8 THE VARSITY ENGINEER. October, ig".

The Chlorination of Gold Ores.

By Mr. Donald Clark, M.M.E., B.C.E.

The ideal method for extracting fine gold would be to dis-solve out the gold with some solvent which would not attack other minerals present. The solvents known 6o years ago were chlorine, bromine, and cyanide of potassium. Yet in spite of this knowledge it was only comparatively recently that such solvents were successfully used in Australia.

The history of the chlorination process is interesting, and not commonly known, so that a brief account of it will not be out of place.

Dr. Percy—rightly called the father of English Metallurgy —in 1848 read a paper on the value of chlorine as a means of extracting gold from ores ; in the same paper he suggested the use of hyposulphite of sodium as an after treatment for the extraction of silver. It is interesting to note that both these processes subsequently became very important.

The sulphides are themselves readily attacked by chlorine, so he recommended that roasting should precede chlorination, since oxide of iron is not readily attacked.

Curiously enough, during the same year Plattner, who was Assay Master of the Royal Frieberg Smelting Works, carried on some experiments with chlorine water on roasted ores from Reichenstein, in Silesia. These residues were left from cer-tain arsenical pyrites, which had been crushed and roasted so as to expel their arsenic, which was recovered as asenious oxide, or white arsenic. They consisted mainly of oxide of iron, and contained about half an ounce of gold per ton—too small a quantity at that time for profitable smelting. Platt-ner's experiments on a small scale showed that the gold could be successfully extracted, and many others set to work and evolved methods which were not Plattner's at all, but still bear his name.

Dr. Duflos, of Breslau, took r51bs. of ore and i5lbs. of water, which three-quarters filled a bottle ; he then passed chlorine gas in, and corked tightly, and having rolled the bottle for two hours on a table, he decanted the liquor and again charged the bottle with water and chlorine; on adding the liquors together and filtering the gold chloride from the sand he obtained a good extraction. As a modification of this ex-periment he put the ore in funnels and allowed a stream of chlorine water to drain through. He obtained exactly the same result as with the bottle experiment.

His next test was to take one part of chloride of lime and 5o parts of water and to add hydrochloric acid. This solution was used as before, and the same results obtained.

October, i9ii. THE VARSITY ENGINEER. 9

These three results really cover what are to-day known as three chlorination processes.

He also tried ferrocyanide of potassium and chloride of lime, but got no gold. He did not try cyanide of potassium because he knew the ore contained ferrous salt, which would have given him ferrocyanide of potassium, and from which he only obtained negative results. Had he oxidised the ferrous compound and then applied cyanide of potassium, he would have really given us not only the chlorination, but also the cyanide process.

Duflos also used wooden barrels, but since his extractions in vats were just as good, he discarded the former and adopted the latter.

In 1849 Lange carried out similar tests on the same residues, but went further and mixed chloride of lime with the residues ; he then poured diluted hydrochloric acid on them so as to pro-duce chlorine within the mass. He also discovered, at the sug-gestion of Karsten, that a saving could be effected by the use of gaseous chlorine. The ore was only moistened and chlorine gas was passed into it until no more was absorbed; when the gold dissolved, it could be washed out with much less water. It is this method which is erroneously attached to Plattner's name.

Subsequent to the German discoveries and methods of ap-plying chlorine for the solution of gold, patents were taken out year by year for inventions which disclosed no more than the Germans made public.

Deetkin introduced the so-called Plattner process into Cali-fornia in 1858, and patented it in 1863! In 1864 De Lacy ob-tained a Victorian patent for the use of chlorine under pres-sure. He proposed to hang a barrel vertically on trunnions attached to its greatest diameter, to fill it with ore prepared for chlorination, and then to force chlorine under pressure into it—surplus chlorine to be absorbed by lime, and so form chloride of lime; he also suggested mixing ore and chloride of lime together, and then adding acid, so that the chlorine would be evolved in the barrel under considerable pressure.

De Lacy's patent was patented under several names many times afterwards.

Dr. Mears, of Philadelphia, in 1877, secured a patent for treating ore in a barrel with chlorine under pressure (prac-tically De Lacy's) ; this was in use for some time in America, but went out of fashion.

