Just under three years ago Naledi Foundry, the foundry division of the Naledi Inhlanganiso Group, began implementing a plan that had increased capacity, improved quality and diversification as the pillars of its strategic blueprint.

In order to achieve this the foundry embarked on a restructuring process that included investment in new equipment as part of an approximate R100 million investment programme in the foundry, formerly known as Guestro Casting and Machining that has origins dating back to 1912.

With most of the new equipment now installed and operating, the company’s plan is coming to fruition with a marked influence.

Naledi Foundry has broadened the range of products that it manufactures, over and above its traditional scope of business. This includes the manufacture of cast grinding media for the local energy and mining industries

However, during this restructuring and investment period Naledi Foundry management announced that the company intended to broaden the range of products that it manufactured, over and above its traditional scope of business. The manufacture of cast grinding media for the local energy and mining industries was its stated target.

The culmination of this plan is now being realised with the commissioning of a state-of-the-art gas fired heat treatment plant that includes quenching and tempering.

“One of the basic operations in minerals processing is grinding of the ore to the point where valuable minerals are liberated from the host rock. Subsequent operations then separate the desirable minerals from the gangue or waste. The grinding balls form the major portion of the consumable costs, and they say they can be as high as 40% to 45% of the total costs of extraction,” said Coenie De Jager, General Manager of the foundry.

“Grinding is essential to the efficient separation of minerals and is the key to good minerals processing. To produce clean concentrates it is necessary to grind the ore fine enough to liberate the minerals, but over-grinding inflates energy costs and can lead to the production of untreatable fine ‘slimes’ particles. Grinding is therefore a compromise between recovering clean high-grade concentrates, and minimising operating costs and fines losses.”

Naledi Foundry has recently commissioned a state-of-the-art gas fired heat treatment plant that includes quenching and tempering of cast grinding media. The hardening section includes a furnace size of 15 metres by 900 by 700mm. The gas heated travelling furnace with a power voltage of 380 Volts (3 phases + neutral) is heated by means of six overhead gas burners divided into three heating sections rated 90kW, 150kW and 230kW. The maximum temperature is 1 100° C and the working temperature variance across the furnace is ±10° C. Temperature control is by means of PID (Proportional Integral Derivative) controllers with an accuracy of ±2° C

Balls are loaded into one piece cast baskets that measure 660 x 575 x 450mm and can take up to 400 kilograms of product. Time duration in the furnace is six hours per basket and there are 24 baskets in the traveling/conveyor furnace at any one time

“Annually, large tonnages of grinding media are consumed around the globe, costing several million dollars. Highly abrasive ores, such as those in the gold industry, can consume about one to two kilograms of grinding media per ton of ore milled, according to Mintek, South Africa’s autonomous research and development (R&D) organisation specialising in all aspects of mineral processing, extractive metallurgy and related technology. As a result, grinding media replenishment is one of the highest above-the-ground consumable costs for mines. Therefore, an improvement in wear characteristics and life, or decrease in price of grinding media, can represent a significant saving.”

“Many intrinsic and extrinsic factors are involved in the optimum selection of grinding media. The intrinsic factors include ore hardness, feed size, product size, pulp density, pulp level, mill size, mill speed and feed rate. Often, the mines have a good grip on these and exercise the necessary controls. The major extrinsic factor that has a huge effect on the milling efficiency is the quality of grinding media. The use of inferior grinding media does not only compromise the milling efficiency, but also comes with a cost penalty due to the premature breakage of balls in service.”

“Grinding media fail if there is a significant variation in chemical composition, hardness and microstructure of the balls. Common failure modes of grinding balls observed in different semi-autogenous grinding (SAG) applications are misshapen, surface spalling, shelling, capping and half balls.”

“Traditionally there has been one manufacturer of cast and forged grinding media in South Africa. This dominant position has been challenged in recent years with the emergence of new manufacturers in the cast grinding media arena in South Africa, and of course imports. We also believe that another new player in this market is in the final stages of planning before investing in new equipment to manufacture cast grinding media. This will take the local manufacturers of grinding media to five entities that we know of.”

