Diamond Extraction Methods

by Tyler Ringler

Mineralogy: Fall 2007
www.emporia.edu/earthsci/amber/go336/
ringler/diamondmining.htm



Source: The Hindu Newspaper, www.hindu.com/mag/
2003/09/07/stories/2003090700360800.htm

Introduction
Introduction and Properties

Extraction Methods
Occurrence in Nature
Open Pit Mining
Underground Mining
Processing
Finishing Touches
Interpretations and Conclusions


    Diamonds: Introduction and Properties

    Diamonds and precious gems have fascinated humans since they were first discovered as having unique properties. For centuries now, diamonds have been marketed for these properties and are now extremely pricey and symbolize many things. Diamonds indicate wealth and can also have meaning as being of great emotional bonding between people. Diamond jewelry given between significant others are considered to be an indication of a strong and meaningful relationship in today’s standards. Despite their popularity, few know the lengthy and expensive methods that must be done to bring the gems to their final appearance as seen in the jeweler’s store.

    The exact chemical composition of the diamond was a relative mystery for many centuries. In 1704, Newton hypothesized that diamonds were made up of carbon. Newton was proved correct by various experiments during the late 18th century (Orlov, 1977). Due to the rarity and beauty of the gem, several properties of diamonds have been studied extensively. Probably the most notable property of the stone is its breathtaking adamantine luster. This property is dependent on the perfectness of the crystal surface. Another interesting optical property is their reaction to ultra-violet light, or X-rays. When exposed, diamonds exhibit luminescence which can also vary in intensity (Orlov, 1977).

    The hardness of the diamond is an extremely noteworthy physical property. On the Mohs hardness scale, diamonds are ranked as 10, making it the hardest material found in nature (Figure 1). This essentially means that no other material but diamond can scratch diamond. However, diamond is still quite brittle and cleaves easily from the stress of a well placed hit from a hammer. This is caused by seemingly small defects in the crystalline structure that weaken the crystal (Orlov, 1977). Density of diamond ranges from 3.50 to 3.53 grams per cubic centimeter and are present in a wide variety of colors ranging from blue, yellow, orange, green, or even black. Most diamonds occur as the popular transparent coloration (Wenk and Bulakh, 2004). According to Wenk and Bulakh (2004), diamonds have a wide range of uses in industry including the "manufacturing of drilling heads, cutting tools, and abrasive instruments" (p. 344).

    Figure 1. Mohs Hardness Scale

    Picture taken from Life Science
    http://www.infowest.com/life/projgeo.htm .

    Extraction Methods

    Occurrence in Nature

    Meteorites

    Diamonds were initially discovered in meteorites by Erofeev and Lachinov in 1888. The exact origin of these diamonds in meteorites was highly hypothesized. Some scientists believed that the diamonds formed due to the high pressures in depths of planetary bodies (Orlov, 1977). Another popular hypothesis suggested that the diamonds form upon impact with the Earth, which would submit the material to high pressurization (Orlov, 1977). However, the extent of diamond occurrence in extraterrestrial rocks is minute. For diamond mining on a large scale, we must look within our planet.

    Terrestrial Igneous Rocks

    The most prominent geologic occurrence of diamonds is in kimberlites found in the upper sections of the Earth's crust. According to Orlov (1977), "the first diamond bearing kimberlites were found in South Africa by Kimberley and De Beers" (p. 160). Kimberlites characteristically are only found in the tectonically stable center of continents known as shields or platforms. They occur as pipes, dikes, veins, and sills composed of an alkali rich igneous rock. Researchers believe that these intrusions can be attributed to alkalitic magma from the upper mantle pushed into large fault systems to the upper areas of the Earth's crust (Orlov, 1977).

    Studies have shown that kimberlite pipes (Figure 2) occur more readily near the marginal parts of the earth's crust undergoing bending. This is indicative of deep fault systems that are undergoing severe stress from previous upwelling of magma. Kimberlite pipes are usually funnel shaped and are known to contain high levels of xenoliths of sedimentary rocks. This is caused during the breaching of the upper part of the platform when deep seated rocks occur along the path of the rising magma. Dominant times of diamond-bearing kimberlite volcanism occurred during the Precambrian and early Paleozoic (Orlov, 1977).

    Figure 2. Kimberlite Pipe

    Picture from All About Gemstones
    http://www.khulsey.com/jewelry/
    kh_jewelry_diamond_mining.html
    Click for larger picture.

    Open Pit Mining

    The most commonly used extraction method for diamondiferous material is an open mined technique. Open pit procedures are to loosen and haul the diamond rich material for further sorting and processing. The very geometry of the kimberlite pipe supports this method in which layers of the earth are removed and then benched down to a lower level connected via roads (Figure 3). The kimberlite pipe narrows with depth, which supports open mining processes which need to also narrow as the mine becomes deeper (Legrand, 1980).

    Figure 3. Open Pit Mine: Russia

    Picture from Diamond Mines of the World
    http://www.khulsey.com/jewelry/
    kh_jewelry_diamond_mines_russia.html
    Click for larger picture.

    The top layer of the kimberlite pipe is usually weathered and appears brown to yellow in coloration. Surficial strata is soft enough for direct digging and ripping while underlying layers become harder and explosives must be utilized (Legrand, 1980). There are several factors that must be considered before beginning an open mine operation. Legrand (1980) states that "local geology, rock strength, and equipment type must be considered when designing the mine" (p. 107). Also, stability of the mine must be maximized by the correct width and height of individual benches (layers) being mined. Usually the slope of the mine is relatively flat during the early stages of mining and steepens towards the final stages of the mines life (Legrand, 1980).