Munktell, a Swedish engineer, proposed to use chlorine water by dissolving sulphuric acid in one vat and chloride of lime in another, and running the two solutions through a Y pipe on to the surface of the ore; he claimed that the chlorine would be nascent, and so dissolve gold more readily. This of course is absurd, for only an ordinary solution of chlorine

zo THE VARSITY ENGINEER. October, 1911.

would form in the pipe, and only chlorine water with salts dissolved would penetrate the ore. The method, as you may remember, is practically the same as the German proposals of 185o; it was formerly used by the Mt. Morgan Company and by several Victorian companies, and is still in use at Cassilis, Victoria.

In 1886, Hargreaves and Robinson proposed to treat hot dry ores with chlorine at 400°F., but so far as can be learnt, noth- ing came of the proposal. During the same year J. S. Mac-Arthur and Forrest patented a process for the treatment of powdered ores with chlorine, bromine or iodine, in a solution carrying such salts. This patent, like a good many others, was a kind of dragnet device to obtain patent rights for solvents whose properties were to be found in any dictionary of solu-bilities ; they also suggested that the work be carried out in total darkness!

Hannay and Ballantyne, in 1887, proposed the use of liquid chlorine or chlorine compressed at more than four atmo-spheres, or chlorine evolved by electrolysis; they also aimed at overcoming the precipitation of any gold within the ore by the use of cyanide of potassium, and the subsequent precipita-tion of the gold by electrolysis. I have made use of com-pressed chlorine and found it satisfactory, but more expensive than chlorine generated in other ways ; also have used cyanide of potassium solutions to dissolve out more gold and chloride of silver left after treating the ore with chlorine, with good results.

It was not until 1887 that the Newberry-Vauten process was patented. One essential part of this process consisted in treat-ing the prepared ore with the ordinary amount of chlorine and then pumping in air. The object of the air pressure being to liquify the chlorine. The inventors state that "when chlorine is used, great advantage is gained by employing air pressure at about 4 atmos., for thereby any chlorine that may exist in the form of gas in the vessel containing the material under treatment will be liquified and dissolved in the water added in which condition we find its action greatly. accelerated." Un-fortunately the law of partial pressures was entirely overlooked and if any advantage was gained with air pressure, it was not for the reason they stated. I do not want to suggest that this process should be ridiculed because of an error in principle, for the process patented contained a number of valuable work-ing details, and included the precipitation of the gold by means of charcoal. The process was adopted at Mt. Morgan and elsewhere, and worked as patented, but later on, the air pres- sure was omitted, and shortly afterwards the process itself was discarded in favor of simpler ones. It is of interest to note that in 1888 a patent was granted to MacArthur and Forrest for dissolving gold from ores by means of cyanides or cyan- ogen yielding substances; and still more interesting to note.

4

October, i ii. THE VARSITY ENGINEER.

how the proposal was Iooked upon by some of the mining en-gineers only twenty years ago. A notable writer states that ` Cyanide of potassium is one of the chemicals frequently used in the arts for keeping gold in solution ; but its practical ap-plicability to metallurgical operations on a large scale is ques-tionable from almost every point of view."

During the saine year as Newberry took out his patent for increasing the air pressure in ores treated by chlorine, James Holms Pollock, Assistant to the Professor of Chemistry at the University of Glasgow, took out a patent for applying water at a pressure of loolbs. per square inch to the chlorinated ore. In 1888 Sir Wm. Crookes patented a process for the treat-ment of auriferous ores with perchloride of iron solutions in the presence of some oxidising agents, such as nitre, or man-ganese dioxide. The ore and solution were to be heated to boiling point, the gold and silver would go into solution.

Comparatively recently Etard patented a process for the use of a dilute solution of permanganate of potash, and hydro-chloric acid as a source of chlorine, and Professor Black, of New Zealand, modified this by taking sulphuric acid and salt, making a dilute solution, and then mixing this with a dilute solution of permanganate of potassium. This affords an easy and simple method for the production of chlorine water.

The tendency in Australia has been to return to the simpler methods of chlorination, such as were described by the German investigators of 185o.

Mr. Edwards, of Ballarat and Bendigo, has returned to a modification of the so-called Plattner system, that is, moisten-ing the ore and sifting it loosely into a vat provided with a sand and gravel filter bottom. When filled, the vat is closed by a lid, and chlorine gas passed upwards through the ore and under sufficient pressure to ensure the permeation of the gas into every patch of ore. The Mt. Morgan Company, after using the Newberry-Vauten process, threw it out and adopted the modified Munktell process of. adding dilute solutions of chloride of lime and sulphuric acid to the ore. Subsequently the Company prepared chlorine water by generating chlorine from manganese dioxide salt and sulphuric acid, and later by electrolysis of salt solution, and absorbing the gas in scrubber towers. Chlorine water so prepared was run on the one in large, shallow, open cement vats, until the gold was dissolved. The Cassilis Company adopted the so-called Munktell process, and has used it since the, inception of the Company.

