Success for kiln specialist with redesigned heating system

Furnace and carbon regeneration kiln specialists Custom Furnaces says its redesigned heating system for carbon regeneration kilns has been well received by the gold mining industry in South Africa, as it reduces downtime by at least 30 hours.

“In 2010, we redesigned the heating system using candle-type elements. These are fitted through the roof and hang perpendicular, heating the retort tube from two sides,” says Custom Furnaces owner John Lee.

He explains that replacing a conventional strip-type element required cooling down the kiln to gain access into the heating chamber, which takes about 24 hours.

“One can only enter the kiln once it has cooled to below 40 ºC,” notes Lee.

The roof section of the kiln then needs to be replaced and the kiln must be heated to 750 ºC before production can resume.

“With the new candle-type elements, replacement is possible during production. The affected zone simply needs to be electrically isolated and the failed element disconnected. It can be extracted with a ‘T’ spanner and replaced with a new element, a process that can be completed in under an hour,” he says.

The new elements are made from purified resistance material (PRM), which maintains form stability at high temperatures and forms a surface oxide layer that provides protection against deposits of ash and carbon, which is prevalent in the carbon-regeneration process.

PRM is made through a metallurgical purification process which involves electro slag melting, resulting in a pure material that offers 30% higher loading than other iron-chromuim-aluminium materials.

“The low watt density used in the redesign, and the elements that are freely radiating from all sides, ensures a long element life,” says Lee.

He notes that the redesigned heating system was installed on a new kiln for mining company Harmony’s Kalgold openpit mine, 60 km south of Mafikeng, in the North West province, which was so well received that the client asked Custom Furnaces to design similar elements for its elution system.

The mine, which was acquired by Harmony in 1999, is situated in the Kraaipan greenstone belt.

It treats tonnages at a carbon-in-leach plant on site, and produced 1 041 kg of gold in the 2012 financial year at an average grade of 0.78 g/t.

“The system was also so well received in 2012 by mining giant AngloGold Ashanti at its Great Noligwa mine where we have recently received an order to convert a second, larger kiln, with a third to follow,” says Lee.
The mine is located about 160 km south-west of Johannesburg, in the Free State.

Great Noligwa is one of four AngloGold Ashanti mines, comprising its Vaal river region, and operates a twin-shaft system that serves eight main levels at an average depth of 2 400 m.

The underground mine, which started production in 1972, employs a scattered- mining strategy, owing to the geological complexity of the orebody. Access to the reef is through footwall haulage and return-airway development, with cross- cuts developed every 180 m to the reef horizon.

Raises are then developed on the reef to the level above and the reef is stoped out on strike, with an average stope width of 150 cm. About 4 000 m of development is performed each quarter.

The mine comprises four gold plants, one uranium plant and a sulphuric-acid plant. It incorporates crushers, ball mills and carbon-in-leach (CIL), carbon-in-pulp and electrowinning facilities. The Great Noligwa plant has conventional crushing, screening, semiautogenous grinding and CIL processes, which are followed by milling and treatment.

Lee notes that the heating system was redesigned after management at Custom Furnaces noticed the costs saved when these type of elements were used for heat treatment furnace applications, owing to their reliability and reduced downtime.

“We noticed the system would also be suited to externally heated rotary kilns. With low grades of ore being treated, proper regeneration of carbon is important to ensure good recovery rates. Downtime means loss of income, as well as gold,” he states.

Lee notes that the elements can be used in any type of externally heated rotary kiln and in conjunction with protective tubes for processes with aggressive atmospheres.

“The cost-saving benefit soon covers the cost of retrofitting the equipment. We have also recently started to replace conventional, coil-type elements for calcining furnaces in the gold mining industry,” says Lee.

Owing to the steady growth of the heat- ing system since 2010, Custom Furnaces plans to offer the system to mining clients outside the gold mining industry, such as deammoniators for the vanadium industry, calciners for the uranium industry and numerous chemical applications.