a slide-gate valve into a tundish that feeds the water atomiser.

Inside the atomiser, high-pressure jets impinge on the molten stream to break it down into fine particles, which can vary in size, depending on the alloy and the pressure of the jets.

The solidified particles are initially separated from the water by a magnetic separator, followed by a dewatering screw. The final drying of the powder takes place in a rotary kiln.

“The furnace operation is something we are familiar with and have been doing for decades, but transferring the hot, molten alloy into a circuit that involves contact with water is new to Mintek,” states Jones.

Mintek’s pyrometallurgy division is well known internationally for its work in dc arc furnace technology. The technology is well established for use in the production of ferrochromium from chromite, and titania slag and pig iron from ilmenite. It can also be used for a number of other applications.

“The 3 MW unit is Mintek’s largest furnace and has been used for extensive demonstration smelting campaigns since 2008. These campaigns can last from six months to a few years,” says Jones.

He notes that long demonstration campaigns allow the real engineering issues to surface and be tackled, more so than a few days of pilot plant test work.

“Factors reviewed during this time include electrode consumption, refractory wear and tap-hole replacement frequency,” says Jones.

He adds that the demonstration period is especially important at the water atomisation plant, as it is a new technology and process for Mintek.

“The aim is to demonstrate the effectiveness and safety of the technology over the next two years.”

Jones says the extended time of the operation of the plant is well suited to the platinum smelting industry, which is by nature conservative because of the large amounts of money at stake, should something go wrong.

“However, large-scale testwork is extremely expensive and most other industries can only afford to test for about a month before financial constraints set in. Fortunately, if the feed material is sufficiently valuable it is possible to allow the demonstration campaign to offset costs by selling the alloy or matte product that is generated,” he explains.

Although, Jones says most other smelting operations may not have the financial backing to demonstrate technology like this for a few years, the plant can be used for a variety of metals.

“Perhaps, after the initial two-year project, we can consider atomising other metals on an ad hoc basis,” he adds.