Many different processes were in use in the United States of America, but the tendency has been to adhere to the barrel method of treatment. This has been simplified by improve-ments in the barrel itself. The barrel has been increased in size so as to hold from Io to 20 tons, a permanent filter bed has been introduced so that when the ore has been saturated with chlorine, and the gold dissolved, water

12 THE VARSITY ENGINEER. October, 191I..

can be run in and the leaching and washing out of the gold performed within the barrel itself. Afterwards, the sand de-prived of its gold, can be hosed out of the barrel. Chlorine is usually generated by adding chloride of lime, and then sul-phuric acid in excess in such a way that the two only come into contact when the barrel is rotated, after sealing it up.

With regard to the different methods of chlorination I can only say that when comparatively small lots of rich ores re-quire to be treated, that I prefer the so-called Plattner method; when the moistened ore is sifted loosely into a vat, chlorine is readily absorbed by the water present; if it is consumed by the ore, more gas readily takes its place, and it is not until the whole of the layers of ore are saturated and satisfied that excess of chlorine will rise to the surface of the ore. When it gets there, then we can be sure that all the gold capable of solution will be dissolved in from 24 to 48 hours. Next, with regard to the washing out of the gold, it can be done with less water than is used in any other process. There is no difficulty about filtration, even with very finely ground ores.

With regard to the methods in which chlorine solutions are used, there is not much to choose between them. Provided the gold is fine and the ore suitably prepared, chlorine solutions will act readily. ; An aqueous solution of chlorine is preferable to that laden with sulphate of lime, such as forms when chloride of lime and sulphuric acid are run in together. The sulphate of lime tends to separate out and form a crust on top of the ore, also to clog the filtration spaces. The permanganete method is clean, but came too late for general adoption. All these chlorine solution processes are open to the objection that even when the liquid is saturated with chlorine they are dilute and chlorine is often absorbed or destroyed before it reaches the bottom of the ore in the vat; in some cases the gold is dissolved from the upper layers and precipitated on the lower ones. It, is only by repeatedly passing chlorine solutions through that the gold can be extracted, and, as is well known, the oftener a solu-tion passes through an ore, the more impervious it becomes, and towards the finish many patches of ore are not acted upon. This is so well known in cyanide treatment that very often a double treatment is given ; that is, the ore is leached in one vat, then emptied into another, and again subjected to solvent treatment. The time taken, and the quantity of liquor used are both much greater than by the Plattner method. The only advantage I can see is that much larger quantities can be dealt with when the ore is low grade and the gold is fine, and that when copper is present, by using excess of sulphuric acid the oxide formed on roasting will be dissolved out, and the metal may be recovered as well as gold.

The barrel system as carried on in Australia, had little to recommend it, and it has practically become obsolete. The new American method is better than the Australian, but even

October, 191J. THE VARSITY ENGINEER. 13

in this case there is nothing to•be gained by the use of a barrel, for no better extraction can be obtained with it than with a simple closed vat, which can easily be made to hold io tons. By using more chlorine or chlorine under pressure, a s.aving of time can be effected, but it is doubtful whether this pays for the extra cost and the greater inconvenience of working.

We want now to consider what ores are suitable for chlor-ination. The necessary conditions are :-

i. That the gold present must be finely divided. 2. That it must be separated from gangue. It must not

be protected by an impervious coat (a) Such as gangue enclosing fine particles of gold. (b) A glaze such as often forms in roasting auriferous

ores or by (c) Salts formed by the action of chlorine on alloys or

compounds associated with gold, such as Argenti-ferous gold, or lead compounds.

3. That minerals and substances which destroy or use up considerable quantities of chlorine should be absent.

Dealing with examples of such ores, if the gold present is coarse, then there are simpler and cheaper methods for saving it; but even comparatively coarse gold is rapidly dissolved by chlorine, and as a rule after an ore has been treated success-fully by this means, little or no coarse gold remains. The ordinary concentrates saved on Wilfley tables and other ap-pliances, as a rule contain visible specks of gold, and when mercury is used a large amount of gold becomes amalgamated during the passage of the concentrates over the table, and gathers together in small pieces of amalgam. This amalgam should be caught and removed.

