Molten metal being poured into chrome sand moulds

Chrome Sand Applications in Mutiple Industries

Chrome sand is a naturally occurring mineral that consists mainly of the oxides of iron and chromium. The properties of chrome sand include:

1. A high melting point of 2150oC
2. A high thermal conductivity.
3. Resistance to acid slag attack
4. Increased dimensional stability
5. Increased resistance to thermal shock
6. 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. The following are the uses of foundry chrome sand:

Green Sand

The name “green sand” is misleading. It is rather a mixture of which chrome sand is a chief component. Others may include silica sand, zircon sand, olivine, staurolite, or 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. The temperature at which copper and iron are poured causes the clay to become inactivated such that montmorillonite is converted to illite. Newly mixed sand replaces the old sand, which is either recycled or thrown away; this happens as the clay burns out. Among the various sands used, silica happens to be the least desirable.

Metamorphic grains of silica tend to explode; thereby forming microscopic particles when this material is thermally shocked. These particles likely enter the workplace and put employees at risk of silicosis. This is not the case with chrome sand which does not undergo a phase transition, responsible for the rapid expansion of the grains, and cools the metal faster; finer grains are produced consequently. Additionally, chromite is not a metamorphic mineral and do not form microscopic particles which are responsible for certain health hazards.

refractory chrome sand oven

Ladle Filler Sand

This is a mixture of raw materials used in the nozzle of a ladle in continuous casting of steel. 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 doesn’t open freely, the process might be delayed. The slide gate system is responsible for obstructing the flow of molten steel from the teeming ladle to the turn dish. In due course, oxygen lance cutting process is applied in restoring the flow of steel which is disadvantageous in the sense that it is intrinsically slow, has certain operational safety risks, and causes damage to the refractory material.

A granular, refractory material known as filler sand prevents contact between the molten steel and slide gate system. Chrome sands are the most widely used materials. Good filler sand must have the following properties including refractoriness, particle-size distribution, particle-packing density, low thermal expansion, flowability, and above all, 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 must tally for ideal filler sand; creating a sintered surface, though not 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.

moulds

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 it suitable for its applications. Features of these refractory bricks include high refractoriness, high-temperature strength of about 1700oC, strong basic slag erosion resistance, excellent thermal shock resistance, and a certain resistance to acid slag. The applications of magnesite chrome bricks include:

  1. Factories for metal production use magnesite chrome refractory bricks for building electric furnace top, open hearth furnace top, finery forge, and other furnace used for non-ferrous metals.
  2. Fused cast magnesite chrome brick is used in areas within the wall of ultra-high power electric arc furnace where the temperature is high.
  3. It is adopted for burning zones rotary cement kiln and regenerative chamber in glass furnace.
  4. “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 furnace.
  5. 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:

  1. It is widely used in zinc smelting electric furnace and volatilizing kiln.
  2. It is also used in a copper smelting furnace.
  3. It is generally used in areas within metallurgical furnaces prone to increased abrasion and high temperature.

Casting Moulds

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.

Produced goods from moulds

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.

Metal pouring out of crucible into moulds

Nichrome Alloy

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.

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