PROFESSOR BROWN ON THE NEW METHODS OF TREATING THAMES ORE.

New Zealand Herald, Volume XXV, Issue 9131, 13 August 1888, Page 11

PROFESSOR BROWN ON THE NEW METHODS OF TREATING THAMES ORE.

The following important letter has been sent by Professor Brown to the Minister of Mines, and is published by the Minister's permission :—

University College, Auckland, July 28th, 1888.

To the Hon. the Minister for Mines. — Sir,—l learn from the newspapers that you are interesting yourself in the question of the extraction of gold and silver from the refractory ores, so-called, of the Thames district. As I have for some time devoted attention to this subject, and have just paid a visit to several of the chief mining centres, I venture to think that the following remarks may be of service:—

The ores found in the Thames district maybe roughly divided into three classes—(l) Free milling auriferous quartz; (2) auriferous silver ore, consisting of antimonides, sulphides, telluridea, and other compounds of silver mingled or combined with a variable but generally small quantity of gold, and a very large quantity of quartz; (3) complex sulphides, containing gold, silver, lead, copper, zinc, and other metals, and much less quartz than the first two classes of ore. The ores of the first-class are found chiefly at Coromandel and near the mouth of the Thames River ; those of the second at Marototo, Karangahake, and Waihi; those of the third at Te Aroha and Waiorongomai. In order to extract the precious metals from these ore.-?, three different processes should be employed, one for each class of ore.

The ores of the first-class tire probably treated in a satisfactory manner by the stamp mills and bcrdans in general use at the Thames. It is, however, to be noted that the percentage of gold obtained by these means is unknown, owing to the fact that much of the finely-divided metal may be carried away by the water from the mill, and not retained in the tailings. It follows that analyses of the tailings, which show that these contain but little gold, do not prove that the loss by the battery process is inconsiderable. There have recently been erected on the foreshore at the Thames some pans of peculiar construction, termed AskhamMolloy pans. These machines are designed in accordance with the soundest scientific principles,are mechanically excellent, and are doubtless destined to increase the amount of gold saved from ores of the first class. Their proper use is to extract the fine gold from the material escaping from the stamp mill. In the case of free milling auriferous quartz there are two questions which require an answer : Firstly, what is the loss in the ordinary stamp mill ? Secondly, can the mill be replaced efficiently by a rotatory grinding machine? Neither of these questions can be regarded as urgent. The stamp mills and the bcrdans are good, though doubtless capable of improvement.

The ores of the second class, which are found in large quantities in the Ohinemuri Valley, are properly treated by roasting and amalgamation in pans or other contrivances having the same end in view. There is some di.iicuity in deciding as to the best method of roasting to be employed, that in general use resulting apparently in a heavy loss of the precious motals, especially when salt is employed. In other respects the reduction of these ores presents no difficulty. A magnificent plant for carrying out this process of roasting and amalgamation is now in course of erection at Waiht, by the Waihi Goldmining Co. (a company registered in London). When they have overcome the roasting difficulty, they will doubtless extract the precious metals from the ores of their mine in an efficient and economical manner. I anticipate that the plant will be in full operation in about three months.

An alternative to amalgamation is chlorination, by gas and leaching. Machinery for the extraction of gold and silver in this manner has been landed at the Thames, and is now being erected. This system, for which patent rights are claimed by Messrs. Newbery and Vautin, is sometimes regarded as relatively expensive. Experience alone will prove whether it can compete successfully with the amalgamation process in New Zealand.

A gentleman of great metallurgical experience is now erecting a reverberatory furnace in the village of Karangahake. The furnace will be completed and ready for use in about three weeks, when the ores of the district will be submitted to a smelting process apparently similar to that employed in the reduction of copper ores. I fear that the process is not adapted to the reduction of the ores of this district, but it could be utilised for the treatment of the ores of the third class, which are common, as before said, at To Aroha and Waiorongomai. The inexpentive nature of the plant required is greatly in favour of this process, and it is possible that the small first cost may bring it into use even in the case of ores which are more properly treated by amalgamation.

Passing now to the ores of the third class, it has recently been demonstrated by the Mount Atoha G.M. Co. that these ores are very easily concentrated, and that the concentrates may he smelted without difficulty. Machinery for this purpose has been ordered by the company, and will be added to the extensive and admirable plant already in operation at Waiorongomai. The smelting is to be carried out in a blast furnace similar to those in use in Colorado and to those erected at the Thames at the instigation of Mr. LaMonte. In this connection it is to be remarked that the previous failure of these furnaces in New Zealand resulted, not from any imperfection in their design, but from ignorance of the class of ore to which they are adapted.

The foregoing statements, which are all derived from personal observation, will convince you that in a short time the necessities of the whole district will be fairly provided for, and that the problem of the extraction of gold and silver from the ores will be completely solved by the companies engaged in the various enterprises enumerated. The obstacles which have previously existed were not due to any troublesome peculiarity in the ores, but simply to the want of familiarity with the processes in general use in America and elsewhere.

1 understand that the Government propose to oiler a bonus of Jt'SOiK) in order to encourage persons to undertake the reduction of the socalled refractory ores. Had this bonus been otlercd some years ago it would doubtless have hastened the erection of machinery similar to that now on the tield, but the work is now to a great extent achieved, and the companies engaged in it will not be induced by such a bonus to do anything but that which they arc already intent on doing. If they succeed i'sooo will be but a small item in their profits ; if they fail it will not extricate them from their difficulties. Further, it is evident that inasmuch as three processes are required, three sums of money should be offered ; otherwise it will be impossible to decide to which of the three classes of possible candidates the single bonus should be awarded. I would submit that in any case the patent rights which might be connected with any process receiving a bonus, should be transferred to the State. It does not seem right first to grant a profitable monopoly, and then to subsidise the fortunate possessor of the same.

You will have gathered from the description given above that there are certain questions which require an answer before the operations on the Thames goldtields are placed on a thoroughly satisfactory basis. Of these the most prominent at the present moment is : How may the ores of the second class be most economically roasted The solution of this problem, and of many others which will arise from time to time, may easily be found by means of the experimental plant which exists at the Thames in connection with the School of Mines there. Unfortunately the School possesses no funds, and is therefore not in a position to make use of its machinery unless the cost of the experiments is furnished from some external source. It is essential that the men employed in the testing mill should be always the same, and therefore that there should be some fund from which their wages may be paid when no special experiments are being carried out at the cost of private individuals or by companies. This fund should in my opinion be supplied either by the Central Government, or, better, by the counties and boroughs interested. I understand that a Bill recently brought before the House of Representatives provided, inter alia, that the counties should be permitted to furnish the necessary supplies, and this appears to me a most reasonable provision, inasmuch as it is based upon the principle that those who chiefly benefit by an undertaking should pay the cost of it. I am informed that you have expressed some doubt as to the expediency of thus allowing county councils to contribute to the funds or Schools of Mines. I trust that this letter will lead you to re-consider the question, as I am convinced that the cost of technical education generally should be borne by the county and borough councils, and not by the Central Government.— am, &c., Fred. D. Brown.