The second condition, that it must be free from enclosing gangue, is much more difficult. Some of you know that gold is excessively fine, but the fine state of division of the gold is such that even the finest trituration is not sufficient to free it from its associated matrices. Mr. Mickle, in his researches in our laboratory, attempted to dissolve the gold from some Cassilis ore by powdering it, and sifting it through the finest sieves, and then treating the finely divided material with solvents until no more gold would dissolve. The result was that there still remained about io dwt. per ton. He then ground it until the ore was so fine that it would remain sus-pended in water for some time ; there was still 7 dwt. per ton left in. Some mines produce such fine gold that it would be hopeless to attempt to liberate the gold mechanically.

Taking another . case, the well known Mt. Morgan Mine. The gold there was so finely divided that although tons of gold were extracted annually, no gold was visible in the ore, and very often it was so fine that it could not be extracted by

i4 THE VARSITY ENGINEER. October, 911.

crushing and panning off, yet the ore would assay ounces per ton. I have seen similar material in Victoria which would assay over 50 ounces per ton, yet no gold could be discerned with the aid of a lens.

I might mention incidentally in passing that I consider, as stated over ten years ago, that the bulk of gold liberated by the erosion of strata has become disseminated through our sediments in such a fine state o f division as to be commer-cially irrecoverable. The recovery of such fine gold at Mt. Morgan was a most difficult problem. Battery treatment, fol-lowed by grinding in pans with quicksilver, only gave 2 ozs. out of 8 or io ozs. per ton ; the escaping sands were settled as far as possible, but the water running away carried a large amount of gold. In course of time 20,000 tons of tailings ac-cumulated, and these assayed 5 to 6 ozs. per ton—worth from 4.0o,000 to £500,000. The water was settled in dams, and the boilers were supplied with water from a dam nearly 200 yards. from the tailings heap, yet the scale which formed in the boilers assayed II ozs. of gold per ton.

If a piece of the brown iron ore were placed in hydrochloric acid and left for a few days, the iron oxide would all dissolve out and leave as a residue a friable, spongy mass of gelatinous silica, of nearly the same size and shape as the original lump in this, minute spangles of gold could he seen.

Inventors got to work, and many concentrating devices were tried and failed ; amalgamation methods failed, smelting the silicosis ore was too expensive; the ore was crushed and chlorinated, and returns of only 50% at best obtained. It was not practicable to grind the ore sufficiently fine to liberate the gold. At this stage the knowledge of one little fact in mineralogy served to solve the problem, and only served to show that apparently useless scientific facts, if acquired and stored away, may ultimately be of great importance. Now, Cosmo Newberry, who did excellent pioneering work in prac-tical metallurgy, in Victoria, as a mineralogist, knew that limonite and hydrous silica lose their combined water by being heated to below redness. Every molecule of these substances is affected by such a change, and the mass is changed from an impenetrable solid to a spongy material. The molecules of water liberated would burst every grain of material, and ,thus open up the ore and expose the gold more effectively than would be possible by the finest crushing. This simple dis-covery was worth millions of pounds to the Mt. Morgan Com-pany.

Very few ores are capable of being treated direct by chlorine, for even if the gold particles are freed by crushing and "triturating the ore, there is usually some chlorine consum-ing compound present, which would use up so much of the element as to make the operation unprofitable.

