History, Uses and Applications of Garnets
Far from just finding use as a gemstone, modern industry relies on high quality garnet in a host of different milled specifications for applications as varied as sand blasting, solvent filtration and industrial flooring. African Pegmatite is the leading supplier of fine garnet to industry - processed to precise sizes, for virtually any use case.
Garnet is the general name given to any member of a group of common silicate minerals with similar chemical compositions and crystal structures, but more properly are silicate materials which belong to the nesosilicate group, regardless of what other elements are present. The nesosilicate part means that the bulk mineral is built up of isolated silicon tetrahedra bound together by other cations.
They are widely used as abrasives and gemstones. Gemstones, because they exist in several shades of green and red, as well as black.
Origin and Occurrence of Garnets
Rock-forming garnets are the most common type of garnet found in metamorphic rocks. A few others such as granite used in construction and granitic pegmatite exist in igneous rocks. In sedimentary rocks, they occur in clastic sediments. Most often than not, garnets contain many inclusions which could be fragments of other minerals or rocks. Garnets whose inclusions look to have been rotated are known as pinwheel garnet or snowball garnet. They occur randomly in foliated metamorphic rocks. Garnets rich in iron are usually weathered to rusty masses which consists of mostly limonite. Its spessartine alternative also weathers to form various masses of manganese oxide ores when subjected to tropical conditions.
The name garnet is derived from the word “gernet”, an ancient word which is translated as deep red color. This is because it was believed to exist only in red colors. Until the late 20th century, garnets were believed to exist in every color except for blue. Not until a recent discovery in Madagascar proved that garnets could change from greenish blue into blue-green in daylight or purple instead when lit by incandescent light.
Applications of Garnet
Garnets have several industrial applications owing to some of its properties like relatively high specific gravity, angular fractures, nontoxicity, chemical inertness, and perhaps the most important of these characteristics is its ability to be recycled. Many of the applications of garnet derive from its impressive hardness profile - in the 7.8 to 8.0 Moh range. In addition to the standard measure of hardness, African Pegmatite supplied garnet bears a bond work index rating of 19.7 kWh per tonne. Such a value is commensurate with high durability (and thus longevity) and chemical resistance. The bond work index is a measurement of the resistance of a material to being ground. Higher values are superior.
2010 figures for the industrial end use in the United States are as follows:
- Water jet cutting 35%
- Abrasive/blasting media 30%
- Water/solvent filtration 20%
- Abrasive powders 10%
- Other 5%
Abrasive and cutting applications
Aside from decorative uses, garnet is most associated with its abrasive properties. Whilst garnet is far from the only material used as an abrasive (particularly in sandblasting), it is regarded as one of the best (due to the aforementioned bond work index and hardness) and one of the safest. Traditional abrasive methods may have used sand or silica - the latter of which is highly problematic if inhaled by humans.
Water Jet Cutting
Water jet cutting is one of the latest cutting technologies embraced by steel companies and employed in the cutting of metals. It has become a popular choice because it cuts metals precisely and accurately. Its growing popularity can also be attributed to low costs, non-heat generation, and faster productivity. Garnets are used in waterjet cutting machines for the cutting of concrete, aluminum, high strength steel, marble, granite, automotive glass, plastic laminates, titanium, aerospace composites, steel bridge decking, and textiles, among others.Water jet cutting is a process whereby a finely milled abrasive is introduced in a mixing chamber to a high pressure, narrow stream of water. The abrasive-water mixture exits the chamber at sufficient force to cut materials such as glass and sheet metal. African Pegmatite supplies garnet suitable for water jet cutting applications in 125 to 300 μm and 250 to 600 μm grades for broad cutting use cases.
Owing to its grain structure, garnet grains can be used as a filtration media in water treatment facilities. Its high specific gravity, as well as, chemical and abrasive resistance makes it an ideal filter media for water purification purposes. The fact that it is chemically inert and non-metallic also makes it a good option for use as a filtration media.
In multi-media filters, it is used as the bottom layer and improves water filtration by about 10-20 microns. Also, garnets decrease backwashing frequency and increase filter run times. In general, garnets are considered excellent filtration media for the following reasons
- They do not contaminate sea beds or disturb marine wildlife when they are used offshore.
- Because of relatively high densities, less garnet needs to be used compared to other material; disposal and recycling are easier compared to when other slag abrasives are used.
- They produce very low dust emissions; therefore, they won’t pollute the air or atmosphere.
- Since its solids penetrate the entire bed, they create longer filter runs not mentioning increased storage.
The two most common other materials that garnet is used as a water filter alongside (i.e. in a multi media filter system) are sand and anthracite.
Garnet supplied for water filtration applications is typically in the range of 0.6 to 1.0 mm grades, with an effective filter size of 0.3 mm, easily filtering particulate matter as small as 10 to 20 μm in diameter.
Solvent Extraction and Electrowinning
Garnets are high-performance media that function effectively and are consequently used between electrowinning and solvent extraction. In general, electrowinning and solvent extraction are hydrometallurgical processes especially used for the production of copper ore. The quality of the cathode plate can be improved if a garnet is used as one of the beds in what is to be a multimedia filter. Using garnets can also significantly reduce costs due to their supreme longevity relative to other filter media.
Garnet is a superior choice for solvent extraction and electrowinning for the following purposes.
- It reduces expansion and abrasion
- It is generally chemical resistant
- It is stable when used in a multi-media design
- It needs lesser materials when backwashing
- It is also known to provide great support for high-density media
- High resistance to acid which means not a lot of the material will leech into the solution
In modern solvent extraction electrowinning plants, several sequential deionised filter beds containing alternately anthracite and garnet are used. Anthracite is a coarser top-layer filter which is charged with removing coarse contaminants and organic residues. Garnet is the primary size exclusion filter which is able to remove solids down to the micron scale owing to its high density and efficient packing size, making channel formation difficult.
