Notes on lead and copper smelting and copper converting

Preface

This book is precisely what is indicated by its title — a series of notes on the practical work in lead and copper smelting, includino; the converting of copper matte. It is by no means a treatise or an attempt at a treatise. No effort has been made to trace in order all the steps in beneficiating ores by smelting from the crude material to the marketable product. This has already been done ably by other writers. It has seemed, however, that the experience gained in the everyday operation of three large works, extending over a period of ten years, might be useful to others who are engaged in similar work. Progress in any art is helped by an interchange of ideas. Hence these notes are offered.

Acknowledgement is due Mr. Wm. Braden for the reproduction of drawings of the settlers used at the Arkansas Valley Smelting Works, Leadville, Colo., from his paper in the Transactions of the American Institute of Mining Engineers^ Vol. XXVI ; and to L. S. Austin for the illustrations of the matte-pots and slag-trucks employed at the Omaha & Grant Smelting Works, Denver, Colo., which are taken from his paper on the separation and disposal of slag in The Engineering and Mining Journal of November 23, 1895 ; also to Julius A. Dyblie and John Bendixen for valuable services in preparation of plans and drawings.

Chapter I.— Copper Matte Smelting

Percentage of copper required — Alteration of lead furnaces for copper matte smelting at Leadville, Colo.— Effect of fine ore in the charge — Kind of material smelted at Leadville — Composition of slag — Proportion of matte-fall necessary in copper matte smelting — Danger of a hot-top^ and its remedy — Amount of wind required — Comparative merit of Roots and Baker blowers — Relation between typo of furnace and volume of blast needed — Experience with forehearths at Leadville — Use of speiss in the furnace charge — Breaking lump speiss — Speiss roasting — Danger of poisoning workmen in speiss roasting — Size of water-jackets at the Arkansas Valley works 1

Chapter II.— Extraction of Gold and Silver from Matte. The Hunt & Douglas process— Method employed at Belleville, 111. — Separation of gold from copper bottoms at Argo, Colo. — An experience at Leadville 15

Chapter III. — The Calculation of Furnace Charges. Charge used at the Arkansas Vail oy works, Leadville — Example of a slag calculation — Relation between matte and slag assays — Charge used at Aguas Calientes, Mexico — Weighing up furnace charges — Best type of slags ... 18

Chapter IV. — Types of Furnaces.

Adaptability of certain slags to certain furnaces — Best position of matte-tap — Furnaces with outside forehearths — Furnaces with internal crucibles — Method of tapping matte — Type of furnace at San Luis Potosi, Mexico — Smelting converter slag and refuse 24

Chapter V.— Spouts, Settlers, and Jackets.

The Schumacher spout^ — The Hixon spout — Design of tap-jacket — Other kinds of slag-spouts; their defects — Kind of settler used at Anaconda, Mont.— Settlers at Aguas Calientes, Mexico — Difference in matte and slag separa-

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TABLE OF CONTENTS.

tion in large and small settlers — Arrangement of water-jackets — Advantages of a small number of Jackets — Size and number of tuyeres — Proper height of jackets — Protecting a brick shaft with water-pipes — Tuyere-bags 28

Chapter VI.— Blowing-in and Barring-down a Furnace. Blowing-in a lead furnace — Barring-down — Effect of wall accretions on the furnace-running — Running down charges preparatory to barring-down or blowing-out— Barring into the crucible — Blowing-in a copper-matte furnace — Importance of a heavy matte-fall in blowing-in 37

Chapter VII. — Handling Blast-Furnace Slag.

Method employed at the Arkansas Valley works, Leadville — At the Omaha & Grant works, Denver — At the San Luis Potosi works, Mexico — At the Pueblo Smelting & Refining Co.'s works, Pueblo, Colo.— At the East Helena works, Montana — Handling slag from copper-matte furnaces—Amount of water required for granulating slag . . 42

Chapter VIII.— Design of Lead Blast Furnaces.