October, i9ir. THE VARSITY ENGINEER. i5

It is well known that chlorine does not tend to attack higher oxides in the same way as it attacks protoxides, sulphides and metals, and therefore if we convert the latter compounds into higher oxides they will not, as a rule, be attacked. The oxida-tion is brought about by heating them in a current of air, and is commonly known as roasting. If such ores are roasted, then the sulphides and arsenides of iron will be altered mainly to volatile oxides of .sulphur and arsenic, while the iron will be converted into ferric oxide, any gold present remaining unaf-fected. Now one danger in roasting such material is that the operation may be carried on too rapidly at first, and the temperature may be too high ; in this case the outer crust of the pyritic particles will oxidise rapidly, but the inner core of sulphide will melt and molten sulphides will absorb and dis-solve gold as easily as hot lead will dissolve it. If the temper-ature is very high, even the magnetic oxide of iron first formed may melt and seal up the gold, or if it is in contact with grains of silica, a compound of the two, or a slag may form and the gold will be more effectively locked up than in the Mt. Morgan limonites. Fortunately the roasting of bisulphide of iron is comparatively easy. As you are aware, if gently heated, part of its sulphur is expelled, and if roasting is very slowly con-ducted, the open, porous particles become quietly oxidised to ferric oxide, which will be open and loose in texture, without any sign of fusion or glazing. Such an ore can be easily dealt with. If, however, we take some of the other sulphides, the action is entirely different. Some such as those of antimony and bismuth, and to a lesser extent galena, melt so readily that the temperature must be very low, and must be kept low until all danger of fusion is past. When very small quantities are present, this can be done, but if larger amounts be present then it is almost impossible to prevent locally overheating, fusion, or fritting. These minerals are usually caught on modern concentration tables, which separate them into bands, or streaks, depending on their specific gravities, and they are dis-charged as banded products. In consequence of this, they enter the furnace without being thoroughly mixed, and it be-comes a much more difficult matter to roast them than if they were diluted with more or less inert material, or thoroughly intermixed. A much better roast and a better extraction can as a rule be obtained by dry crushing the whole of an ore, and then roasting it, than by crushing the ore, concentrating it, and then roasting the concentrates alone. The absorption of gold by the metals, for no doubt metals are produced transiently in a roaster, and sulphides, the glazing of gold bearing par-ticles retards or prevents solution. It is well known that even after as much gold as can be extracted by chlorine solutions is taken out of a roasted ore that by grinding it up, and re-treating with solutions, an additional amount can be obtained. The losses owing to sealing up and glazing of ore particles is often large; for instance, at the Cassilis Mine the tailings often assayed an ounce or more per ton, and very seldom

16 THE VARSITY ENGINEER. October, t9ii.

under io dwts. Mr. Mickle devoted a considerable amount of time to the investigation of these and had his results been properly considered, a large sum of- money could have been saved and sanguine estimates as to extractions by grinding and cyanide methods discounted.

The third trouble with regard to the sealing up of particles is due to the formation of some insoluble compound formed by chlorine, which protects the gold; all native gold, as you no doubt are aware, is really an alloy of gold and silver; some-times the amount of silver is only from a fraction of one per cent., as at Mt. Morgan, to 3, 4, and 5% as at Ballarat and Bendigo, or 20, 3o and 4o per cent., as in several places in Gippsland. If a piece of alluvial gold running even 90% gold and io% silver is boiled with aqua regia, the gold at first rapidly dissolves, but after a time the action slows off, and a white coat of chloride of silver soon forms, which prevents further rapid action. From this experiment, no doubt, it has been assumed that during the chlorination of gold ores that chlorine would have a similar action and so be prevented from attacking the gold. It has been further stated by the barrel process advocates that the rotation of the ore in the barrel would cause the removal by attraction of the coat of chloride of silver, and the gold would be more readily attacked. I must say, however, from the investigation of a number of samples of tailings from chlorination works that so far as Victorian ores go, there is little or nothing in this contention, and some years ago took strips of alloyed gold and silver from 6o% to pure gold, and found that in chlorine water the gold would dis-solve at nearly the same rate as pure gold. It must also be borne in mind that the gold in ores treated by solvent as a rule is very fine, and that the roasting operation tends to remove part of the silver; for instance, in Bendigo, the gold won by amalgamating roasted ores, is always poorer in silver than the gold won by amalgamation of the same ore. Practically the chloride of silver theory may be dismissed.

The last case is the most serious, that is when chlorine de-stroying compounds are present. These chlorine destroying compounds may consist of any materials which can be chloro-dised or oxidised; for instance, free lime and magnesia will absorb very large amounts of chlorine. After the success of the Mt. Morgan works, another mine in the district put up roasting and chlorination works, but unfortunately a large part of the gangue consisted of calcite; this, when heated in a current of air, became lime ; the lime when moistened and subjected to the action of chlorine gas absorbed it and became chloride of lime, every 21bs. of lime taking nearly 1lb. of chlorine. The result was that no gold was extracted, and the works were a failure. A little chemical knowledge would have prevented such a blunder. Magnesia behaves somewhat

October, igi I. THE VARSITY ENGINEER. I?

like lime, so that when ínâgnesian minerals are present, the chlorine is usually absorbed.

Talc is often mentioned as a source of loss. It is only when comparatively large amounts of such minerals are present that this consumption takes place; for with small amounts the sul-phur dioxide evolved from the pyritic minerals at once com-bines with any strong bases, and converts them into sulphites, which subsequently become oxidised to sulphates. It is often stated that sulphates are detrimental to successful chlorina-tion, and even in Rose's well known work, he gives as a test for roasted ores the barium chloride test for sulphates. Such a test is apt to be misleading, for an ore may be full of sul-phates, such as those of zinc, magnesia, lime and others, with-out affecting successful treatment at all. Further, since in some methods of chlorination sulphuric acid and chloride of lime or other chlorine supplying compounds are added to the ore, it is rather absurd to test it preliminarily for sulphates. The only sulphate which is sometimes formed, and which would be injurious is ferrous sulphate; but this, as a rule, does not remain long as such, but becomes ferric sulphate or a basic sulphate which is non injurious, or is split up into sul-phur tri. and dioxides and ferric oxide.

