Table of Contents
The work of the past two years at many mills in the United States, Mexico, and South America, has done much to prove the suitability of flotation processes to the recovery of the sulphide ores of copper, and to indicate the best reagents. In a general way, it may be said that oils of mineral origin, such as coal-tar and fuel-oil, give better results on copper ores, while oils of vegetal origin, such as the terpenes, pinenes, wood-tars, etc., are better adapted for the treatment of zinc and lead ores.
Coal-Tar Products.—Among these cresylie and carbolic acids are the best known reagents. Commercial cresylic acid is an oily refractive liquid, generally with a red or yellow tinge, has a specific gravity of about 1.044, and consists of approximately 40% metacresylic, 35% orthocresylic, and 25% paracresylic acids, the properties of these three isomers being, according to Lunge and Keane:
This acid is much less soluble in water than its homologue, carbolic acid, and is still more easily broken down by sulphuric, which probably accounts for the statement that sulphuric acid may not be used along with either of these weaker acids. My own experiments give conflicting results, and I cannot speak with confidence on this point.
I have found a marked difference in the behavior of different brands of cresylic acid, and I suggest that, in conjunction with tests run, the different brands be analyzed to see what bearing the differing amount of the three constituents has on the action of the various acids. I had very poor success in treating a certain chaleocite ore with a dark colored cresylic acid, and on changing over to a light colored brand, I had immediately surprisingly good results.
Lunge and Keane give a method (Rascheg’s) for the estimation of the three isomers of cresylic acid. For the benefit of those who may not have access to this book, I give it in full:
On treating m-cresol with excess of HNO3, at 100° it is quantitatively converted into trinitrocresol, while its isomers are completely oxidized to oxalic acid. The following directions, which must be most carefully observed, give reliable results: Exactly 10 grammes of the cresol mixture are weighed into a small conical flask mixed with 15 c.c. ordinary H2S04 (1.84), then treated for 1 hour in a steam-oven, and the contents poured into a wide-necked flask of 1 litre capacity. The flask is cooled under the tap, shaking it round meanwhile in such a manner that the sulphonic acid, which is a mobile liquid while hot, settles as a thick syrup on the sides of the flask during cooling. 90 c.c. of HNO3 (1.38) are then first poured into the small flask in which the sulphonation was conducted, in order to remove any sulphonic acid adhering to its sides, rinsed well round, and then poured, all at once, into the larger flask. The contents of the latter are well shaken immediately, so that all the sulphonic acid is dissolved, which takes about 20 seconds. The flask is then placed in a draught-cupboard. After one minute a violent reaction occurs, red fumes are evolved, and the liquid boils; then it suddenly becomes turbid; oily drops of trinitrocresol form and collect on the bottom of the flask; and after five minutes the reaction is apparently ended. The whole is allowed to stand for at least another five minutes, then poured into a dish containing 40 c.c. water and the flask rinsed out with a further 40 c.c. water into the same dish. On mixing with water the trinitro-m-cresol solidifies with liberation of nitrous fumes to a crystalline magma. It is allowed to stand for at least two hours while the liquid cools; then it is crushed with a pestle, and filtered on the pump through a filter that has been tared against another one. The crystals of trinitrocresol are washed with 10 c.c. H2O, dried at 95 to 100° and weighed. If these instructions are carefully followed, 1.74 gm. of trinitro-m-cresol are obtained for each gramme of metacresol present in the mixture whatever the composition of the latter. The presence of even 10% phenol does not diminish the accuracy, as the picric acid that is formed remains in solution, but the method must not be applied to mixtures containing large amounts of phenol. This, however, does not often occur in practice. In such samples the presence of phenol is detected by the B.P. and also by the fact that the nitro compound does not remain solid in the steam-oven at 95 to 100°, but melts, or, at any rate, forms a soft paste. But a cresol that distils for the most part between 190 and 200° and, therefore, contains scarcely any phenol, always yields a pale yellow crystalline mass, the weight of which divided by 1.74 gives the weight to within 1% of the m-cresol in the mixture. It may be well to repeat that not less than 90 c.c. of HNO3 is used, and poured all at once into the flask as quickly as possible, a flask having a very wide neck being used.
To determine all three isomers Rascheg separates the o-cresol completely by repeated fractional distillation; the distillates being composed roughly of 60% m- and 40% p-cresol in which the m-cresol is determined as above. This operation, however, is entirely beyond the skill and resource of the average chemist. It is rendered unnecessary from the fact that the three acids mentioned bear a fairly constant ratio, as before stated, in any commercial cresylic acid; from the percentage of meta-cresylic acid formed, the others may be calculated. These three acids can be obtained in a pure state. I suggest a trial of them on a small scale, and I venture the opinion here that the ortho-cresylic acid is the one that does the work.
Cresylic acid should be handled carefully, as it gives rise to painful skin-wounds, and may easily splash into the operator’s eyes. It is well to keep a bottle of olive-oil handy as a remedy.
