AS 4156.1.1:2018 Coal preparation
6.3.2.3  Preparation of solutions
Caesium formate is readily soluble in water and therefore the solution can be diluted with potable tap water to give densities in the normal working range of float and sink testing.
6.3.2.4  Sample rinsing
Particles shall be rinsed with water while vacuum filtering to remove caesium formate. The dilute rinsings may then be processed to recover the caesium formate.
6.3.2.5  Recovery of media
If required, the recovery of caesium formate from dilute washings can be achieved by either distillation (e.g. using an immersion heater), vacuum evaporation or reverse osmosis.
6.4  Aqueous suspensions
6.4.1  General
An insoluble material with a high relative density and correct particle size distribution shall be used to give a relatively stable suspension of low viscosity. An example of an aqueous suspension would be zirconium dioxide.
6.4.2.1  General
The float sink procedure for aqueous suspensions shall be the same as that outlined in Section 9 of this Standard. The major differences compared to using organic liquids or inorganic solutions are the preparation of the dense media, sample rinsing and recovery of dilute media. Zirconium dioxide is chemically inert and has a particle density of 5.75 allowing a range of aqueous suspensions to be prepared.
NOTE  The physical properties of the aqueous suspensions used in float and sink testing are shown in Table 5.
6.4.2.2  Limitations on accuracy
The use of aqueous suspensions such as zirconium dioxide shall be thoroughly validated before its use to ensure washability results are accurate and any subsequent laboratory testing of float sink fractions is not compromised.
Particular attention shall be given to the following concerns, which may limit the use of zirconium dioxide:
(a) Zirconium dioxide may contaminate particles even after rinsing. This could affect subsequent properties to be tested, in particular ash.
(b) Some coal or mineral matter particles may disintegrate in a water based solution.
(c) Zirconium dioxide suspensions are opaque. Therefore, the time required for the completion of the separation of floats from sinks cannot be judged by visual observation and has to be pre-determined by experiments combined with theoretical calculations.
(d) The stability of the aqueous suspension should be known and monitored. Note that lower density suspensions are the least stable.
(e) Zirconium dioxide is not a suitable media for separation of fine particles (<1 mm).