Pyrites themselves would destroy large quantities of chlorine.

FeS, -{- 15Cl 8H2O = FeC13 -f- 2II,SO4 -}- i2HC1 Or if the action takes place as in the above case, one part of pyrites will destroy about 4/ parts of chlorine. Ferrous oxide, such as is present when ores are imperfectly roasted, ferrous sulphide, and any other sulphides, will if present in more than minute amounts, render the successful chlorination of an ore a hopeless task.

One advantage of the Plattner method is that the metallur-gist is able to tell at once whether his ore has been properly roasted, for the gas will rapidly rise through a suitable ore, but if anything is wrong, it has to be forced through inch by inch, and sometimes the consumption is so great as to make chlorination unprofitable.

In fact, one of the most direct and reliable tests as to whether an ore is properly roasted or not is to take some dihtte permanganate solution and acidify it and then shake up with a sample of the roasted ore. If the colour remains, it is right, but if the permanganate is used up in large quantities then further oxidation is required.

The successful chlorination of ores, provided the ore is suit-able, depends almost entirely on the roast it has received. It would occupy too much of your time to deal with this matter thoroughly, but a few points might be dealt with. If the ore is pure pyritic, then if this mineral is heated in a current of air, the ultimate product left is ferric oxide or Fe,O,. The

18 THE VARSITY ENGINEER. October, 3gii.

equation representing this change is usually written 4FeS2 + 1102 = 2Fe2 03 + 4S02.

Although ferric oxide is certainly a final product in a good roast, you must remember that the above equation only repre-sents molecular action, and that in each particle of the pyrites you are roasting that there are myriads of such molecules. The finer the material is ground, the closer is the approximation to our equation ; but on the other hand by grinding very finely we are apt to increase loss by dusting.

I have collected samples from many roasting furnaces, and find that the progress of a pyrite roast may be gauged by the amount of magnetics formed at any time. After a very short time nearly the whole of the pyrites turns into magnetic oxide, with a smaller amount of magnetic sulphide, the magnetics become less and less as roasting proceeds, and finally the ma-terial is transformed into non magnetic oxide of iron.

There is not time to deal fully with the changes which go on when roasting other ores, but it may be stated that pyrr-hotite or magnetic pyrites is much more difficult to roast than -pyrite, for the reason that it does not open up on gentle heating like pyrite, but is slowly converted from the solid outside to the solid core by the action of heated air to ferric oxide.

Galena is slowly converted to sulphate and oxide, but a pro-portion of sulphide of lead usually remains as a core, and this is apt on heating to react with the sulphide and oxide of lead, according to the well known equations.

PbSO4 -{- PbS = 2Pb + 2S02 and 2PbO PbS = 3Pb + SO2

If gold happens to be in contact, it is immediately dissolved by the lead, and as roasting proceeds, the oxidation and scori-fication of the tiny bead will either leave a rounded prill of gold, which only slowly dissolves in the chlorine, or an alloy of gold and lead which may not dissolve at all.

Lead compounds are always injurious, in that silicates and fusible compounds so readily form, which by their impene-trable glaze may effectively seal up gold.

As I pointed out before, the modern concentrating tables which bring the heaviest minerals together invariably have a rich gold galena streak, and the gold in contact with the galena is more likely to be locked up than if mixed through pyrite or inert material.

Blende, or zinc sulphide, commences to oxidise at a higher temperature than either pyrite or galena, and roasts slowly and perfectly to oxide or. sulphate. Since both the sulphide and oxide are infusible, it serves to stiffen the charge, and also to oxidise and sulphatise it, when zinc sulphate is heated to a 'high enough temperature to be decomposed. I only mention this for many erroneous statements have been made concerning

-October,19ii. THE VARSITY ENGINEER. 19

this common material. Copper pyrites are somewhat like pyrrhotite, they roast slowly, and are converted into sulphate and oxide. Since the amount is usually small, one or two per cent. of copper in a parcel of concentrates does not give trouble, and the bulk of the copper may be recovered from the chlorine solutions after the gold has been extracted.

