Heavy Liquids

We can define heavy liquids as liquids that have densities much higher than water and are also referred to as High Density Solutions.A heavy liquid has a  known Specific Gravity (SG) and are mainly used in the quick separation of substances such as stones with similar appearance.Buoyancy is the principle for the separation of  materials with different density by the use of heavy liquids. 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.

It is important to note that some heavy liquids need to be stored in the dark to prevent light from influencing their physical properties. Their containers also need to be stored closed with a stopper to prevent evaporation.

Various types of heavy liquids include:

Organic Heavy Liquids

Organic heavy liquids were the first generation of heavy liquids to be put in use. Organic heavy liquids most commonly used are Bromoform and Tetrabromoethane (TBE) with Bromoform having a lower viscosity of 1.8 cP. However, Bromoform has a higher vapor pressure of 5.9 mm Hg at 25oC thus considered more hazardous to work with. TBE has a higher viscosity (9 cP) and a lower vapor pressure of 0.02 mm Hg at 25oC.

Methylene iodide is another organic heavy liquid that is used when higher densities are required. Methylene iodide has a density of 3.31 g/mL, a vapor pressure of 1.2 mm Hg at 25oC and a low viscosity of 2.6 cP.

It is important to note that these three organic heavy liquids are highly toxic and must be handled in a fume hood. TBE is poisonous by inhalation or ingestion and is a moderate irritant to the skin while Bromoform has been shown to cause severe liver damage and is potentially carcinogenic. The process of phasing out organic heavy liquids is ongoing due to their toxic nature.

bromoform.PNG

Bromoform

Tungsten Based Heavy Liquids

These are inorganic based low toxic heavy liquids that are currently in use, substituting the highly toxic organic heavy liquids. Tungsten based heavy liquids include

  • Sodium polytungstate (SPT)
  • Lithium metatungstate (LMT)
  • Lithium heteropolytungstates Solution (LST)

SPT and LMT have very high viscosities estimated to be more than 25 times that of water. For this reason, both SPT and LMT are not preferred for use in mineral laboratory experiments which always require heavy liquids with densities equal to or more than 2.8g/ml. At this density, SPT and LMT are too viscous to be used in the separation of minerals as the higher viscosity of SPT leads to significantly slower and less efficient separations, as well as slow filtration. Also, it has been found that SPT and LMT lack thermal stability at temperatures above 80oC.This means that solutions of SPT and LMT cannot be evaporated by boiling thus the recirculation and regeneration of these liquids requires special equipment, or is quite time consuming.

The inefficiencies of the organic, SPT and LMT heavy liquids led to development of Lithium heteropolytungstates solution (LST), originally developed through a research programs between CCC in Australia and AMIRA International Ltd .LST Heavy Liquid is less toxic and has a low viscosity and excellent thermal stability. LST Heavy Liquid 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. At its optimal working density at 2.85 g/mL, LST heavy liquid has a viscosity of 10 cP and so has less of the viscosity related separation problems associated with SPT and LMT.

Generally, LST Heavy Liquid allows faster grain separations and easier filtration as illustrated below.

LST animation

We therefore recommend the use LST as the best Heavy Liquid for use in various separation activities.

 

1 litre LST.PNG

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