A Textbook of Assaying | Page 9

weights of this kind would only tend to confusion, even if they were not unnecessary. With a set of gram weights and its subdivisions anything may be done. If it is desired to report as pounds to the ton, then, since there are 2240 lbs. to the ton, a weight of 2.240 grams may be taken as the assay ton, and each 0.001 gram yielded will equal 1 lb., or 22.4 grams may represent the ton, and each 0.01 gram a pound. Similarly, since there are 32,666.6 ozs. troy to the ton; if we take 32.6667 grams as the assay ton, each 0.001 gram will equal 1 oz. to the ton. In some cases it may be convenient to have, in addition to the usual gram weights, one or other of the "assay tons" mentioned above, but generally it is better to work on a purely decimal system, and convert when required into ounces per ton, &c., either by actual calculation or by reference to a set of tables.
PRACTICAL EXERCISES.
The student should practise such calculations as the following:--
1. Calculate the percentages in the following cases:-- (a) Ore taken, 2 grams; copper found, 0.2155. (b) " 1.5 gram; iron found, 0.8340. (c) " 30 grams; lead found, 23.2.
2. Calculate the parts per thousand in the following:-- (a) Bullion taken, 1.1 gram; silver found, 1.017. (b) " 1.14 gram; silver found, 1.026. (c) " 0.6 gram; gold found, 0.5500.
3. Calculate parts per 100,000 in the following:-- (a) Ore taken, 20 grams; silver found, 0.0075. (b) " 50 grams; gold found, 0.0026. (c) Water taken, 500 c.c.; solids found, 0.1205.
4. Calculate cwts. to the ton in the following:-- (a) Ore taken, 5 grams; tin found, 2.816. (b) " 5 grams; tin found, 3.128. (c) An ore with 68.2 per cent. of tin.
5. Calculate lbs. to the ton in the following:-- (a) An ore with 3.28 per cent. oxide of tin. (b) Ore taken, 20 grams; oxide of tin found, 1.67.
6. Calculate ozs. (troy) to the ton in the following:-- (a) Ore taken, 50 grams; gold found, 0.0035. (b) " 20 grams; silver found, 0.0287. (c) " 25 grains; silver found, 0.0164.
7. Calculate in grains per gallon:-- (a) 0.51 gram per litre. (b) 24.6 parts per 100,000. (c) Solution taken, 100 c.c.; copper found, 0.0045 gram. (c) " 50 c.c.; iron found, 0.165 gram.
8. Convert into ozs. (troy) per ton:-- (a) 7 loths per centner. (b) 30 grams per quintal. (c) 15 parts per 100,000.
FOOTNOTES:
[1] Ether or carbon bisulphide.
[2] Such substances are best dried by pressing between folds of dry filter-paper.
CHAPTER II.
METHODS OF ASSAYING.--DRY GRAVIMETRIC METHODS.
The methods of assaying are best classed under two heads, Gravimetric and Volumetric, in the former of which the final results are weighed, whilst in the latter they are measured. A commoner and older division is expressed in the terms much used in practice--wet assays and dry assays. Wet assays include all those in which solvents, &c. (liquid at the ordinary temperature), are mainly used; and dry assays, those in which solid re-agents are almost exclusively employed. Dry assays form a branch of gravimetric work, and we shall include under this head all those assays requiring the help of a wind furnace. Wet assays, as generally understood, would include not only those which we class as wet gravimetric assays, but also all the volumetric processes.
~Gravimetric Methods~ aim at the separation of the substance from the other matters present in the ore, so that it may be weighed; and, therefore, they must yield the whole of the substance in a pure state. It is not necessary that a metal should be weighed as metal; it may be weighed in the form of a compound of definite and well known composition. For example, one part by weight of silver chloride contains (and, if pure, always contains) 0.7527 part of silver; and a quantity of this metal can be as exactly determined by weighing it as chloride as by weighing it in the metallic state. But in either case the metal or its chloride must be pure.
Exact purity and complete separation are not easily obtained; and methods are used which are defective in one or both of these respects. It is well to note that an impure product increases the result, whilst a loss of the substance decreases it; so that if both defects exist in a process they tend to neutralise each other. Of dry methods generally, it may be said that they neither give the whole of the substance nor give it pure; so that they are only calculated to show the amount of metal that can be extracted on a manufacturing scale, and not the actual quantity of it present. Their determinations are generally rough and always low. The gold and silver determinations, however, will compare very favourably with