Antimony sulphide is troublesome, especially when not as-sociated with pyrites ; it fuses at such a low temperature that it behaves like other fusible sulphides in collecting and sealing up gold ; if roasted at a very low temperature, lower than that required for any other mineral, it will quietly be transformed into a powdery oxide, without showing any tendency to melt; the antimonious oxide so formed will at a higher temperature partly volatise, but part, when sulphur dioxide is present, is converted into tetroxide, which remains, and part unites with metallic oxides forming first antimonites, then antimoniates. It is probable that the bad extractions obtained from anti-monial ores is mainly due to the fusibility and sealing up of gold compounds. If the antimony sulphide is rapidly oxidised a large proportion of oxide is volatized, and this carries gold with it, probably mechanically or sympathetically, as it has been fancifully termed. Practically the whole of the antimony can be volatised by roasting the sulphide with finely powdered coal, the oxide thus formed is the volatile antimonious oxide ; this, however, takes gold with it.

Arsenical pyrites are roasted with ease provided they are admixed with pyrites. Sulphur has a greater affinity for nearly all metals than arsenic has, consequently the tendency on heating sulphides and arsenides together sulphides of the metals tend to form, and arsenic is expelled ; if there is excess of sulphur the arsenic combines with this and forms com-pounds corresponding to orpiment and realgar, which readily volatize.

If a cold iron bar is placed over mixed sulphide and ar-senide ores in a roaster, a beautiful yellow or red sublimate of these compounds will condense on it. As in the case of antimony, some arsenic on becoming oxidised unites with basic oxides and forms arsenite, and later on, arseniates. Such compounds differ from sulphates in that they are not decom-posed at a high temperature, but like the antimoniates if fully oxidised, they do not interfere with the chlorination of ores and are certainly not the bugbears many of your text books would lead you to believe. As a rule some of these compounds dissolve when chlorine is applied to the ore and some arsenic compounds, probably basic arseniate of iron, is precipitated with the gold when sulphate of iron is added to the gold bear-ing solutions; but apart from this arsenical ores may be roasted and chlorinated as successfully as purely pyritic ores.

The type of furnace which has such a grade as to allow of a natural flow of ore when disturbed by rabble from one end

20 THE VARSITY ENGINEER. October, 19m.

to the other seems to be the best so far as output and quality of the roast is concerned. This method of roasting ensures the flow of the upper layer of ore from the feed end towards the discharge end, always leaving the unaltered material ex-posed. There is little or no dusting, and if the rate of flow is- properly arranged, the quality and quantity conditions are satisfactory.

THE PRECIPITATION OF GOLD FROM CHLORINE

SOLUTIONS.

The ore containing the gold should be washed free from the metal ; so long as the water used for washing contains no re-ducing agent, there is no danger of gold becoming precipi-tated. It is better, however, to have some free chlorine present in the wash water to obviate any danger of precipitation of gold on patches of partly oxidised ore near the bottom of the vat. Washing should be conducted after the same manner as washing a"precipitate in a- filter paper, that is allowing the solution to drain away as far as possible, then stirring up the surface of the ore and applying another wash.This should be repeated until the effluent liquor will show no reaction for gold. The best test to apply is to make a solution of sulphate of iron by dissolving cast iron in dilute sulphuric acid, and de-canting off the clear solution (cast iron solution will give a more sensitive reaction than pure sulphate of iron). The gold solution is best tested by taking some in a white basin or cup and pouring in the sulphate solution; if much gold is present,. a brown or black ppt. will separate, but if only a small quan-tity, a bluish colouration is all that will be seen; the latter would be invisible in a glass vessel.

If the ore is exceedingly fine, then it would not be possible to wash it after the manner suggested; the first liquor would go through; but if the surface were allowed to become ex-posed and another wash put on, the ore would pack and be-come impenetrable. In this case the surface must never be-allowed to uncover, and the first liquor should be applied be-low, this causes the washing upwards of the very fine ma-terial. As soon as the ore surface is covered with liquor, the solution can be run off, and at the saine time a stream of wash liquor must be run on to the surface so as to keep it always covered. If this is done, there is no difficulty in washing out all the gold from an ore, which would otherwise be impervious to solution.

Prior to the precipitation of the gold, it is best to free the solutions from dissolved chlorine by exposing them to light. and air and blowing air or steam into them. This is usually carried -out in a small stoneware vessel, intermediate between the vat and the precipitating vessels.