Tendency to increase height of furnaces — The Aguas Calientes plant — Strange result of the first smelting campaign at Aguas Calientes— Effect of bell and hopper feed on a lead-smelting furnace — Deductions from the Aguas Calientes experience — Comparison between stack and top-feed furnaces — Methods of feeding lead and copper blast furnaces 47

Chapter IX.— Lead Slags and Losses in Lead Smelting. Example of a lead-furnace charge — Types of lead slags — Characteristics of high-lime slags — Losses of lead and silver in smelting — Losses in preliminary roasting-slagging — Percentage of fuel required in smelting — Effect of different kinds of coke 54

Chapter X. — Improvements in Roasting Furnaces. Proposed combination of roasting, smelting, and converting plants — The O'Harra furnace — The Hixon roasting furnace 59

Chapter XI. — Smelting Raw Concentrates with Hot Blast

AT Anaconda.

Construction of stoves — Temperature of blast — Unsatisfactory results of the experiment at Anaconda 65

Chapter XII.— Copper Converting at Anaconda.

Difference in conditions governing copper converting and the Bessemer steel process — Reactions in copper converting — Inapplicability of basic linings to copper converters

VI LEAD AND COPPER SMELTING.

— Result of an experiment on protecting the converter lining with water coils — Experiment with a basic lining — Amount of silica required for a converter charge — Composition of converter slags — Old practice of relining converters at the Anaconda and Parrot works, Butte, Montana — Composition of lining material — Cost of converting in the first Anaconda plant — Design of a converter plant — Cost of converting — Losses in converting at Anaconda, 66

Chapter XIII.— Blowing a Converter Charge.

Condition of charge indicated by flame coloration — Danger in over-blowing — Slag skimming — Practice at Aguas Calientes — Regulation of temperature in the converter — Management of the tuyeres — Indications of a finished blow — Method of developing heat in a cold charge — Difference in amount of heat furnished by high-grade and low-grade matte — Use of silicious ore for converter lining at Aguas Calientes— Kinds of finish on converter copper — Distribution of silver in converter bars— Sampling copper bars for shipment 74

Chapter XIV.— Design of Converter Plants.

The Parrot works, Butte, Montana — Arrangement of the new Anaconda works — Dimensions of copper converters — Objection to very large converters — Mechanical devices for handling converters — Method employed at Aguas Calientes — Moulds for casting converted copi)er — Coj^e and core apparatus for making copper moulds 84

Chapter XV. — Lining a Converter.

Effect of lining material on composition of converter slag — Preparation of lining material at Anaconda — Method of lining at Anaconda — Life of a converter lining — Experiment on patching linings at Anaconda 94

Chapter XVI.— Casting Anodes Direct from Converters. Trial at Anaconda — Effect on electrolyte — Method of casting — Difficulties to overcome — Grade of converter copper — Casting anodes from converters at Great Falls, Montana 97

Chapter XVII.— Cost of Producing Copper at Anaconda. Details of estimate— Losses in various processes of treatment—Details of copper-converting plant at Anaconda, 99

Appendix

Specifications of buildings and machinery for copper-converting p)lant. Anaconda INIining Co., Anaconda, Montana, and accompanying working drawings 102

LIST OF ILLUSTRATIONS.

Fig. 1. Furnace and forehearth No. 1, Arkansas Valley

smelting works, Leadville, Colo 3

Fig. 2. Furnace and forehearth No. 1, Arkansas Valley

smelting works, Leadville, Colo., front view ... 3

Fig. 8. Forehearth, furnaces ISTos. 2 and 3, Arkansas Valley

smelting works, Leadville, Colo 5

Fig. 4. Forehearth, furnaces No. 2 and 3, Arkansas Valley

smelting works, Leadville, Colo., front plate .... 5

Fig. 5. The Hixon slag-spout, cross-section 29

Fig. 6. The Hixon slag-spout, longitudinal section 30

Fig. 7. Plan and section of settler used at Anaconda .... 32 Fig. 8. Plan of furnace and settler at Aguas Calientes ... 33

Fig. 9. Design of water-jackets 34

Fig. 10. Matte-pots used at Arkansas Valley smelting works,

Leadville, Colo 42

Fig. 11. Matte settling pots at Omaha & Grant works, Denver,

Colo. ; general view 42

Fig. 12. Matte settling pots used at Omaha & Grant works,

Denver, Colo. ; longitudinal section 43

Fig. 13. Matte settling pots used at Omaha & Grant works,

Denver, Colo. ; cross-section . 44

Fig. 14. Slag-truck used at Omaha & Grant works, Denver,

Colo 45

Fig. 15. Slag-truck used at Omaha & Grant works, Denver,

Colo 45

Fig. 16. Proposed combination of roasting, smelting, and converting plant, elevation 60

Fig. 17. Proposed combination of roasting, smelting, and converting plants, plan 61

Fig. 18. The Hixon roasting furnace, cross-section 62

Fig. 19. The Hixon roasting furnace, details of carriage and

plows 63

Fig. 20. Converters at Aguas Calientes 85

Fig. 21. Converters at Aguas Calientes, details 87

Fig. 22a. General arrangement of 8-foot copper converter ... 88 Fig. 226. General arrangement of 8-foot copper converter ... 89 Fig. 23. Cope and core apparatus for making copper moulds . 90

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Vlll

LEAD AND COPPER SMELTING.

