Chrome Sand Applications in Mutiple Industries
Chrome sand is one of the most widely used refractory materials, and is available in high purity from African Pegmatite as part of a wide catalogue of high performance refractories.
Chrome sand is a naturally occurring mineral that consists mainly of the oxides of iron and chromium. The properties of chrome sand include:
- A high melting point of 2150 °C
- A high thermal conductivity.
- Resistance to acid slag attack
- Strong dimensional stability
- Great resistance to thermal shock
- Improved resistance to metal penetration
These properties allow for the application of chrome sand in heavy duty grey iron and steel foundries as a core and mold making sand, amongst many other applications. Alongside high thermal conductivity, chrome sand has low thermal expansion(1). The following are the uses of foundry chrome sand:
The name “green sand” is perhaps misleading. It is rather a mixture of which chrome sand is a chief component. Other components include silica sand, zircon sand, olivine, staurolite, graphite, bentonite (clay), water, inert sludge and anthracite. Choosing the type of sand is highly dependent on the temperature at which the metal is poured.
Process Outcome When Using Green Sand
Green sand casting is a simple, scalable and resilient method for the casting of metals. The outcome of the casted product is dependent upon many factors, including the grind size of the sand, the addition of cereal biners, and wetting (see later). Escape of silica is a known issue if silica sands are used, when hot steel is poured into the mould. A higher temperature tolerance across the breadth of the green sand mould may alleviate this - strongly refractory materials such as chrome sand may help.
Ladle Filler Sand
This is a mixture of raw materials used in the nozzle of a ladle in continuous casting of steel often containing chrome sand. The process of continuous casting involves forwarding the steel ladle from tapping to teaming until a semi-finished product is formed. If the slide gate system (which controls egress from the ladle) doesn’t open freely, the process might be delayed and a premature hardening of the moment metal may occur. The slide gate system is responsible for obstructing the flow of molten steel from the teeming ladle to the turn dish.
A granular, refractory material known as filler sand prevents contact between the molten steel and slide gate system, with chrome sand being the most widely used material. Good filler sand must have properties including refractoriness, uniform particle-size distribution, consistent particle-packing density, low thermal expansion, flowability and the ability to form a sintered crust having the appropriate thickness when brought in direct contact with the molten metal.
The refractoriness and the surface melting temperature must tally for ideal filler sand; creating a sintered surface, albeit not one with a large thickness level. A significant amount of pressure is needed to break through a thicker sintered surface which causes a reduction in “free opening.”
Conversely, rapid sintering of the surface of filler sand limits the permeation of liquid steel into the inner aspect of the nozzle. In 2012, Farshidfar and Kakroudi published a study regarding the use of the foundry chrome sands in the continuous casting process. They concluded that a suitable particle size distribution has a positive influence on flowability and permeability of filler sands. Suitable particle size and distribution are easily attainable with chrome sand.
In the making of these sands, some kind of resin or other material is used as a binder. A classical choice is liquid glass, but carbonaceous additives are often added to this, such as one study where ultradispersed pyrolytic carbon was used. Whilst the chrome sand was found to be tolerant of both novel and traditional binders, as its temperature tolerance and general stability allows for uniform heat transfer and thus the better formation of a surface film (i.e. between the mould and the casting), the carbonaceous additive left more small defects on the surface, rendering it a poorer choice(2).
Refractory Fire Brick Technology
Chrome sand is used in the production of magnesite chrome refractory bricks which usually contains over 33% of chromium (iii) oxide. Magnesite chrome bricks have specific properties that make them suitable for a variety of applications. Features of these refractory bricks include high refractoriness, high-temperature strength of about 1700 °C, strong basic slag erosion resistance, excellent thermal shock resistance, and a certain resistance to acid slag. The applications of magnesite chrome bricks include:
- Factories for metal production use magnesite chrome refractory bricks for building electric furnace top, open hearth furnace top, finery forge, and other furnaces used for non-ferrous metals.
- Fused cast magnesite chrome brick is used in areas within the wall of ultra-high power electric arc furnaces where the temperature is high.
- It is adopted for burning zones, rotary cement kiln and regenerative chamber in glass furnaces.