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Precipitation is best effected by means of charcoal. The charcoal should be free from dirt and grit and charcoal dust. In Victoria it is used in lumps of about Iin. diameter, at Mt. Morgan in smaller fragments. The charcoal is packed in stoneware jars, or similar vessels, and the gold bearing solutions allowed to run through in a steady stream. Gold is almost instantly precipitated, and forms an adherent coat or film over the surface of the charcoal lumps. Ior a long time the action of the charcoal was uncertain. It was supposed by some that the precipitation of the gold was due to hydrocarbons, but Mr. Avery, after extensive experiments, proved that the carbon of the charcoal was the precipitating agency. In other words, carbon water and gold chloride re-acted, giving gold hydrochloric acid and carbon dioxide.

4AuClq + 3C + 6H20 = 4Au -}- i2HC1 -}- 3CO2

Theoretically a small amount of charcoal will precipitate a large amount of gold, but in practice the whole of the charcoal is never consumed in this way, for after a time the gold forms protecting films on the charcoal core inside, and if the solutions are run on, after a time the charcoal will be stripped, or the deposited gold will commence to dissolve off. However, the amount precipitated is large, and from a single jar about eft. high and a foot in diameter, some hundreds of ounces of gold can he deposited. Safety jars filled with charcoal are always placed in the series, and a final vat also packed with the same material receives all solutions before they run to waste. The last is only cleaned out at long intervals, and it usually con-tains only very small quantities of gold but most of the silver, since the small amount of silver chloride soluble in the saline solutions accompanies, the gold, but is not so readily precipi-tated. The excess of charcoal is burnt off in cleaning up and the gold obtained is smelted, with borax as a flux. It is almost pure.

The other method of precipitation followed is by means of sulphate of iron. The gold solution, after having been drained from the vat, should in most cases be either settled or clarified by some method of filtration. The clear solution should be then run into another vat, and a solution of sulphate of iron acidified with sulphuric acid, added. This will serve to pre-cipitate the gold, as a brown powder. If the sulphate of iron is put in first, and the gold solution run into it, the gold will be precipitated in such a fine state of division that it settles with great difficulty. The solutions and precipitates should be well stirred.

Sulphate of iron is not satisfactory as a precipitant, for the gold is tóo finely divided ; sulphuretted hydrogen is much better, but the objection to it is that it throws down other metals in an acid solution as well as gold. However, if frac-tionally used, the whole of the gold can be precipitated with a

22 THE VARSITY ENGINEER. October, 1911..

small amount of copper, and has the advantage of being a heavy voluminous precipitate, which separates and settles well.

The third precipitant which was used in Victoria was a cur-rent of sulphur dioxide. This precipitates gold in a very fine state of division, but at Bethange, where it was used, the whole of the solution after excess of sulphur dioxide was allowed to filter through sawdust, which caught practically the whole of the gold.

The sulphur dioxide in this case also served to reduce the. cupric chloride present in the solutions to cuprous chloride, and as such it passed into tanks filled with scrap iron, where the copper was precipitated, and subsequently recovered.

Having now dealt with the subject of chlorination, I must confess that it has almost been displaced by cyanide methods.

i. In the first case, there are many sulphide ores which can be treated direct by cyanide solutions, and as good an extraction obtained as after roasting and chlorinating the ore.

2. An ore even half roasted, may if neutralised and after-wards exposed to the air or ærated, be treated successfully by cyanide, whereas it would not be possible to chlorinate such half roasted material.

3. The amount of chlorine used is generally from 4 to i21bs. per ton of ore; sometimes much more, and the whole of this is either used up or wasted, whereas the total amount of cy-anide destroyed is usually not more than ilb. per ton of ore.

4. Any silver present in the ore is usually lost, when chlorin-ation is practised, but is recovered with the gold in cyaniding.

The cases in which it will still reign are i. Those in which copper is present in the ore. The gold and copper can be both obtained in solution and both recovered.

2. When small parcels of rich sulphides are to be treated, gaseous chlorine or chlorine under pressure, will act more speedily than a solution of cyanide of potassium, and the gold may be extracted in 48 hours as against a fortnight when treated by cyanide ; but this again is discounted if the ore after roasting is finely ground, any coarse gold extracted by amalga-mation, and the finer gold by agitation with potassium cyanide followed by filter pressing.

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Library Digitised Collections

Author/s:Clark, Donald (1864�1932)

Title:The Chlorination of Gold Ores.

Date:1911

Persistent Link:http://hdl.handle.net/11343/91389