ASSAYING. GENERAL DIVISION. 1. OBJECT OF THE ART OF ASSAYING.1 THE art of assaying (docimacy, from dozuale, to test) is a branch of analytical chemistry. Its object is the quantitative determination, in the shortest possible time, of the products of mining and metallurgical operations, as well as the quantitative examination of many natural and artificial products derived from other sources, such as coins, fuels, etc. Formerly, in order to reach the result with the greatest expedition, the dry method (dry assay) was chosen for producing the chemical reaction; but, as this was frequently done at the expense of accuracy in the result of the assay, the wet method (wet assay, gravi metric analysis, analysis by measure or volumetric analysis, and colorimetric analysis) is also used in modern times. But it has by no means entirely displaced the dry method, for the latter is employed in all cases where results sufficiently accurate are more quickly reached, or where suitable, simple wet assay methods (as in assay 1 Kerl, Eisenprobirkunst, Leipzig, 1875. Balling, Probirkunde, Braun schweig, 1879. Mitchell, Manual of Practical Assaying, London, 1868 Ricketts, Notes on Assaying and Assay Schemes, New York, 1876. ing lead, cobalt, nickel, gold, and silver) cannot be substituted for it. Sometimes a combination of both is employed (assays of lead, gold, etc.). Recently the method of precipitating metals by electrolysis' has been employed to great advantage (copper, nickel, cobalt). Volumetric assays can mostly be performed in a shorter time, which is an important item where much assaying has to be done. The results they yield are either very accurate, or at least sufficiently exact2 for metallurgicotechnical purposes; they are less expensive, but require greater experience and more chemical knowledge on the part of the operator, and special apparatus of accurate construction. While by the dry method the metal assayed, or one of its alloys having a known composition, is weighed directly, in volumetric assays it is calculated from certain reactions of the reagents employed, and the result may possibly be vitiated on account of the presence of foreign substances, of whose presence there is not always an indication. Colorimetric assays are chiefly employed for determining very small quantities of metals which either could not be detected by other methods, or, if so, then only by very tedious processes (copper, lead); but recently they have been developed so as to adapt them for substances rich in metal (copper). The blowpipe is frequently used for a preliminary assay.3 1 B. u. h. Ztg., 1869, p. 181 (Luckow) ; 1875, p. 155; 1877, p. 5 (Schweder). Grothe, polyt. Ztschr., 1877, p. 11 (Bertrand). 2 B. u. h. Ztg., 1869, p. 330. (Compare gravimetric and volumetric assays of Cu, Fe, Zn, Sb.) 3 Berzelius, Anwendung des Löthrohrs, Nürnberg, 1828. Scherer, Löthrohrbuch, Braunschweig, 1857. Birnbaum, Löthrohrbuch, Braunschweig, 1872. Simmler, Löthrohrchemie, Zürich, 1873. Hirschwald, Löthrohrta I. Mechanical Manipulations. 2. SAMPLING. It is absolutely necessary that the small quantity of sample with which the assay is made, should represent the average composition of the ore-heap, etc., from which it is taken. The manner of taking samples varies according to the character of the substances to be assayed, viz. :— A. Non-alloys (ores, matt, speiss, slag, etc.). 1. Substances in fragments, either homogeneous or heterogeneous in composition. a. Homogeneous fragments, many iron ores, lead and copper ores, etc. a. Samples from the heap.-Pieces are taken at random (it is best to do so with bandaged eyes) with the hand or a shovel, from different places on the circumference of the heap, and also from the interior, after the upper layer which has been dried by the atmosphere has been removed. The collected lumps (about 100 kilogrammes, 220.54 lbs.; in Freiberg, for certain ores, one-tenth of the heap) are comminuted to pieces of the size of a bean, either by means of rollers or stamps, or with a sledge-hammer. A square or conical heap is then formed of the pieces, and this is divided into four parts. One of these is taken, the pieces forming it are still further comminuted, and then again formed into a heap, which is divided as before. The comminution and reduction are repeated several times, finally upon an iron plate bellen, Leipzig, 1875. Landauer, Löthrohranalyse, Braunschweig, 1876. Kerl, Löthrohrprobirkunst, Clausthal, 1877. Landauer, systematischer Gang der Löthrohranalyse, Wiesbaden, 1878. Plattner-Richter, Probirkunst mit dem Löthrohr, 4 Aufl., Leipzig, 1878. B. u. h. Ztg., 1868, p. 26; 1872, p. 59. provided with a rim (reducing-board), until at last from to 1 kilogramme (1 to 2 lbs.) of the sample remains, in such comminuted form that it will pass through a sieve having 30 by 30 meshes to the square centimeter (about 75 meshes to the inch). B. Samples taken while the ore, etc., is being weighed. -Pieces are taken at random from every lot weighed, and the collected pieces comminuted and reduced according to paragraph a (Upper Harz copper pyrites). 7. Samples by rasping.-Fuels, etc., which cannot be pulverized are comminuted by a rasp, and a reduced sample made from this. 8. Slag samples.—A piece of slag is taken every time the slag is tapped or run off, or a piece is broken off from every cone formed. The pieces of one charge are comminuted and reduced in the manner above stated. b. Heterogeneous fragments.—(Gold and silver ores, many copper ores, coal with slate and pyrites, etc.) a. Sampling by the crossing method.'-When the grains are too dissimilar and too coarse, the entire heap is broken up. The broken fragments (as large as a walnut for less. valuable ores, and about the size of a hazelnut or bean for the more valuable) are passed through screens or cylindrical sieves. An oblong or square heap, 30 to 40 centimeters (about 12 to 15 inches) high is then formed. Ditches about 20 to 30 centimeters (about 8 to 12 inches) wide, and crossing each other, are dug out with a shovel, and the samples are then taken by digging them out from the top down to the bottom of the squares, which remain standing between the ditches. These samples are comminuted to the size of millet-seed, thoroughly mixed, and formed into a new rectangular heap, which is again 1 Preuss. Ztschr. xvii. 137 (Mansfeld); xviii. 223, 224 (Swansea). crossed, and samples taken from it in the manner stated. This operation is repeated, finally by using the spoon, until the samples are reduced to a powder. (Method in the great ore markets of Swansea and Liverpool, for American silver ores in the Upper Harz and in Freiberg, etc.) B. Sampling by dropping the ore.' (Sturzprobe.)—The ore is dropped through a funnel standing over a pyramid of sheet iron, which is divided into four divisions by partitions projecting over the edge, into which the ore is distributed. The ore from one of the divisions is comminuted and dropped through a funnel into a similar but smaller pyramid. This operation is repeated, smaller pyramids being used every time, until a sufficiently reduced sample has been obtained (method in Chili and Colorado). 2. Small ore and pulverized substances. a. Sampling while weighing.-The small ore must be carefully mixed, and, in case it should be rich, it is best to have it in such a condition that not over 15 per cent. of coarse material will remain behind in passing it through a sieve having 10 meshes to the square centimeter (25 meshes to the inch), but, otherwise, it may be coarser. It is generally weighed in quantities of 50 to 100 kilogrammes (say 100 to 200 lbs.). Three spoonfuls are taken from different places of every lot weighed, and placed in wooden troughs standing near the scale. All the samples taken from one lot are mixed together and a heap is formed from them. This is reduced by quartering, or by the crossing method.2 1 Preuss. Ztschr. xxiv. 49. B. u. h. Ztg. 1872, p. 232 (Chili). Ann. d. Mines, 1878, XIII. 606 (Colorado). 2 Kerl, Oberharzer Hüttenprocesse, 1860, p. 195. |