REFERACTORY ORES.

New Zealand Herald, Volume XXVI, Issue 9307, 9 March 1889, Page 5

REFERACTORY ORES.

HOW THEY ARE TREATED AT" FREIBURG. PROFESSOR AUSTEN'S REPORT. VARIOUS PROCESSES DESCRIBED. [BY TBLKOKAPH.—OWN CORRBSI'ONDKNT.] Wellington, Friday. The Minister of Mines has received from the Agent-General the report of Professor Robert Austen, assayer to the Royal Mint, upon the processes and methods adopted in Freiburg, with special reference to the treatment of refractory ores in New Zealand. It is an interim report, and very voluminous. The following extracts appear to me to possess a special reference to the Northern goldfields, the Thames and Coromandel districts being mentioned. Professor Austen says:—"From tho various reports made by direction of the New Zealand Government, as well as from the statements appearing in the New Zealand press, it would appear that besides being rich in free milling auriferous quartz, New Zealand possesses considerable deposits of antimonides, tellurides, and other refractory ores, both of gold and silver, as well as complex sulphides, containing, in addition to the precious metals, lead, copper, zinc, and iron. With reference to the applicability of the Freiburg process to the treatment of ores occurring in Now Zealand, it may be pointed out that in order to render such a process a success, there must either exist a sufficient amount of suitable ores and fuel within a reasonable distance of the works (as is the case in Freiburg), or the ore must be sufficiently rich and abundant. The further means of transport must be sufficiently cheap to counterbalance the expense of carrying ore from mines distant from the smelting works. The network of railways in the United States has led to the system of sending ores to the ' Custom' works, which works are situated in the centres of mineral districts, and obtain their supplies of fuel by railway 'carriage at reasonable rates. Such ' Custom' works appear to have been erected in several places in New Zealand, one example being given in the report on the mining industry of New Zealand (1887), but they do not appear to have been commercially successful. The Lamonto process employed at the works referred to did not essentially differ from the ' cupola' treatment of argentiferous and plumbiferous material in use at Freiburg. The ores suitable for such treatment appear at present in New Zealand to be limited, and the colony is wanting in the means of transport, which render ' custom ' works profitable elsewhere." Professor Austen next describes the Freiburg works and tho general processes as follows :—" The ores treated at Freiburg consist chiefly of sulphides of lead, but silver and copper are always present in the mixture of ores which constitute the furnace charges. The plant also includes a furnace for the treatment of ores of zinc, arsonic, and antimonial fume as an incidental process/* Sulphuric acid is made both by the ordinary lead chamber method and by a patented and secret process in which the gases containing the sulphurous anhydride are stated to be passed over perforated clay elabs coated with platinised asbestos. Bismuth is also extracted from the portions of the cupellation hearths in which tho greater part of the bismuth collects. Provision is also made for the extracting of copper, nickel, and cobalt present in the ore. The treatment tie a whole centres round the lead ores, the various residues from the treatment of other ores, frequently rich in precious metals, being added to the lead smelting charge. It is in this latter treatment that the distinctive features of the Freiburg smelting processes are found, the lead ores being smelted in admixture with tho copper ore and argentiferous and auriferous residues. The furnaces are water jacketed cupolas. They were introduced originally in the year 1865. The extraction of silver by tho classical Frieburg amalgamation process, conducted in barrels, was also abandoned about this period, and was replaced by a method still in use, by which the precious metals are collected in tho smelted lead. In order to impart the necessary degree of strength and coherence to enable them to be treated in the blast furnace, as well as to obtain them in pieces of a size adapted to cupola treatment, they are first roasted at a temperature sufficient to clot the mass when roasted. This is effected by subjecting a suitable mixture of ores to an oxidising roasting in a long-bedded reverberatory furnace. The sulphur in the ore is in this way oxidised, and the metals are left chiefly in the form of oxides. Towards the end of the roasting the temperature of the furnace is raised to a sufficient degree to partly fuse the oxidised charge then present. The roasted material can in this state be withdrawn from the furnace into sheetiron wheelbarrows, in which it is allowed to solidify. The solidified material is broken to pieces and classified, according to the completeness with which the roasting has been effected, as indicated by , the presence or absence of unroastod lead sulphate. It is then molted down in the cupolas in a mixture with coke and brown coal. Large quantities of iron are present in tho material charged into the furnace, and the cold blast has therefore to be employed to prevent too great a reduction of that metal, and the consequent formation of unfused masses of reduced iron in the furnace. The products of this smelting are (1) lead, (2) regulus of fused sulphides of lead and copper, (3) a slag, (4) lead, and fume. A small quantity of speisse containing nickel cobalt is also occasionally obtained. The lead produced contains the greater part of the precious metals. The above articles are again treated by various processes for results indicated." Professor Austen gives the various methods with their separate results, which are of a strictly technical character, and interesting only to those practically acquainted with metallurgical processes of reduction and separation. _ Professor Austen gives a minute descripiion of special processes, methods, and results of a great variety of ores treated at Freiburg, mostly lead and silver or composite ores containing several kinds of mineral and metalliferous substances. On the 22nd of January, 18S9, after returning from Freiburg, he writes as follows to the Agent - General, in London :—" Anyone in the older countries familiar with mining and metallurgical practices, who read these reports of the mining industries of New Zealand, cannot fail to be impressed with the great anxiety of miners to possess trustworthy information as to all improved methods and processes. I have read tho various reports and handbooks bearing on the subject, and which contain statements of much interest and importance. I understand that your main object in visiting Freiburg was to ascertain how far the conditions which prevail at the great Saxony works correspond with New Zealand, and in accordance with your request, and that of Sir Saul Samuel, I append a report of the processes adopted there. It is impossible not to fully recognise the value of tho strenuous and successful efforts which are being made by Professor Black and other technical oficers of the New Zealand Government to meet the wan ts of this industrial population. There is evidently no lack of accurate metallurgical knowledge in the colony, but it would seem that the ablest men feel that the most pressing claims on their attention arise from the needs of the miners for more or less elementary technical instruction, they cannot, therefore, devote themselves to the study and practical development of processes suited to the actual treatment of complex ores in their neighbourhood. For instance, in the report to the Minister of Mines, dated May, 1886 (paper C 46, pp. 15 and 16), Professor Black calls attention to the great variety of valuable metals that are found in the Coromandel Peninsula, and also to the complexity of the ores containing the metals. He points out that ' processes for treating such ores are well known theoretically in the colony, but what is needed is the presence of some one connected with the Mines Department wlio has a practical knowledge of the processes, and has seen them at work ;' and he suggests that a qualified person should be sent to the Pacific Slope to collect and bring to New Zealand the information that is so urgently required on the Coromandel Peninsula, as well as at Collingwood and other parts of New Zealand. In a later paper (C 45, 1888, p. 8), Mr. H. A. Gordon, Inspecting Engineer to ; the Mines Department, observes that'll is not so much the want of capital as the want of knowledge of how to deal with our