In sandblasting processes, garnet abrasives are lauded to be better alternatives to silica sand. Unlike silica sand, garnets produce less dust and is generally safer than its more hazardous alternative. Garnet used for this purpose is usually produced from Andradite deposits or crushed Almandite. It may come in rounded edges or rough edges, with a Mohs hardness of about 7 to 8. Since it is a low-dust option compared to silica sand or coal slag, it can be used around water. As a consequence, it can be used for enclosed tanks, aluminum, fiberglass, and drydocks.
When the right methods are used, it produces comparable production rates as when silica sand is used.
Garnets are excellent for sandblasting purposes because:
- They increase productivity
- They are non-toxic
- They do not attract moisture
- They are suitable for either dry or wet abrasive blasting
- They are also recyclable for as much as 5 times.
Generally, they can be easily used to make preparations for coating or industrial painting.
African Pegmatite supplies garnet in 250 to 600 μm grade for sandblasting applications.
This was the first industrial use of garnets. In industries, they are used to produce sandpapers. This is possible because of the hardness properties of garnet. With a Mohs hardness of about 6.5 to 7.5, they function effectively as abrasives in different manufacturing processes. When garnets are crushed, rough edges are formed for cutting and sanding. The small granules gathered from the crushed garnet are bonded to paper to produce reddish color sandpaper which is used as a popular abrasive in woodwork shops. The crushed garnets can also be screened to several sizes so that they can be sold as abrasive powders and granules.
Non-abrasive based applications
The applications for garnet that do not rely on hardness may use a lower quality sample of garnet - whilst still very hard, the value derives from supreme resistance to chemical attack, excellent packing ability and the fact that it is relatively heavy and therefore particularly well suited to high volume fluid applications. Garnet’s longevity in the settings listed below mean that it doesn’t need to be replaced often, meaning longer term use, giving rise to long term savings in terms of labour and material costs.
Garnets are used in textile industries for stonewashing denim. An advantage of using garnets is that it can be used to wash textile without causing any damage whatsoever to it. Stonewashing is used in the manufacturing process of textiles to produce a worn-out appearance on the textile material. It helps to increase the flexibility and softness of rigid materials like denim and canvas. Here, garnets can be used in place of large stones for washing the fabric. On rotation of the wash cylinder, the fibers of the textile are repeatedly beaten and pounded by the garnet. This is overall a relatively gentle process which does not significantly weaken the fabric.
Ship and Aircraft Building and Repair
Garnets are used in shipyards and on flight lines for several purposes. They are used to refit and repair coatings, rust or in some cases, tight adherent mill scales. In a process similar to sandblasting, garnet is used to finish the outer skins of ships and aircraft prior to painting. A smooth finish is crucial to ships and aircraft, where excessively turbulent flow of air or water over the surface can have a dramatic effect on performance - and in the case of aircraft, safety. Its low dust levels make it easy to work with. With garnet blast media, you can ensure precise control of feathering when weld seams are being blasted.
Garnets are used in blast-room applications. They are used where sensitive substrates, aluminum surfaces and installed electromagnetic components are used before the use of steel shot or grit.
Garnets are used as abrasives in the fine grinding of glass to produce very smooth surfaces. They can also be used to cave out several shapes for artistic purposes. They can also be used to clean glass surfaces to get rid of things like fingerprint imprints or dirt, etc. The glass polishing application is expanded to include scratch free lapping of semiconductor materials such as silica for the production of computer microprocessors.
Garnets can be used for industrial flooring purposes. It is an ideal option for flooring because of its anti-skid characteristics. This is an essential characteristic because the floors within an industrial facility are meant to be safe for the movement of people and machines. Industrial floors made with garnets are usually durable, safe and easy to clean.
Geological Indicator Mineral
Although it is general knowledge that most of the garnets found on the earth’s surface have been formed within the earth’s crust, some garnet types are pushed up from the mantle during some massive volcanic eruptions. Most often than not, these eruptions bring along with them pieces of mantle rocks known otherwise as ‘xenoliths.’ These xenoliths are then delivered to the earth’s surface through a structure known as a pipe. These xenoliths usually contain a large number of garnets for every diamond. These deep-source garnets are very much different from those that form in the shallow depths of the earth’s crust. In essence, these deep-source garnets give an insight into the presence of diamonds in a particular area. These unique garnets are used as indicator minerals by geologists when mining diamonds.
Radiation Shielding Cement Mortar
Although only demonstrated experimentally so far, radiation shielding concrete that has used ground garnet as between 25 and 100% sand replacement have been shown to be effective. Research showed that mortars with higher garnet compositions exhibited the highest density and the most compressive strength. Neutron and gamma ray transmission is reduced with increasing garnet content in the cement.
- Garnet is a material composed of silicate materials that belong to the nesosilicate group - they are typically formed in metamorphic rocks (extreme heat and pressure)
- Known for its hardness, garnet finds many uses in abrasive scenarios such as sandblasting and jet cutting
- Additional applications rely more so on garnet’s longevity as a hard material and packing ability, in areas such as water filtration, solvent extraction and industrial flooring
- It is a low dust and non toxic material, meaning a safe workplace without having to compromise on abrasive, cutting or filtering ability
African Pegmatite is a leading miner, processor and supplier of garnet - milled to virtually any specification, for any application. Combining the widest reach, the best in house milling technology and the broadest experience, African Pegmatite is the go to industrial partner for garnet and other similar materials and minerals for abrasive, filtration and specialist use cases.