Rocks, digestion methods

Comparative studies were performed to evaluate microwave digestion with conventional sample destmction procedures. These included the analysis of shellfish, meats, rocks, and sods. Generally, comparable accuracy at much shorter digestion time was found for the MAE vs the classical digestion method (39). 

Developments of recent years include plants designed to precipitate the calcium sulfate in the form of the hemihydrate instead of gypsum, hi special cases, hydrochloric acid is used instead of sulfuric acid for rock digestion, the phosphoric acid being recovered in quite pure form by solvent extraction. Solvent-extraction methods have also been developed for the purification of merchant-grade acid, which normally contains impurities amounting to 12 18% of the phosphoric acid content. Processes for recovering part of the fluorine in the phosphate rock are in commercial use. 

Nitrophosphate fertilizer is made by digesting phosphate rock with nitric acid. This is the nitrophosphate route leading to NPK fertilizers as in the mixed-acid route, potassium and other salts are added during the process. The resulting solution is cooled to precipitate calcium nitrate, which is removed by filtration methods. The filtrate is neutralized with ammonia, and the solution is evaporated to reduce the water content. The process of prilling may follow. The calcium nitrate filter cake can be further treated to produce a calcium nitrate fertilizer, pure calcium nitrate, or ammonium nitrate and calcium carbonate. 

Analytical Methods Whole rock and LA-ICPMS analyses were carried our at Universite du Quebec a Chicoutimi (UQAC). Sulfur was determined by combustion and IR analysis. Ni and Cu were determined by atomic absorption spectrophotometry after aqua regia digestion. Gold and PGE were determined by Ni-sulfide fire assay followed by Te co-precipitation followed by ICPMS. 

The Re- Os method was first applied to extraterrestrial samples in the early 1960s when Hirt et al. (1963) reported a whole-rock isochron for 14 iron meteorites that gave an age of 4 Ga. Further development of this system was hindered by several technical difficulties. Rhenium and osmium each exist in multiple oxidation states and can form a variety of chemical species, so complete digestion of the samples, which is required to chemically separate rhenium and osmium for mass spectrometry, is difficult. In addition, accurate determination of rhenium abundance and osmium isotopic composition requires spiking the samples with isotopically labeled rhenium and osmium, and equilibration of spikes and samples is challenging. A third problem is that osmium and, particularly, rhenium are very difficult to ionize as positive ions for mass spectrometry. These problems were only gradually overcome. 

Mineral samples such as rock, soil, and sediments require more aggressive digestion. Total sample dissolution may be done by several methods, and... 

Typically, these will be alloys, rocks, fertilisers, ceramics, etc. These materials are taken into solution using suitable aqueous/acid media, according to solubility hot water, dilute acid, acid mixtures, concentrated acids, prolonged acid digestion using hydrofluoric acid if necessary, alkali fusion (e.g. using lithium metaborate), Teflon bomb dissolution. Fusion and bomb methods are usually reserved for complex siliceous materials, traditionally reluctant to yield to solubilisation. 

Chlorination using chlorine attack can be an attractive method for determination of noble metals in samples of complex composition. All noble metals undergo chlorine attack. Binary chlorides or salts dissolved in diluted mineral acids are the products of chlorination. Dry chlorination by chlorine passing over the sample, either alone or mixed with a small amount of NaCl, in an open mbe at 500-600 °C, is the most promising method because of the low amounts of chemicals introduced into the sample, low blanks, and the possibility of direct combination with the detection technique. The effectiveness of such a digestion procedure compared with FA in the analysis of rocks was reported [63]. 

Inorganic Samples. Wet digestion, fusion, and pressure dissolution are common methods for the dissolution of metals, slags, ores, minerals, rocks, cements, and other inorganic materials and products. If the final solutions contain more than about 0.5% of dissolved material, the standards should also contain the major constituents in order to match the viscosity and surface tension. 

Some materials are particularly resistant to acid digestion, e.g., certain rocks, mineral oxides, phosphates, and some iron alloys. For these samples, high-temperature fusion with an acidic or basic flux such as lithium metaborate (LiBOs) in the molten state can be used to render such materials soluble in water or dilute acid. Fusion decompositions are the most rigorous methods available and all silicate materials, including refractory substances like zircon and cassiterite, can be dissolved completely when fused with an appropriate flux. However, there are several disadvantages to this method including the introduction of additional salts into the final solution... 

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