COAL-TAR CRESOLS
Cresylic acid is an expensive flotation reagent, costing at least $1.25 per gallon delivered at “Western American mills in peace-time. It comes principally from Germany and England. A search for a cheaper substitute has shown that crude coal-tar creosote, which is a by-product of gas-works, blast-furnaces, and gas-producers, is promising. Samples from different sources vary greatly in liquidity and chemical composition (proportion of phenols and cresols present) ; they well merit investigation. Being generally viscous they emulsify imperfectly, especially in the cold, and while some solvent like pine- oil or cresylic acid can be employed, such solvents are expensive and tend to mask the effect of the original reagent. It is probable that the employment of the more liquid blast-furnace creosotes, with preliminary heating, would be attended with good results.
Carbolic acid (phenol) is a homologue of cresylic acid. It is difficult to distinguish between them, the smell and color being so much alike. Carbolic acid has a solubility varying from 4.83% at 11° to 11.83% at 77° in 100 parts of water. It is easily broken down by sulphuric acid, yielding oxalic acid. It appears to be much less selective than cresylic acid in its action on metallic sulphides, and a slight excess brings over a concentrate high in insoluble matter.
FUEL-OILS
My investigations cover Mexican, Texan, and Californian crude oils. From the known difference in composition, it is not surprising that on any particular ore the results are widely different. The metallurgist should have samples of all three on hand when running tests. Fuel-oils are not highly selective like cresylic acid, pine oils, etc., but are strongly emulsive; they serve the purpose of giving body and mineral-carrying power to the relatively weak but more selective froths; they are cheap, quickly obtainable, and, when used in moderation, bring over little gangue. It is well to increase their fluidity by steam-jacketting the container from which they are fed to mixing-compartments.
Gas Oil (stove-oil)
This is one of the distillation products of crude oil. It is a strong emulsifying agent, which is, at times, most useful. It is worth a trial in running tests. It must be used in very small quantities.
Crude Wood Turpentine
This is not the ordinary spirits of turpentine. It is a dark reddish-brown liquid with a pungent smell. On gravity-flow machines I have found it of little use, as its action in slight excess is to bring over gangue freely. As an emulsifier, I much prefer fuel-oil or tar-oil. On machines through which the flow of pulp is maintained by mechanical means, it has been found a valuable reagent for the purpose of controlling the levels of pulp, through its physical action on froths, but this result is achieved at the expense of impure concentrate, unless the agent is used in the strictest moderation.
PINE-OILS, WOOD-TAR OILS, FIR-OILS, WOOD-CREOSOTES
The destructive distillation of soft woods yields a large number of products, and possible reagents. I have found the oils derived from pine and fir to be more selective on chalcopyrite than on chalcocite ores. Wood-tars and tar-oils are excellent emulsifiers, but it appears that the series in general gives better results on zinc and lead than on copper sulphides. I have found pine-oil used in conjunction with crude sulphuric acid to give excellent recoveries on weathered chalcopyrite ores where cresylic acid had been a complete failure. There may be some significance in the fact that the action of sulphuric acid on terpenes (pine-oils) and phellandrenes (crude eucalyptus- oils) is to give in both cases dipentenes and terpinenes. I mention this, as eucalyptus-oils, which are prohibitive in cost in America, are, I understand, universally used in conjunction with sulphuric acid on zinc and lead ores in Australia.
APPLICABILITY OF THE PROCESS
T. J. Hoover in the latest edition of his book on flotation repeats the statement made in the first edition, that there is a doubt if chalcocite can be recovered successfully by flotation. This surely is an oversight; no one is more cognizant than Mr. Hoover of the recent work done in flotation, and I have only here to refer to the very high recoveries that have been made on a working scale on chaleoeite ores, in Arizona, at several large mills.
Cuprite is considered a difficult mineral to recover by flotation. In the case of one ore that has come under my notice, in which the cuprite occurred as a subsidiary mineral, the saving amounted to a small percentage only, certainly not as much as an efficient slime-table would have recovered. On the silicate and carbonate ores there is probably no appreciable recovery.
MECHANICAL SIDE OF THE PROCESS
In my opinion, the development of what may be termed pneumatic-flotation processes by Callow, Flynn, Towne, and others, constitutes the most distinct advance of recent years. They consist of a directly and cheaply applied supply of air-bubbles in a finely divided state, to assist in bringing to the surface of the pulp the already prepared sulphides. The agitation is cheaply and easily performed, and is quite subsidiary to the action of aeration discussed above. I have found a shallow trough-agitator, with beaters only partly submerged, quite sufficient. The introduction of a jet of live steam into the mixing compartment is an advantage. The pulp may flow through the mixing and aeration compartments by gravity at the expense of a trifling loss of head-room. This loss is more than compensated by decreased power and labor costs, and simplicity of working. Finely divided air-bubbles can be directly and perfectly applied through many forms of porous media such as canvas, corundum stones, silica-tiles, sandstone slabs, etc. The mineral particles are seized upon and at once removed as concentrate, without having to be repeatedly subjected to a sort of ‘fractional distillation’ as in the older systems. It is, in a way, the converse of Elmore’s vacuum process, and has the merit of being positive in action and under perfect control. Remarkable results have been shown in the economy of reagents, power, and labor; also in the ease with which such machines can be started after any of these sudden stops incidental to milling operations.
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