Drawings of the copper-converting plant of the Anaconda Mining Co. accomi3anying appendix.

Plate I. General arrangement,

Plate IE. General plan.

Plate III. Converters.

Plate IV. Details of converters.

Plate V. Details of trunnion ring, etc., for converters.

Plate VI. Details of tuyeres and tuyere-boxes for converters.

Plate VII. Details of converter-stand, etc.

Plate VIII. Details of foundations for stands.

Plate IX. Details of hydraulic cylinder, rack and gear for converters.

Plate X. Hydraulic four-way valve.

Plate XI. Details of runners from cupola to converters.

Plate XII. Details of converter flues.

Plate XIII. Cross-section of buildings.

Plate XIV. Details of copper cupola.

Plate XV. Details of charging-fioor.

Plate XVI. Water-jacket furnace.

Plate XVII. General plan of blast pipe.

Plate XVIII. Details of matte-carts.

CHAPTER I.

Copper Matte Smelting.

The matting of ores without the presence of copper to serve as a carrier to collect the silver has been attempted on many occasions, but has only resulted in failure, the losses being too large under ordinary conditions to admit of treatment in this way. A small percentage of copper can be used with success, but if the resulting matte runs below 5 per cent, copper it is very doubtful if the process can be made successful. In cases where a large tonnage of ore is to be treated with a small amount of copper, it would be advisable to crush and roast a part of the matte produced and smelt it along with the charge to supply the copper needed. But in cases where the ores are sulphides, without the presence of either copper or lead to act as a carrier, better results can be obtained by leaching in localities where the conditions will not admit of marketing the ore to custom smelters. The reasons are that the tonnage of matte produced is not easily transported, and when sold has to pay treatment charges as well as allow for losses in subsequent treatment. If it is possible by the concentration of the matte to save enough copper to overcome these objections, then it becomes a question of costs to determine which is the most economical, and each particular case is an independent proposition and should be treated accordingly.

Emergencies arise, and it may be necessary to convert a lead-smelting plant into a mixed one of lead and copper, or vice versa.

In 1890 at the works of the Arkansas Valley Smelting Co., in Leadville, copper ores of sufficient quantity to justify separate treatment were received, and accordingly two of the

Z LEAD AND COPPER SMELTING.

old lead furnaces were changed to copper furnaces by the very simple process of filling up the lead crucible and placing an overflow pot under the slag-tap, which pot was later replaced by a forehearth of the type shown in Figs. 1 to 4.

The running of these furnaces was at the time a novel feature in Leadville practice, as prior to that time all the blast-furnace work had been lead smelting and no strictly copper smelting had been done. The copper ores were mainly Irom the Maid of Erin, with a considerable quantity of iron sulphides from the Mahala and Wolftone mines.

The greater portion of the ores that could be used on the charge were sulphides of such a character as to require crushing and roasting, and, as a consequence, the charge was about 70 per cent, calcines, including roasted matte from the lead furnaces known as furnace or lead matte, which, however, contained about 7 per cent, copper, the result of concentration of small amounts of copper from the ores fed to the lead furnaces. This material was consequently fine enough to make an excessive quantity of flue dust and have a serious effect on the running of the furnace.

It might appear to the inexperienced that the size of material to be smelted would have no great effect on the cost of treatment, losses and tonnage of a furnace, but if any such persons should have to contend with the complications arising from an exceedingly fine charge, they would soon come to the belief that there is a very close relation between the size of the charge material within certain limits and the tonnage that can be put through.

There was a twofold object in running the copper furnace ; first, to treat separately the lead ores and such ores as contained no lead and some copper, and, second, to desilverize an accumulation of foul slag of long standing which previous administrations had left on hand. This slag was in part the bottoms of pots or that portion of the slag next to the layer of matte, and of old make, and the other part was the shells resulting from the Devereaux pots in use on the lead furnace, of which six were in blast most of the time.

It is due to state by way of explanation that lead was rather scarce in all the Colorado smelters from 1889 to 1891, and, as a consequence, the lead furnaces were running on a

COPPER MATTE SMELTING.

Scale M incli = l foot.

Fig. 1.— Furnace and Foeehearth No. 1, Arkansas Valley SMEiiTiNG Works, LEADViiiLE, Colo.

Scale 1 inch = 1 foot.

Fig. 2.— Furnace and 1^ orehearth No. 1, Arkansas Valley Smelting "Works, Leadville, Colo.; front view.

short allowance of lead, 8 per cent, to 10" per cent, on the charge, with a long allowance of zinc and magnesia, and the conditions were such that the slag assays were seldom below 1.5 ounce Ag and more frequently 2 and 3 ounces Ag per ton. As a consequence of the high character of bullion — 300 to 400 ounces — and low percentage of lead this was to be expected, and to remedy it as far as possible and to prevent an excessive silver loss the greater portion of the shells was fed into the copper furnaces. If it had not been for this addition of coarse, fusible material to the charge, the furnaces would have been even more difficult to manage than they were. Being only 36 by 84 inches, the furnace was not large enough to put through a tonnage that Avould keep a constant flow open, and, as a consequence, intermittent tapping, the same as on lead furnaces, was the practice. The slag composition attempted was 34 Si02, 33 FeO, 20 CaO, but owing to the feeding of so much slag of unknown and varying composition the analysis of the resulting slag shoAved up somewhat incorrectly owing to the presence of some lead, and did not permit of the formation of as clean slags as can be made when the charge contains no lead. It is a well-known fact that in matte smelting lead is scorified, and, owing to its strong affinity for silver, will carry it into the slag, whereas if only copper and silver are present the slags will be much cleaner.

The furnaces were arranged with a double tap in front, and every time the overflow pot was changed the lower tap was opened and the accumulation of matte in the furnace drawn off. The amount thus obtained would depend very much on the condition of the furnace, and ran from one to six or eight pots of clean matte before the slag would make its appearance. The condition of the furnace was mainly dependent on the amount of matte produced by the particular charge the furnace was running on.

In cases of reconcentration before shipment, when the matte production was very heavy, perhaps 40 per cent, of the charge, the furnace would cut ont all accumulations of crust in the shaft and below the slag-tap and get into excellent condition. On the same composition of slag and percentage of fuel, with the regular charge on, where the matte produc-

COPPER MATTE S:MELT1NG.

Fig. 3.— Forehearth, Furnaces Nos. 2 and 3, Arkansas Valley Smelting Works, Leadville, Colo.

-21^—

\U

43^

For >^" Bolts

For K" Bolts

Mi.

C

Fig. 4.— Forehearth, Furnaces Nos. 2 and 3, Arkansas Valley Smelting Works, Leadville, Colo.

6 LEAD AND COPPER SMeItING.

tion was necessarily small to effect a concentration of ten into one or more, the zone of fusion would travel up and the bottom of the furnace become crusted to such an extent that the crucible would hold not more than one pot of matte between the slag and matte-taps.

As a consequence of small matte production and high concentration, it frequently happened that the matte-tap could not be opened until a charge was put on the furnace that would produce more matte, either by making lower grade or by feeding back some of the matte already produced.

As a general thing less than 15 per cent, matte production in a strictly matting furnace will result in the tuyeres becoming hard and black and the rising of the zone of fusion to the top of the charge, which, of course, results in the fuel being consumed before it should, and the consequent loss of the heat escaping up the stack, as well as the inevitable loss of silver that will follow smelting in a furnace hot on top. Many furnaces have been, and doubtless are now, run on less matte production, but to run steadily at its maximum capacity is what is expected of a well-behaved furnace, and in order to do this it is essential to have the matte production in excess of rather than below 15 per cent. Likewise the production of matte in a lead furnace has a very marked influence on the minimum amount of lead that can be used on the charge. For example, it might be quite possible to run successfully on as low as 7 or 8 per cent, of lead, reinforced by a heavy production of matte, and altogether unxDrofitable to attempt it without the matte. Unless the corrosive effect of something beside slag is acting in the furnace the cold blast will chill the slag, resulting in crusts and the elevating of the zone of fusion. With the zone of fusion high above the tuyeres in a lead furnace the losses by volatilization become higher until the maximum is reached, when the entire amount is lost.

To return to the subject in hand, the handling of small furnaces with intermittent slag-tap and with matte production as low as 10 per cent, or lower, it must be said that it requires a very careful handling on the part of the furnacemen, even allowing that the slags are of the best possible composition. To keep the zone of fusion down at the tuyeres is the essential condition of success, and to attain this is not entirely in the hands of the man who figures the charges. The charge as it comes to the furnace may be either too fine or too coarse to obtain the best results, and while in the first case nothing but the coarsest of coke should be used, in the latter the use of a sledge hammer on the large pieces will materially improve matters.

The proper placement of the charge in the furnace is a matter of the greatest importance, and it is hard to decide where the more experience is required, on the feed floor or in handling the furnace below.

In a case where the furnace temporarily becomes cold, as is likely to happen through the variation in matte production, it may become advisable to increase the fuel or put back a charge of straight matte in order to repair the threatened bad effect, and if the feeder does not know his business, or if the furnaceman does not properly attend to his duties, the result may be slag in the tuyeres, a disagreeable experience with sledge hammers for a time, and what is known as muscular metallurgy.

When a furnace has from any cause, be it fine ore, a bad slag, small matte production, or any other fault, become dark at the tuyeres and hot on top, to know the quickest way in which to remedy the difficulty is what is required of the man in charge. If it is a copper furnace the easiest way is by putting on matte charges until the tuyeres get bright and the crusts or accretions have disappeared sufficiently so as to warrant the matte charges being taken off or reduced in number by putting on one matte charge to one, two. or three, or more, of ore charges. It frequently happens at shift change, especially early in the morning when the night men are tired and are doing their work in a somewhat lazy and careless fashion, that the furnace is allowed to run down and get very hot on top. At such time the operation of the furnace assumes the character of pyritic smelting, the oxidation of sulphur being much greater than when the furnace is fed at its proper level, with the result that the matte production in proportion to the charge falls off* rapidly and consequently increases in grade of copper contents. The excessive loss of heat up the stack results in the furnace running cold, and when suddenly filled up again the consequence is bad running and probable freeze-up.

It is not the purpose of the writer to refer only to ideal conditions where everybody connected with the furnace knows his business and attends to it properly, but to take from the experience of the past that which may be valuable to others in the future. Too much blast will result in driving the fire to the top of the charge, and, providing the furnace will stand it, a reduction of the blast will assist the matte charges in bringing the zone of fusion down. On the other hand, tonnage must be maintained, and in order to make a furnace run on a good charge above 100 tons per day it is necessary to drive the wind into it at a lively rate, and a No. 7 Roots blower will have to make 140 revolutions per minute to furnish blast enough. This amount of blast would be all right for a copper furnace 42 by 120 inches at the tuyeres, but would be too much for a smaller furnace on the same charge, and entirely too much for a lead furnace of the same size.

It is natural to discuss at this point what is the approximate amount of wind to blow in in order to obtain the best results in copper and lead smelting. Taking the listed displacement of a Roots No. 7 blower at 65 cubic feet per revolution, and allowing 140 revolutions per minute for a copper furnace 42 by 120 feet at the tuyere, we get 9,100 cubic feet per minute for a furnace with cross section of 35 square feet, or 26 cubic feet of blast to each square foot of furnace section. Under ordinary conditions 70 revolutions per minute would be about the proper amount for a lead furnace of the same size to run on and would give 13 cubic feet per square foot of furnace section.

These figures are of necessity only for normal conditions of charge and tonnage, and if conditions arise for greater or less tonnage or to reduce the loss by volatilization, the amount of blast supplied has to be regulated to suit those conditions.

Also, the blowers may through wear become leaky to such an extent that they will not deliver the calculated amount of air to the revolution, and, as a consequence, would have to be run faster. I have never personally tested my blowers to find out how much they would deliver per revolution and at varying pressure, except at Anaconda, where a No. 4 Roots was run with a closed outlet at 20 revolutions per minute and gave a pressure of four pounds to the square inch in the blast pipe, which would mean that if run against a pressure of four pounds it would deliver no air at all, or, in other words, the leakage at four pounds was 100 per cent.

With Baker blowers the leakage is much higher, and I do not believe that they could be made to generate a pressure of more than two pounds before the leakage would be 100 per cent. This would indicate what experience is slowly teaching all blast furnacemen — that the efficiency of Baker blowers, or blowers of that type, is much less at any pressure than blowers of the Roots type. Fifteen years ago, when Leadville was a new camp and when the writer received his first baptism of fire in the smelting business, there was not a Roots blower in any of the smelters that were in operation, and this tendency to oppose change has been maintained to a great degree even in the construction of new plants. It is an easily demonstrated fact that if the efficiency is lower the power required to deliver the blast to the furnaces must necessarily be larger than would be the case if blowers of the Roots type were used. As there are now two competing firms building this class of positive blowers, the writer can not be accused of unjust prejudice.

The number of moving parts and the closer contact required for a more positive delivery of the air, are the reasons for the greater efficiency and consequent lower consumption of power. The diflPerence in power consumption will amount to fully 50 per cent, in favor of the Roots type, and regardless of first cost would soon pay for entirely new blower plants for many works now in operation. The listed speed for a No. 7| Baker is only half that of a No. 7 Roots of the same displacement per revolution, so that on a basis of cost alone one Roots type is worth two Bakers, and on a basis of operation the ratio is two to one in favor of the Roots type, so that finally it would appear that the relative values of the two types is about four to one in favor of the Roots.

The type of furnace has also much to do with the amount of blast required for successful running. If, for instance, it is a constant discharge, the flow of slag must be rapid enough to keep the slag-spout open, and this can not be done on much less than 60 tons of furnace charge per day. If the furnace is small and the character of charge such as not to admit of rapid smelting, it may be better to run an intermittent tap with a moderate blast than to try forcing the tonnage with high blast and run the risk of driving the fire up and running the furnace cold at the tuyeres.

These were the conditions at Leadville. The ore was fine, the matte production small, and the tonnage that could be handled not enough to keep a constant discharge open. A forehearth of the Herreshoff type with water- jacketed sides was constructed, but when the furnace was started it was found that the cooling eflPect of the water was too great and the entire contents of the hearth soon became solid. The hearth was changed and the water-jackets allowed to remain empty with better results, but the neck connecting the hearth with the furnace would become closed and slag would be forced out of the tuyeres after about twelve hours' run, and it was necessary to change the hearth at least that often.

The hearth was then constructed of cast plates with a pipe cast around the hole which connected with the furnace, and a similar plate with water circulating around the hole in the furnace front. This construction worked satisfactorily as long as the matte production was small, but if as much as 20 per cent, matte was produced it would cut out and discharge the contents of the furnace on the floor. This type of furnace was not adapted to steady running under varying conditions, since the changing of the hearth necessitated a shutdown which was unavoidable and frequently of an hour's duration.

After running some time under the conditions stated, with a matte production of about 10 per cent, and varying in grade from 15 to 40 per cent. coi3per, and in silver from 150 to 600 ounces, the experiment was tried of mixing in a small percentage of speiss with the ore before roasting. At first the quantity used was one-fifth of the calciner charges, and as no bad effect was observed on the calciners or blast furnace the proportion was increased until one-half of the calciner charges Avas arsenical speiss that had been produced several years before and was regarded as a waste product.

This speiss contained an average of 18 ounces Ag, one-tenth ounce An, 2 to 5 per cent. Pb, 15 to 20 As, about the same amount of sulphur, and the remainder iron. It had been made when the ores of the camp were chiefly of carbonate or oxidized character, and was a great annoyance at the time of its production as well as entailing a heavy loss of silver and gold in smelting. The accumulation of ten years' operations was stacked up in one corner of the slag-dump in the hope that by some undiscovered process it might some day be treated and the silver and gold recovered. There were about 3000 tons of it at this time on the dumps of the Arkansas Valley works, representing a large sum of money in gold and silver as well as the value of the iron excess as a flux for silica. At first this experiment did not promise to turn out well, as only the finest portions of the old pile could be used, and it was thought that Avhen they were consumed the golden eggs would be gone. It will be understood that the crushing of speiss in lumps, varying in weight from 20 to 80 pounds and of a hemispherical shape, is attended Avith great risk of breakage to the machinery, and especially when there is some metallic lead to be found associated with it. The speiss was therefore thoroughly culled before sending it to the crushing mill, and all large pieces broken in two or more pieces by sledging to avoid as far as possible the risk of getting lead into the crusher.