- “Magnesite chrome brick composed of synthetic compost“ and “fused cast magnesite chrome brick” and are used in areas where a substantial erosion is likely to occur in nonferrous metals flash smelting furnaces.
- Composed of a synthetic compost, Magnesite chrome brick is employed in areas with the risk of a substantial erosion in the finery forge.
Chrome sand is also used in chrome conundrum refractory brick. This refractory brick has the same properties as that manufactured using magnesite chrome. The applications of chrome conundrum brick include:
- Zinc smelting electric furnaces and kilns processing volatile materials.
- Copper smelting furnaces
- It is generally used in areas within metallurgical furnaces prone to increased abrasion and high temperature.
Foundry chrome molds the external shape of castings, as well as the internal void spaces. In normal circumstances, sand grains do not stick to each other. As such, binders are added to cause sand grains to adhere to each other. The bonding of sand particles allows a shape to take form when molten metal is poured into a casting mold and cooled off.
A different form of sand with specific physical and chemical features is used in molding certain castings. For instance, a high thermal conductivity which characterizes foundry chrome sands is required for automotive castings including engine blocks and camshafts.
A high thermal conductivity ensures that the castings cool off rapidly; thus, reducing the molten metal’s potential to penetrate the mold’s surface. A low thermal expansion which is a crucial feature of foundry chrome sand allows for increased dimensional stability. Additionally, resin bonding systems and inorganic binders are used alongside chrome sand due to its basicity being closer to neutral.
Because of its higher cost relative to other refractory materials, chrome sand is only used in those applications which produce the highest quality alloys, or in the manufacture of reactor cores. Chromite is not easily wetted, meaning a better surface finish is ensured. Unlike silica, chrome sand is basic in nature(3). For castings where a high level of chemical abrasion is likely, such as the production of high manganese ‘Hadfield’ steel, chrome sand is preferred over silica sand owing to the former’s superior chemical resistance at high temperatures.
Unlike conventional sand moulds, chromite moulds can be hard to recover and reuse should there be moderate-to-high amounts of silica present. This is due to a greater likelihood of the chrome becoming contaminated by the silica, and thus its refractoriness is reduced(4). Chrome sand refractories are typically used as a ‘facing sand’ in silica moulds(5). Contemporary research has, however, displayed some process to allow for release and reuse of chrome sand castings when using an alkali phenolic binder. In terms of economics and environmental sustainability, this is an important step forward. Researchers found that the order of operations in dismantling a chrome sand casting system was important, as was paying attention to the temperatures throughout the process(6).. Chemical analyses performed on the chromite matrix and at the surface/interface - naturally - suggested some chemical change post heat treatment. Whilst more research is needed, the concept that a chrome sand casting is a one time only event may soon be disappearing.
Another study that looked at reusability of chrome sand noted that the higher proportion of silica present in the sand mixture, the less reusable the material is(7). They also observed that chrome sand castings produce fewer finings than an equivalent silica mold would - meaning less new sand needs to be added for subsequent castings. Overall, they regard the material as “green” from an environmental standpoint. This method refers to the repeated use of a single casting mold, as opposed to the earlier reuse method of dismantling and then regenerating another mold.
Casting molds are often made in a traditional way, a box is placed over a shape for which a negative is required, sand is poured in and then the assembly may be heat treated. Green sand - and other types of - molds, on the small scale, have been produced via 3D printing methods(8) with high degrees of precision and success. 3D printing has become a popular choice for laying down chrome sand based molds, with intricate detail available in short times(9). Lower quality chromite - such as that reclaimed from other industrial processes - does not have the same levels of thermomechanical robustness as freshly mined and milled chrome sand. It is likely that sintering will occur during the casting process if low quality chrome sand is used, thereby producing an inferior final product(10).
With regards to sintering - the process of forming a solid mass through heating without melting - chrome sand in its purest form is generally regarded as a material that sinters at a high temperature(11). If sintering occurs at a lower temperature, this is due to the presence of impurities, such as excess silica or other fine particles. Such effects can lead to a higher prevalence for casting defects. Therefore, it is imperative to use the purest chrome sand possible.