silver ores that is keeping the field back just now, for if people could satisfy themselves by actual trial of the efficiency of any process they would find the capital to pat up all necessary machinery.' He (Mr. H. A. Gordon) refers to the preference which is shown to ' sending the ores all over the world' instead of putting up plant that may prove useless, pointing out that_ help from abroad is costly. It is impossible for me to say how far it may prove to be practicable to adopta system of smelting complex ores in blast furnaces with a_ view to the concentration of the precious metal in lead, as is done in Freiburg. It may, however, be pointed out, that what is called the Latnonte process, which is based on the same principle as that adopted at Freiburg, does not appear to have been successful. On the other hand, a similar scheme proposed by Mr. Parkes for the treatment of the very complex ores of the Thames district would appear & have given better results, but with reference to this Mr. Gordon, in the report to which I have referred says that he is very doubtful whether the smelting process will ever be carried on with New Zealand ores at a cheap rate, as the cost of obtaining the necessary fluxes will always make it an expensive process in the colony. I cannot offer suggestions as to the treatment of ores until I obtain further information respecting the nature and relative abundance of the ores, the variety of fuel it is proposed to employ, the cost of transit, and the local conditions generally. I have only at present described the smelting processes for complex ores as conducted at Freiburg, but I propose to send later on some further observations, pending the arrival of the particulars to which I have just referred. In the meantime I may say that I am of opinion that if lead ores are sufficiently abundant it ought to be possible at the future, if not at the present, time to establish in New Zealand and New South Wales some large central works similar to the Muldenhutte in Freiburg, to which the complex auriferous and argentiferous ores of your colonies might be transmitted for treatment."