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LST Heavy Liquid | LST Heavy Liquid for density separations
General information on heavy liquids General Information on Heavy LiquidsHeavy liquids are dense fluids or solutions used to separate materials of different density through their buoyancy. Materials with a density greater than the heavy liquid will sink, while materials with a density less than the heavy liquid will float on the liquid surface. In the mineral industry, heavy liquids are commonly used in the laboratory to separate the “light” minerals such as quartz and clay from the “heavy” minerals. The density used for this type of separation is about 2.85 g/ml, nearly three times the density of water. Heavy liquid separations are carried out for varied reasons, depending on the industry. The mineral sands industry uses heavy liquid separations to check the grade of samples in their process, and to determine the efficiency of their industrial hydrocyclone separations. The diamond exploration industry uses heavy liquids to separate the dense indicator minerals from sand and clay. Another use of heavy liquids is in paleontology. Typically, these heavy liquid separations are conducted at a lower density (e.g. 2.2 g/ml) since the separation is not between minerals of different types, but between fossil bones and minerals. Organic heavy liquidsThe organic heavy liquids form the "older" generation of heavy liquids. Two of the organic heavy liquids most commonly in use are bromoform and tetrabromoethane (TBE). Of these, bromoform has the lower viscosity (1.8 cP) but is considered more hazardous to work with because it has the higher vapour pressure (5.9 mm Hg at 25 C). TBE has a higher viscosity (9 cP) and a lower vapour pressure (0.02 mm Hg at 25 C). Another organic heavy liquid, used when higher densities are required, is methylene iodide. Methylene iodide has a density of 3.31 g/ml, a vapour pressure of 1.2 mm Hg at 25 C, and a low viscosity of 2.6 cP. The three commonly used organic heavy liquids all have problems with toxicity, and must be handled in a fume hood. Methylene iodide is moderately toxic by subcutaneous and other routes. Bromoform has been shown to cause severe liver damage and is poisonous by ingestion. There is evidence that it is a human mutagen and a neoplastigen on experimental animals, so it is potentially carcinogenic. TBE is poisonous by inhalation or ingestion and is a moderate irritant to the skin. It is a neoplastigen and mutagen with animals. These liquids have the problem of being volatile and potentially causing health problems. For this reason, they are being phased out in the USA. Tungsten based heavy liquids: SPT, LMT and LSTTwo low-toxicity substitutes for the organic heavy liquids bromoform and tetrabromoethane (TBE) have been available for some time. They are SPT (sodium polytungstate, or sodium metatungstate), and the lithium equivalent LMT which is lithium metatungstate. These are all inorganic compounds, based on the [H2W12O40]6- polyanion, which is dissolved in water to form very dense solutions. SPT (sodium polytungstate) and LMT (lithium metatungstate) are not used by many mineral labs which need a heavy liquid with a density of 2.8 g/ml or more, because their solutions at these densities are too viscous. Typically the viscosity of these liquids is greater than 25 cP, or more than 25 times the viscosity of water. The higher viscosity of heavy liquids prepared from sodium metatungstate (also known as sodium polytungstate, or SPT) can be seen in the graph below. In practice, the higher viscosity of SPT leads to significantly slower and less efficient separations, as well as slow filtration.
Another disadvantage of SPT and LMT is their lack of thermal stability above 80oC. The lack of thermal stability of sodium metatungstate and lithium metatungstate means that solutions of SPT or LMT cannot be evaporated by boiling, so the recirculation and regeneration of these liquids requires special equipment, or is quite time consuming. A more recent development is the low-toxicity heavy liquid called LST. LST heavy liquid has a low viscosity and excellent thermal stability. LST can be used up to a density of 2.9 g/ml at room temperature, and up to a density of 3.6 g/ml at elevated temperatures. Physical Characteristics of LST Heavy LiquidLST solids contain lithium heteropolytungstates in the form of extremely soluble, colourless hydrated crystals. Concentrated aqueous solutions of LST form colourless or pale yellow heavy liquids. LST heavy liquid is normally used with an aqueous base. However, the LST crystals are also highly soluble in acetone and alcohols, and can form dense liquids in these organic solvents. This provides a range of methods for the use and recycling of LST heavy liquids. Thermal Stability of LST Heavy LiquidLST heavy liquid maintained at a temperature of 140 C for over two weeks showed no observable change. The high thermal stability of LST heavy liquid allows it to be boiled to evaporate water and regain its original high density. The stability of LST heavy liquid means that recycling is rapid, simple and effective. Operating Density of LST Heavy LiquidAt 25 C LST heavy liquids can be used at any density up to 2.95 g/ml by diluting with water or by concentrating to remove water. The recommended operating density as a replacement for bromoform is 2.85 g/ml. At this density the viscosity is low, and the likelihood of crystallisation is reduced. Densities up to 3.6 g/ml can be achieved at higher temperatures. The maximum density of LST heavy liquids is given for a range of temperatures in the Figure below
Viscosity of LST Heavy LiquidLow viscosity is a feature of LST heavy liquids. Grain separations in LST heavy liquid occur at about the same speed as when using TBE, but without the occupational hygiene risk of organohalides. The exact viscosity of the LST heavy liquid will vary with its temperature and density (see Figure below), and is typically about 11 cP.
Storage and shelf life of LST Heavy LiquidLST heavy liquids can be stored in closed plastic or glass containers. LST heavy liquids may cause corrosion of metals, so contact of LST with metals should be avoided during storage and heavy liquid separations. Changes during Storage in ColdIf subject to very cold conditions, crystals may appear in LST heavy liquids. These will redissolve upon warming the heavy liquid for a few hours to 40 C, with occasional shaking or stirring. Shelf LifeLST Heavy Liquid is a solution of stable inorganic ions in water, and has virtually an indefinite shelf life. In extended trials, LST Heavy Liquid which has been stored for over 10 years shows no sign of decomposition. Links to heavy liquids at other sitesLST Heavy Liquid as a calorimeter material , and Cerenkov radiation from muons in the LST Calorimeter Project. General Contact InformationCentral Chemical Consulting Pty Ltd Telephone: 61 8 9248 2739 For more information or purchasingE-mail: info@chem.com.au |
