Clover Alloys SA is an international, privately owned company that operates chrome fines processing facilities within South Africa. The original plant has been operating since 2006 in the Benoni Area, East Rand, close to Oliver Tambo International Airport
Chrome Ore Foundry Sand
Sand is uniquely effective as a moulding material for making metal castings, an application in which it has been used for centuries. Sand casting remains by far the most prevalent metal casting technique, despite the existence of numerous other casting methods such as die casting, investment casting and permanent mould casting. The global foundry and metal-casting industry uses around 100 million tonnes of sand every year which, in modern industry practice, is typically recycled and reused through many production cycles. Once used sand becomes unusable for further use in a foundry, it is often sold for re-use in other industrial applications.
Sand is used in foundries not only to mould the external shape of a casting, but also to form internal void spaces. Sand grains do not naturally adhere to each other, therefore binders need to be added to make the sand stick together and hold its shape for when molten metal is poured into a casting mould and left to cool.
Foundry sand must adhere to strict physical and chemical specifications to ensure that castings are kept free of defects. Both foundries and sand producers have extensive quality control systems to test and maintain consistent specifications for sand, ensuring that all foundry sand from a particular source will generally have a very consistent specification.
Various types of sand are used in foundries, each being particularly useful in certain castings due to their physical or chemical characteristics. One sand extensively used in foundries is chrome ore sand, which is used heavily in moulds for automotive castings. Castings for automotive applications such as engine blocks and camshafts require the high thermal conductivity which is a key characteristic of chrome ore foundry sand. Its high thermal conductivity ensures that the casting cools down quickly, minimising the potential for the molten metal to compromise and penetrate the surface of the mould (this phenomenon is known as “burn-on”). Chrome ore foundry sand also has low thermal expansion, which facilitates a high degree of dimensional stability. As its basicity is close to neutral, chrome ore foundry sand can be used together with a wide variety of resin bonding systems and inorganic binders.
Key properties of chrome ore foundry sand:
- Melting point of 2150°C
- Good thermal conductivity
- Strong resistance to thermal shock
- Excellent resistance to metal penetration or burn-on
- Resists slag attack
- High dimensional stability
Only around 1 million tonnes out of the 30 million tonnes of chrome ore produced annually is used as foundry sand. The overwhelming majority of the chrome ore produced globally is smelted into ferrochrome, for use as an alloy in stainless steel. In order to be suitable for use as foundry sand, chrome ore has to meet several strict criteria. Its chrome oxide (Cr2O3) content must be above 46% and its silica (SiO2) content must be below 1%, otherwise its chilling properties will be inadequate. The size of the sand particles is also of paramount importance – if the material is too fine it will severely inhibit the strength, performance and lifespan of the casting mould, and more resin and bonding agents will be required.
The vast majority of the world’s chrome ore foundry sand is produced in South Africa, where more than 80% of the world’s chrome ore resources are located.
Metallurgical Grade Chrome Ore Concentrate
Around 92% of all the chromium produced in the world is consumed in steel, to which it brings a high resistance to corrosion, temperature and wear. The steel which uses chromium most extensively is stainless steel – stainless steel consumes more than 80% of the chromium used in steel applications. Stainless steel accounts for only 2% of global steel output, however the average tonne of stainless steel has a chromium content of 18%. Most non-stainless steel does not contain chromium, however certain alloy steels and rebar grades do have a significant chromium content.
Chrome ore contains too many impurities to be added directly into a steelmaking furnace. Prior to being consumed in steelmaking, chrome ore must be smelted in an electric furnace together with a carbon reductant, which eliminates most of the impurities in the ore and produces a product known as ferrochrome. Ferrochrome is an alloy containing 48-75% chromium, 0-8% carbon and 0-6% silicon, with most of the remaining content being iron. For making stainless steel, most of the necessary ferrochrome is added directly into the steelmaking furnace, with only a minority of the required ferrochrome being added in the secondary steelmaking stage following decarburisation. When ferrochrome is used in non-stainless steels, it is mostly added during secondary steelmaking. Chromium is highly recyclable from stainless steel scrap. About 30% of the global consumption of chromium in steel comes from remelted scrap, with the remaining 70% coming from ferrochrome.
The global ferrochrome industry is concentrated in four countries, which together account for more than 90% of world ferrochrome production. These countries are South Africa, China, India and Kazakhstan. Almost all producers of ferrochrome in Kazakhstan and South Africa own their own sources of chrome ore, as do most major producers in India. China is the only major ferrochrome producing country where most of the ferrochrome producers do not own their own source of chrome ore, and therefore have to import it from third parties.
More than half of the chrome ore imported into China now comes from South Africa, providing a growing market opportunity for independent South African chrome ore suppliers, such as Commodity Partners.
An increasing percentage of South African chrome ore supply is in the form of fines, concentrated to reduce impurities such as silica. Developments in furnace technology have significantly increased the ability of ferrochrome smelters in both South Africa and China to use fine chrome ore concentrate, which usually trades at a competitive discount to traditional high-grade lumpy ores.
At its plant in Benoni, South Africa, Clover Alloys produces two grades of metallurgical chrome ore concentrate – one grading 44% Cr2O3 and the other grading 42% Cr2O3. Both grades comfortably meet the normal specifications for South African metallurgical chrome ore concentrate, and are produced by reprocessing used high-grade foundry sand from foundries.
Typical specifications for Clover Alloys’ two grades of metallurgical chrome ore concentrate are as follows:
Chrome Ore Nozzle Sand Base
In addition to its use in foundry casting moulds, chrome ore sand is used in the steel industry to protect tap holes in steelmaking ladles. The chrome ore sand used in this application is known as nozzle sand base.
After it has been decarburised in a basic oxygen furnace (BOF) or an electric arc furnace (EAF), molten steel is transferred to a ladle, where the chemistry of the steel is adjusted exactly to the required specifications. Unwanted impurities form a slag which floats to the top of the ladle, with pure molten steel sinking to the bottom. At the bottom of the ladle is a taphole, through which the molten steel is removed once the correct chemical specification has been attained. The taphole is usually covered from the outside by a sliding gate. If molten steel gets into the taphole it can often begin to solidify and block up the hole, therefore it is necessary to fill up the taphole with sand prior to molten steel being placed into the ladle. Sand used to fill steelmaking ladle tapholes is known as “nozzle sand”. When it is time to remove (“tap”) the molten steel from the ladle, the slide gate over the taphole is opened and the nozzle sand falls out, followed by the liquid steel.
The physical properties of chrome ore sand – its high melting point, good thermal conductivity, and resistance to thermal shock, burn-on and slag attack – make it an excellent material for use in nozzle sand, ensuring a wide taphole opening even when the temperature of the molten steel is high and the processing time in the ladle is long. For maximum performance, chrome ore nozzle sand is often blended with silica sand and/or graphite.
Clover Alloys produces nozzle sand, in addition to foundry sand and metallurgical chrome ore, at its Benoni plant in South Africa.
Typical specifications for Clover Alloys nozzle sand are as follows: