Mill Tailings Storage for Reprocessing in Future

Mill Tailings Storage for Reprocessing in Future

Table of Contents

It is not my purpose to write a lengthy article or to attempt the solution of the problem I am presenting, but to call attention to what I believe an important issue, hoping that it may arouse in some one, or in some company, a sense of responsibility. I am referring particularly to modern up-to-date milling plants discharging tailings which under existing conditions cannot be further treated or from which a recovery cannot be made at a profit, but where a quantitative and qualitative analysis shows that a portion of these tailings might profitably be treated at some future time when our metallurgical knowledge has advanced, when we have cheaper power, chemicals, labor or other advantageous conditions.

Early Methods

In our early milling methods very little attention was given to the care of tailings. It was thought that if the mill could be located by a stream or a river where the final disposal of the tailings could be forever lost sight of, it was highly desirable. We can now see and estimate what was lost in these rich tailings, not recoverable from the streams and the great rivers. Fortunately for many, the depositing of tailings in these streams and rivers was objected to by the farmers and finally prohibited by the State Legislatures.

In looking back over our past milling methods, it seems that the first endeavor was to make the maximum amount of money for a particular period, or year, regardless of the future, the depletion of the mine, or the conservation of the total mineral content. The dumping and sluicing of 3000 tons per day, for a period of many years, of a copper tailing carrying 12 lb. of recoverable copper together with its portion of precious metals, the thousands of tons of valuable metal contained in the tailings that have been carried away by Silver Bow Creek, the streams of the Coeur d’Alene, the Flat River and Colorado districts, the enormous quantities of the fine slime tailings that were dumped into Torch and Linden Lakes, illustrate such extravagances.

Companies that have been compelled to impound their tailings for the sake of recovering their water are the ones where the best metallurgical work has been done. Where water has been cheap and abundant, too much has been used, which leads to dilution and poor concentrating work. Those milling companies that seemingly were not so favorably located, or the companies that were forced to discontinue the pollution of the streams, have found themselves in the possession of tailings that are of great value, due to the new methods of concentration and of recovery of these, flotation has taken the foremost position. I am advised that the Canadian Government has taken the stand that the mineral wealth of the country belongs to the country, and the development of it is a privilege granted by the Crown; that the mineral resources of the Dominion cannot be squandered by loose and extravagant methods of extraction. We now have in this country definite laws preventing the scattering of waste products from mills, with the result that today millions of dollars are being recovered from these tailing dumps at a comparatively small expense.

The problem of future treatment of our present mill tailings is a much more difficult one for the future metallurgist and mill man on account of the recent great advancement in our milling methods; yet relatively the valuable content that still remains may have proportionately greater value as our mines become exhausted and our great mineral deposits have become definitely known and their value realized.

I do not share the view of those who consider that flotation is in its infancy, any more than the electrical industry is in its infancy. If so, they are infants of great proportion. In both industries we are fast approaching the maximum efficiencies. Yet the metallurgist will often find residual values from these modern concentrators and if they are not isolated by enrichment and segregation they will be forever lost on account of admixture with valueless material.

Illustration of Segregation

I remember that while I was employed by the Anaconda Copper Mining Co., in 1901, this company made arrangements to segregate the fine slimes from the sands, supplementing the arrangement which their new plant afforded in saving all of the tailings instead of allowing a portion of them to go down Warm Springs Creek. It was realized, in starting this new concentrator, that these slimes were of great value on account of the copper content and, consequently, ponds were made, 300 ft. in width, 600 ft. in length and approximately 14 ft. in depth, to settle these values and decant the water. These tailings ponds were operated intermittently—as soon as the ponds became loaded they were drained and then excavated by a drag line and bucket. The discharge from the bucket was piled along the side of the pond. Part of this slime was made into blast-furnace bricks and smelted, the remaining portion being stored for future treatment.

A more detailed statement of the character of this particular material is found in an excellent paper by Ralph Hayden on The Concentration of Slimes at Anaconda. The effect of saving these slimes for the last 16 years at the Washoe Reduction Plant has resulted already in the recovery of millions of pounds of copper with many millions more yet unrecovered. The present flotation slime plant at Anaconda which is working on these particular slimes recovers more than 1,000,000 lb. of copper per month, which is produced at a cost probably not exceeding 5c. per pound. The Chino Copper Co. segregated the tailings from its mill at Hurley and now has a retreatment plant handling 1000 tons per day with further extension under construction. There are other companies that have segregated to some extent.

There are millions of tons of sulphide tailings that are now becoming slowly oxidized, and cannot be treated without installing expensive grinding machinery plus the flotation equipment; if the slime which carries the high values had been segregated, millions of pounds of copper could easily be recovered by the simple flotation methods of today, as illustrated by the Anaconda and Chino practice. While the sands, which are of lower value, would oxidize faster without the slimes and become available for leaching where coarse material is desirable.

The illustrations that I have given as to our past losses are in no sense given for the purpose of criticising our managers and millmen, for their achievements are all a matter of record that will go down in our metallurgical history; losses and wastes go hand in hand with new ventures and a new country. The thought I desire to set forth is our future responsibility in these matters.

Kind of Tailings that may be Segregated

There are often concentrators in operation where some portion of their middlings cannot be brought to grade or a marketable product made. These should be stored separately and not mixed with the general tailings. We have not been able to recover the oxides and carbonates that are mixed with the sulphide tailings. J. M. Callow’s filming experiments, which he has so carefully described, point out what may be expected. There are millions of tons of tailings, however, of which the oxide and carbonate contents are less than those of the tailings that are thrown away by the present filming process, and it would seem that enrichment for future treatment should be made if possible. I think it is generally agreed that Mr. Callow is most familiar with the subject of the recovery of complex ores, and he has, at my request, made the following statement of his views as to what we may expect as to the probable recovery of such values.

“In the treatment of complex ores, such as those carrying lead, zinc and iron, and in copper ores containing both sulphides and oxides of this mineral, our present methods of recovery are still in embryo, and such tailings must inevitably carry an appreciable portion of the original contents of the ore, in the complex zinc, lead and iron ores especially. Sixty and 70 per cent, zinc recovery is often considered passable work, the rest of the zinc being either entangled or in some other way combined with the iron and lead which future inventions will no doubt show us how to recover. In the copper ores, flotation has added greatly to our past recoveries, but only in the recovery of sulphides; the oxides are still an unavailable asset, which in due time will undoubtedly be recovered by improved methods of extraction.

“In laying out a system for impounding tailings, the plan hitherto has been to impound them without any discrimination as to their values. In some ores it has been found that the principal values lost lie with the sands, and in others, with the slimes; so that a segregation might well be practiced.”

As an illustration, let us assume the accumulative analyses of the tailings from one of the great concentrating mills where the process consists of tabling and flotation. The values given are in the form of copper contained in sulphides and carbonates. We will say that an extraction of 94 per cent, is made on the sulphides and 20 per cent, on the carbonates, making a total extraction of 82 per cent. The total copper remaining in the various screen sizes is given in Table 1.


On such tailings, it is possible with a Dorr classifier to separate the sands from the slime; the slime tailings may be divided again into two parts by deflocculating and decanting, making a fine crystalline mineral and a decanted slime. It is my belief that the decanted material will carry an amount of oxide copper two or three times that of the general mill tailings. While it may not be possible with the present means to make a satisfactory and marketable concentrate, it would appear desirable to store separately rather than to mix with the impoverished tailings which might carry only 0.15 per cent, in copper, and hold such segregated material until metallurgy has advanced or the market is in condition to profitably treat it. I am not offering this as a solution to the problem of segregation of the tailings, but to point out what some preliminary experiments seem to show.

To retreat 15 or 20 millions of tons of unsegregated tailings in which a large portion carries only 0.12 to 0.15 per cent., is no easy problem for the future metallurgist.

There are many milling plants that treat complex ores which carry lead, zinc and copper, and it is not possible to make a satisfactory extraction from all of these values. In such plants, the extent to which enrichment and segregation of tailings can be carried on is limited only to the extent in which the expenditure seems desirable in the way of a future investment. Whether there ever will be a better opportunity for segregation, or greater ease of separation of these values, and whether it can be done at a lower cost than when the tailings were produced, remains to be seen. Fresh and loosely combined material is always the most desirable to treat in all milling processes.

Storing and Importance of Milling Site

In general, tailings that contain probable recoverable values should not be allowed to accumulate over large areas or in horizontal layers. Tailings from our first work in concentration generally contain the greatest values. Such tailings should be stored, if possible, so that those that are richest and most easily recoverable can be worked first, thus avoiding handling a large class of inert material when retreatment is commenced. It is certainly not desirable to pile the present low grade of tailings coming from mills where 90 to 95 per cent, recovery is made upon tailings from which only a 65 per cent, recovery was made.

I believe the segregation of tailings can be advantageously accomplished in many instances so that the weathering action will oxidize them for the ordinary leaching. The piling of tailings so that the maximum oxidation can take place, the elimination of all colloidal and talcy material, would seem desirable, and a greater total recovery could be made from leaching and precipitation with iron.

The excellent results obtained at Anaconda in making building brick from segregated tailings is a subject worthy of consideration. It may surprise many to know the extent to which brick making may be successfully carried on to meet immediate market conditions, and the extent to which these brick may be shipped profitably to other markets.

The Cost of Segregation

Segregation in general can be carried on, I believe, at a low cost, particularly at the time of the initial milling. The operation of tables, Callow tanks, Dorr thickeners, Cole drag-belt separators, excavating machinery and conveying equipment can be done for a very small amount per ton. There are many illustrations of the actual cost of stacking where mills are located on a flat site, as in northern Michigan and the zinc and lead districts of Missouri.

As to the charge for this work of segregation, it seems it is not a proper milling cost. For the production of this material credit might be given to extinguishment of ore reserves or an entirely separate account be kept. In my opinion, a most effective suggestion has been made by C. W. Van Law, who says: “To my mind this is not at all a charge to be carried against current operations, but is properly chargeable to capital as a deferred asset, just as in the case of the ore that is broken and carried in excess of requirements in shrinkage stope operations on a large scale. The cost of carrying the tailings away from the mill to dispose of them, so far as is necessary merely to enable continuous mill operation, is a proper current operating charge, but anything done in excess of this, where there is reasonable expectation of later obtaining more from the tailings than is at present possible, should be, in my judgment, carried on the books of capital account, their aggregate representing the value of the stored accumulation as an asset at exact cost of producing. Later, when these deposits are reworked, this asset should be scaled down, ton for ton as removed, corresponding reductions in the book asset being made at the same rate as they are created, to the entire extinction of the capital charge, such amortized asset being replaced by the aggregate profits realized from their reworking. I believe this to be an entirely logical and legitimate method of handling this matter, and it is what I should apply in my own case.”

This is one of the problems of conservation of our resources and is analogous to many of the problems that have been so recently brought to our attention. The Government no longer allows the careless lumberman to set fire to the brush after he has removed the marketable timber, thus destroying the young trees and leaving the soil exposed and subject to erosion. The importance of a qualitative and quantitative study of these remaining values in tailings and the removal or relative enrichment is a subject for consideration and careful investigation.

If from our up-to-date milling plants millions of tons of copper-bearing mineral are allowed to mix with worthless material, which when so mixed can never profitably be reworked, and if it is possible to segregate a considerable portion of this copper-bearing mineral so that it can profitably be retreated, then the responsibility of trying to save this large quantity of copper must rest with the administration and the board of directors of such companies. It is not sufficient to turn this problem over to the operating mechanical superintendent who may lack metallurgical training, or operating metallurgists who likewise may be handicapped. Therefore, can we not suggest and point out the desirability of segregating and storing such tailings in a fashion that will best facilitate their future working and the saving of valuable metal?