This section is from the book "Leaching Gold and Silver Ores. The Plattner And Kiss Processes: A Practical Treatise", by Charles Howard Aaron. Also available from Amazon: Leaching Gold And Silver Ores.
This book is written in the endeavor to supply, in some small degree, a want which is severely felt on this coast, namely, that of plain, practical books on metallurgy. In the standard works on this subject, especially in regard to gold and silver, there is a great lack of those practical details which are so essential to the success of the operator, while some of them are characterized by a display of scientific lore which is very discouraging to those whose preliminary education does not enable them to understand it.
This condition of affairs is probably due, in part, to the circumstance that books are less often written by practical workers in this branch than by scientific gentlemen who obtain such practical details as they do give us, not from their own experience, but by inquiry and observation, more or less extensive. It is also in part owing to the fact that in Europe, where most of the works alluded to were written, there are fewer men who are called on to conduct metallurgical operations without previous apprenticeship to the business.
The processes selected for description, namely, the Plattner for gold, and the Kiss modification of the Patera for silver, are those which seem the best adapted in general to our wants; the first, for the sufficient reason that it is the only available process for the extraction of gold by lixiviation; the second, because it is more convenient, and requires a less extensive plant than the Augustin process, which depends upon dissolving silver chloride by means of a hot solution of common salt, while the Ziervogel process, depending on the formation of silver sulphate, which is extracted by means of hot water, is only adapted to the treatment of matte.
The Hunt & Douglass silver copper process is a method of lixiviation which has much to commend it, but its mode of operation and field of adaptability are so distinct that it will more properly form the subject of a separate treatise.
In the arrangement of the book, the author endeavors to make the necessary explanations and practical directions as simple and straightforward as possible, while matter which, however interesting or instructive, is not essential, appears toward the end, in a separate division.
While it is deemed necessary to give an outline of the rationale of the different operations described, it is also thought desirable to avoid, if possible, the use of terms which might be in conflict with either the old or the new systems of notation. Thus, "sodium sulphate," though implying a departure from the still older binary system, in which the formula for the salt was Na O S O3, and in which it was regarded as a sulphate of oxide of sodium, and called sulphate of soda, is, notwithstanding, compatible with the use of the old atomic weights, and represents a view of molecular constitution which antedates, by several years, the general adoption by chemists of the new weights.
It is with this intention that the term sulphur oxide is used, in preference to either sulphuric and sulphurous acid, or anhydride, in speaking of the products of the combustion of sulphur, and the formation of metal sulphates; for, while strictly accurate, and sufficiently precise for the purpose, it is the only intermediate term which could be used as applicable to either of the systems of notation.
The statement met with in every work on the subject, that "sulphuric acid," by which must, of course, be understood "sulphuric anhydride," acts directly on sodium chloride, with evolution of chlorine, seems, while doubtless true, to require more explanation than is usually given. It may be that air takes a part in the reaction, by supplying the oxygen neceessary for the formation of sodium sulphate, but it appears to the writer that a clue to the true explanation is furnished by Brande's statement that sulphuric anhydride is decomposed by heat into sulphurous anhydride and oxygen. The decomposition is probably assisted by the affinities of the sodium in the gaseous sodium chloride. In this view the reaction would be Na C1+ 2 S O3=NaS O4 + S O2 + Cl, (2 Na C1+ 2 S O3=Na2 S O4 + S O2 + Cl2) which can take place within the roasting mass, where air can have little to do with it, as well as in the atmosphere above the ore, the sulphuric anhydride being furnished by the decomposing metal sulphates, under the influence of heat, and the nascent chlorine having the best opportunity to act on remaining sulphides.
Chlorine which may be evolved, or may rise, above the surface of the ore will, in presence of sulphurous anhydride and steam, which latter must be produced whenever fresh fuel (wood or coal) is introduced, form hydrochloric acid, with reproduction of sulphuric anhydride.
For valuable assistance in the literary part of the work the author is indebted to his friend, Professor John Calvert, of the California College of Pharmacy, San Francisco. The works which have been consulted comprise those of Regnault, Cooke, Abell and Bloxham, Ure, and Kustel, as well as Lippincott's Encyclopedia of Chemistry.
While the author has not succeeded in entirely satisfying himself, he ventures to hope that the book will be found useful by those who may have occasion for it.