After the hardening process the balls are immediately sent for quenching. Depending on requirements this quenching is either done by oil or air, a unique specification that the company has incorporated. The oil quenching tank size is 3 x 2.8 x 2.5 metres and the estimated quenching medium volume is 25m³. The continuous medium temperature control and monitoring is done by PLC and the quenching system is equipped with a heat exchanger and cooling system. The quenched balls are collected through a shaker with dimensions of 2.5 x 1.75 x 0.20 metres before progressing for tempering. Time spent in quenching is two hours. The air quenching system is equipped with four sets of high pressure fans

After quenching balls are sent to the tempering section that has a furnace size of 13 metres by 900 x 700mm. The gas-heated furnace with a power voltage of 380 Volts (3 phases + neutral) is heated by means of an exhaust heat recycler combined with an auxiliary heater (150kW). The maximum temperature is 450° C and the working temperature variance across the furnace ±10° C. Temperature control is also done by means of PID controllers with an accuracy of 2° C

“We joined that market when Naledi won an order to supply Eskom with over 1 000 tons a month of 40 and 50mm grinding balls that have between 12 and 27% chrome content. We manufacture these on our GF moulding line so existing processes and equipment are used. Not an ideal situation because throughout the foundry’s history it had only produced steel and iron castings, including grey, ductile and SG iron. The introduction of high chrome meant there would be contamination of metals and longer periods between pouring because of maintenance issues.”

New heat treatment plant
“With the installation of the new equipment and processes we have overcome the obstacle of contamination. However, with the diversification into grinding media it meant that we would have to rely on outside assistance for our heat treatment services as we were not set up for heat treatment of the grinding media.”

“Investigations and feasibility studies began and have resulted in the commissioning of our new heat treatment plant dedicated to the heat treatment of grinding media in April 2018.”

“The plant consists of three sections: Hardening (heating), quenching and tempering.”

“The technical parameters for the hardening section include a furnace size of 15 metres by 900 by 700mm. The gas heated travelling furnace with a power voltage of 380 Volts (3 phases + neutral) is heated by means of six overhead gas burners divided into three heating sections rated 90kW, 150kW and 230kW. The maximum temperature is 1 100° C and the working temperature variance across the furnace is ±10° C. Temperature control is by means of PID (Proportional Integral Derivative) controllers with an accuracy of ±2° C.”

A view of the heat treatment plant from above

“Balls are loaded into one piece cast baskets that measure 660 x 575 x 450mm and can take up to 400 kilograms of product. Time duration in the furnace is six hours per basket and there are 24 baskets in the traveling/conveyor furnace at any one time.”

“After the hardening process the balls are immediately sent for quenching. Depending on our requirements this quenching is either done by oil or air, a unique specification that we have incorporated. The oil quenching tank size is 3 x 2.8 x 2.5 metres and the estimated quenching medium volume is 25m³. The continuous medium temperature control and monitoring is done by PLC and the quenching system equipped with a heat exchanger and cooling system. The quenched balls are collected through a shaker with dimensions of 2.5 x 1.75 x 0.20 metres before progressing for tempering. Time spent in quenching is two hours.”

“The air quenching system is equipped with four sets of high pressure fans.”

“After quenching balls are sent to the tempering section that has a furnace size of 13 metres by 900 x 700mm. The gas-heated furnace with a power voltage of 380 Volts (3 phases + neutral) is heated by means of an exhaust heat recycler combined with an auxiliary heater (150kW). The maximum temperature is 450° C and the working temperature variance across the furnace ±10° C. Temperature control is also done by means of PID controllers with an accuracy of 2° C.”

“As an energy saving measure we have incorporated a fitted suction system that collects low-temperature waste smoke and feeds it back to the tempering furnace. The gutter system traps the waste smoke as well as excessive oil from the oil-quenched balls. Because of the oil collection, which is also recycled, the gutter has to be cleaned periodically.”

Naledi Foundry has recently completed installation of a Lauds Foundry Equipment sand reclamation plant. The L10 primary reclamation plant can process material at 10 tons per hour

“Heat treatment equipment, oil quenching and tempering production is widely used for high chrome grinding media balls or steel balls heat treatment to enhance the balls hardness.”

“By nature of the repetitive heating and cooling cycles, conveying baskets deteriorate over time and as a result require replacement and maintenance. When not replaced and maintained, heating and quenching profiles are inconsistent, resulting in inconsistent metallurgical properties. Basket misalignment and quench delays result in lost production time and equipment damage, and ineffective baskets cannot be loaded optimally.”

“Since grinding balls are subjected to wear, abrasion and impact, the desirable goal of heat treatment should be to ensure that they have properties appropriated to the operational conditions under which they are to be used: Lower residual stress; A high degree of hardness and wear resistance in their surface zone, and an adequate toughness in their central zone to enable them to absorb some impact energy without cracking.”

“The plant is running on two shifts at the moment and we are already producing 600 tons of balls in a month. This tonnage will be ramped up as we progress. By installing our own heat treatment plant we have calculated that we will achieve a two thirds cost saving as compared to outsourcing – a sizeable amount to contribute towards our R15 million investment.”

“Although we have initially been concentrating on the more popular 40 and 50mm size balls we are equipped to manufacture up to 150mm ball size. The material grade specification that we follow is SANS 1338: 2008 Grade 3,” concluded De Jager.

For further details contact the Naledi Inhlanganiso Group on TEL: 011 845 1546 or visit www.ni-holdings.co.za