Mineral Analysis Sample Digestion and Preparation Safety When

Mineral Analysis Sample Digestion and Preparation

Safety • When using muffle furnaces and hotplates, the appropriate gloves should be used. • If fumes are evolved in a reactive process, it must be conducted in a fume hood. • If perchloric acid digests should never be allowed to boil to dryness as they can explode due to physical or thermal shock. • Calcium gluconate gel must be kept nearby when using hydrofluoric acid. Hydrofluoric acid (HF) is extremely dangerous and if 20 ml of 52% HF comes into contact with your skin, it is lethal!!! • If you spill acid on yourself and you are working in a laboratory in which HF is in use, unless you are 100% sure you know what it is, treat it as if it is HF!!!

Sample Digestion and Preparation • Once the analysis sample has been obtained, the sample needs to be digested or prepared as a disc for measurement. – If the sample is pressed into a disc, it is called nondestructive testing. – If the sample is decomposed or digested, it is called destructive testing.

Partial Acid Digestion • Has the following advantages over total digestion: – Takes a shorter amount of time. – Safer to perform. – Cheaper equipment and reagents. – Not as labour intensive.

Partial Acid Digestion • Aqua regia digest: – 3 parts conc. Hydrochloric acid : 1 part conc. Nitric Acid – Weight of sample = 1 -50 g – The metal of interest (usually Au, Pt, Pd) is totally dissolved in the acid medium whilst the useless undigested remainder (gangue) is discarded. – Mixture is usually heated on a hot plate to speed up digestion. The temperature should be carefully maintained at 95 -97 o. C to prevent boil overs.

– Silicates can not be dissolved in aqua regia (normally much of the gangue). – Nitric acid is a strong oxidising agent used to digest sulfide material. The digest will give off strong brown fumes of NO 2 if there is a high content of sulfide in the material.

Total Acid Digestion • Mixed Acid Digestion: – Weight of sample = 0. 15 -0. 3 g – Uses a mixture of Nitric acid (HNO 3), Perchloric acid (HCl. O 4), Hydrochloric acid (HCl) and Hydrofluoric acid (HF). • Perchloric acid is a powerful oxidising agent and will digest minerals that nitric acid can not. • Hydrofluoric acid is the only substance that can digest silica. Teflon containers must be used as HF will attack glass. • Digestion time is a 4 -8 hours. The resulting substance is leached with hydrochloric acid.

Fusion • Fusion is carried out in muffle furnaces at 650 o. C for those minerals that are resistant to mineral attack. • Fluxes: – Lithium borate – for basic ores as it is acidic – Lithium metaborate – for acidic ores as it is basic – Sodium peroxide (base metals) – Sodium carbonate/potassium carbonate (F, Cl) Note: Na 2 O 2 is a strong oxidising agent and can cause instantaneous reactions with paper to start a fire!

• Sample and flux are mixed well and then heated for 30 -40 minutes in crucibles made of alumina or zirconium depending on the metals of interest. • The resulting product is called a “melt” and is normally dissolved in water or acid. • Fluxes are also used to make pressed discs as the flux material is cheap, pure and readily available.

• Pressed fusion discs are often prepared for XRF analysis. Once analysed the samples can be stored for further analysis. – These fusion discs are prepared in inert platinum crucibles. – This non destructive method is more time and cost effective.

• Next topic “Measurement”