Pigmentation Of Porcelain Tiles
Usually, porcelain tiles are produced with different colors and pigmentations. Colors for fast-fired porcelain tiles are derived from black (Fe1Cr)2O3 pigment which is expensive and synthetic. However, chrome sand offers a cheaper option for pigmenting porcelain tiles and do not allow for the alteration of the microstructure and mechanical properties of the tiles when introduced.
Stainless Steel Production
Extracted chromium from chrome sand is used in the production of stainless steel – constituting about 18% of the alloy. Hardening of stainless steel is due to the presence of chromium. Furthermore, chromium ensures that this alloy is resistant to corrosive forces at high temperature.
Tundish Linings And Continuous Casting
A tundish is an open container with certain holes in to allow molten metal to pass at a predefined rate. Such tundishes need to be lined with an insulatory refractory material so no solidification of metal occurs. Magnesia chrome is a popular choice of refractory for tundish linings for the continuous casting of metals. It is composed of magnesia and chrome sand that have been cured into a porous refractory brick, the added chrome sand is responsible for the enhancement of thermal conductivity relative to pure magnesia alone(12).
As a part of magnesia plaster, chrome sand finds further use in the continuous casting space. This plaster, akin to plaster that may be applied to an interior wall in a house, applied as the top layer of refractory (i.e. in constant contact with molten metal). This layered approach to refractories is highly effective and prolongs the life of the system. The plaster is also used to join refractory bricks. Traditional plaster is mostly porous magnesia, but this has a tendency to be destroyed by the presence of calcium oxide or silica from the slag at high heat. Large amounts of magnesia are replaced with chrome sand in modern plaster mixes. The presence of chromite modulates the basicity gap and prevents penetration of the refractory plaster by the slag(13).
Nichrome is made of about 80% nickel and 20% of chromium. Chromium is the primary reason behind the resistance of this alloy to high temperatures – reaching a value of 1250oC. Nichrome alloys are often employed in constructing heating units. Additionally, the ability of nichrome alloys to withstand corrosion and oxidative factors owes a lot to the presence of chromium.
Use Of Chrome Refractories In Gasifiers
Chrome sand based refractories have a burgeoning important use in the lining of gasifiers. In gasifiers, carbonaceous materials (such as coal, coke and biomass) are converted to synthesis gas. Synthesis gas is used as the primary feedstock for the Fischer-Tropsch process for the synthesis of hydrocarbons. The most common gasifier type is the entrained flow type gasifier.
The major side product from gasification processes is the production of ash. This collects on the reactor walls as slag, which then flows down the wall of the refractory and in some cases can penetrate the porous structure of the refractory, at which unwelcome reactions can occur.
Chrome-alumina is the leading choice for refractory materials for gasifiers(14), with laboratory testing suggesting that chrome sand in large particle size that is densely packed performs the best across a range of gasification scenarios(15). Layered or sectional type arrangements of varying chrome percentage refractories may be used through the gasification equipment, tailored to the localised temperature, minimising the amount of high cost chrome that needs to be used. Alumina-chrome refractories are preferred to alumina and magnesia-chrome refractories in the presence of acidic slag due to their better overall performance(16).
As the gasification of biochar is regarded as a “green” process, it has the potential to become a popular choice for the production of synthesis gas. One study looked into the concept of chemical looping as a means of eliminating nitrogen gas from the system and ensuring ample oxygen for the conversion to carbon monoxide, across fluidised beds of graphite, copper oxide or chromite sand. Although not deliberately aiming for chrome as a good result, they noted that the chrome sand was superior to other materials in terms of resistance to agglomeration phenomena, which are common with this kind of reaction(17).
High purity chromite occurs naturally in South Africa(18), the same country in which African Pegmatite is based.
- Chrome sand is a form of a chrome and iron oxide, benefitting from having high refractory properties
- As such, chrome sand is used in green sand castings, ladle fillers and casting mould where the highest quality casting is required
- Additionally, it finds wide use in refractory brick manufacture, in stainless steel and other alloy production as an additive to boost chrome content, and also in gasifiers
As part of the widest catalogue of high quality refractory materials for the broadest range of applications, chrome sand is available from African Pegmatite; milled, processed and packaged to the precise specifications of any customer.
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