    Underground Mining

    Once and open pit mine has become too steep or unstable, a different mining method can be utilized. Underground mining methods are used when manual open pit methods for extraction become too costly. Diamond occurrence is measured in carats per ton of material. This value is known to decrease with depth into the kimberlite pipe (Legrand, 1980). Estimates of a mine's life can be made by initial diamond occurrence values and exactly when (at what depth) it will become inefficient to keep practicing open mining techniques instead of underground mining methods. The process involves essentially dissolving deeper into the kimberlite pipe using intricate underground tunneling systems. Water is an accelerant of decomposition and weathering of kimberlite and is a major hazard in this type of mining. (Legrand, 1980). Tunneling patterns (Figure 4) are dug into the host rock and into the remaining kimberlite deposit of the pipe under the open mine (Legrand, 1980).

    Figure 4. Underground Mining Method

    Picture from First Metal Inc.
    http://www.firstmetalsinc.com/mining.php.
    Click for larger picture.

    Processing

    There are several steps that the raw mined material must endure before it is cut and polished for display. The first step of processing is to haul the diamondiferous material to the modular processing plant (Figure, 5.) This plant is normally extremely close to the open pit mine and material is transported with massive dump trucks and loaders (Legrand, 1980).

    Figure 5. Modular Processing Plant

    Picture from Bateman: Diamond Processing
    http://www.bateman-diamond.com/.
    Click for larger picture.

    Crushing and Milling

    After a kimberlite pipe has been located, the material is extracted from the mine. The processing phase is lengthy and begins with crushing and/or milling the material. The first step is to separate the diamonds from all other material. This is done on a large scale because diamonds occur in extremely small quantities when compared to overall volume of earth that must be moved to find them. The conglomerate is put into a large crushing machine and smashed until the crystals are released. This must be done carefully so as to not harm the precious gems (Bruton, 1978). Milling the material is more suitable for some deposits. Milling is a process where the diamondiferous material is placed in huge rotating drums that contain water. The material slowly disintegrates leaving only raw diamonds. Milling is more efficient at collecting smaller fine diamond crystals (Bruton, 1978).

    Washing, Screening, and Heavy Media Separator

    Washing is the next step for removing yet finer diamonds from the abundance of mined diamond ore. The process is best used with another popular separation technique known as screening. Screening is best known from the gold rush days. A screen of known size is places beneath the material as it is tumbled allowing only a certain diameter of particles to pass through (Bruton, 1978). In 1950, a method known as heavy media separation was popular. This method used fine ferro-silicon powder suspended in water to give the mixture a known specific gravity. Differing levels of S.G. were controlled and used as the crushed material was placed into it. The material that was barely lighter than the mixture would float and then move on to the next level. Material in the final separator would have a density of diamond and be transported (Bruton, 1978).

    The Grease Belt and X-Ray Separation

    The grease belt was established on the property of most raw diamonds to stick in grease because they were non-wettable while all other material would not. This was discovered by F.B. Kirsten in 1896. Using a 3 foot wide belt coated with grease, soaked diamondiferous material is transported in thin layers, revealing raw crystals (Figure 6). The grease is then mechanically scraped from the belt as a fresh layer is added, allowing continual movement (Bruton, 1978). Russian scientists developed a method in the late 1950's that extracted diamonds based on their luminescence. The material is fed into a recovery machine that saturates it and then runs it under X-ray radiation. If a diamond lights up, a photo-electric cell is triggered which opens a corresponding gate that drops the diamond out of the conglomerate (Bruton, 1978.)

    Figure 6. Grease Belt Extraction

    Picture from Oblique Engineering Ltd.
    http://www.oblique.co.za/gb400.htm.
    Click for larger picture.

    Finishing Touches

    Once extracted, the diamond must be polished then can undergo a multitude of cuts. Sawing and cleaving the gems is used during the polishing phase. The modern diamond cutter is focused on creating the best, most flawless diamond in the industry to get maximum economic profits. The cutter must first analyze each diamond, by hand, to observe any inclusions. By doing this, the best possible section of the diamond can be utilized when setting in a piece of jewelry. In modern times, lasers have been utilized in diamond cutting whereas other diamonds had to be used in the past. There are several types of faceting that are most popular. Faceting is the process of creating flat faces on geometric shapes. The brilliant cut of diamonds is widely used and has multiple facets (Figure 7). Once properly faceted, the diamond can be set into a desired jewelry piece (Legrand, 1980).

    Figure 7. Anatomy of a Brilliant Cut

    Picture from Geoffreys Diamonds,
    www.geoffreysdiamonds.com/cut.asp.
    Click for larger picture.

    Interpretations and Conclusions

    There are several other extraction methods that have been tried and failed in the past. Previously mentioned methods were by far the most widely used and successful in diamond processing history. Hopefully, this page has informed many on the large scale and costly processes that bring diamonds out of our earth and onto our bodies. There are numerous practices occurring worldwide that are involved in the lengthy methods needed to produce such a brilliant and respected gem as the diamond. With the future success of diamond extraction in the world, the stone will continue to baffle and astonish us and be a sign of wealth in our society.

    References

    Bruton, Eric. Diamonds (2nd Ed.). N.A.G. Press, London. 1978.

    Legrand, Jacques. Diamonds. Smeets Offset, Weert, Netherlands. 1980.

    Orlov, Yu. L. The mineralogy of the diamond. Wiley-Interscience Publication. 1977.

    Wenk, Hans-Rudolf and Bulakh, Andrei. Minerals: Their constitution and origin. Cambridge University Press. 2004.

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    Webpage for public use created as term project requirement for Mineralogy (GO 336), Emporia State University.