AMERICAN INSTITUTE OF MINING AND METALLURGICAL ENGINEERS

Technical Publication No. 1669 (CLASS D. N o l v w ~ ~ o u s MBTALLVRCY. NO. 7s)

DISCUSSION OF THIS PAPER IS INVITED. It should preferably be presented by the contributor in person at the New York Meeting.,February. 1944. when an abstract of the paper wjll be read., I! this is impos- srble dlscussron In wntrng ( a copres) may be sent to the Secretary Amencan Institute of Mlntng and Metal- lurgical Engineers ag West 39th Street. New York N. Y. Unless bpecial amngement is made, discussion of this paper will cldse June 30. 1944. Any discussion' offered thereafter should preferably be in the form of a new paper.

Changes and Improvements in Modern Copper Smelting BY R. A. WAGSTAFF,* MEMBER A.I.M.E.

(New York Meeting. February 1944)

SINCE the time of the early Egyptians, the use of copper has been a boon to the life of most of the civilized world. I t s use has been varied; in many connections, the art by which it attained its greatest useful- ness has been lost.

In this paper an endeavor will be made to take into consideration some of the many changes that have occurred during the last 40 years of copper-smelting oper- ations. During that period the copper blast furnace has been almost forced out of existence by new methods of ore concen- tration-first, gravity concentration, and then flotation.

These new methods of concentration introduced many new factors into the metallurgy of copper. The product was wet, it was fine and it was high grade. To overcome all these factors many changes have been necessary throughout the smelt- ing agenda.

New equipment has been necessary for transportation, unloading, sampling, bed- ding and moisturing in all smelting plants, especially the custom smelter. During the early days of "flotation," little was known about filtering or the use of an alkaline circuit; often the concentrate going to the smelter contained 20 to 25 per cent mois- ture. I t is easy to imagine the di5culty arising with a product of this nature when it had to be shoveled by hand labor. To meet these conditions, mechanical unload- ing and sampling devices were soon devel-

oped. In many instances such good progress was made that actual costs for handling this unfavorable product were lower than those formerly incurred on the old product.

Today most sampling of copper ores and concentrates is being done by mechanical sampling, pipe or augur sampling, or shovel sampling. Where concentrates are uniform in metal content, a great many of the plants now use pipe or augur sampling. For spotty ores, the old shovel sample with cone and quartering during the final stages is still in use. Where big tonnages are handled, mechanical sampling of concen- trates is being done, with satisfactory results. To curtail slow and tedious cutting and mixing procedure, mechanical mixing boxes, splitters and cutters are used to cut down labor costs.

As crude ores become scarcer, the use of milling operations increases. The few crude ores that are still smelted are used as fluxing types. To use these ores finer grinding has been found advantageous. This has made for better bedding and fluxing conditions, which in turn makes for faster smelting, lower fuel consumption and better metal recovery. The degree of fineness is more or less determined by the nature of the product. Most plants have set a standard of through 34-in. mesh screen for roaster- reverberatory smelting and through )I-in. mesh screen for converter fluxes. For some

This paper was presented a t a meeting of the Utah Section in 1943. Manuscript received refractory ores; even finer grinding has a t the office of the Institute Oct. 18, 1943. been found to be advantageous for good

'Metallurgical Engineer. American Smelt- ing and Refinine Co.. Salt Lake Citv. Utah. reverberatory smelting. - - - ~ - - - - .

Copyright. 1944. by the American Institute of Mining and Metalluieical Engineers,.Inc. MBTALS TBCUNOLOGY. February 1944. Printed in U. S. A.

2 . CHANGES AND IMPROVEMENTS IN MODERN COPPER SMELTING

The term "roasting of ores" is more or less a misnomer when considered in the light of past practice. Since the increased copper content of the charge means less elimination of sulphur, the various types of roasters are now putting through ton- nages as high as 250 to 300 tons per day with an elimination of 6 to 8 units of sulphur. Considerable quantities of fuel are required in this type of roasting, as it is

= zte-Gsential to-have-calcines'as=not=as= possible for good smelting practice. To handle these large tonnages, a l l roaster equipment has undergone changes in con- struction and motive power.

There has been little or no change in actual roaster design, most plants using six to eight hearth roasters with a diameter of from 19 to 2 j ft. Changes have been made in rabble design; some plants are using various types of abrasive resisting material as inserts in the wearing surface of the rabble blades, with considerable success. Air cooling is used in columns and arms instead of water. Considerable vari- ation in arm speed is practiced, varying from one revolution per minute to one in 26 sec., a great deal depending on the type of charge. Where heavy elimination of sulphur is still required, drophole areas have been enlarged to allow faster elimi- nation and less vroduction of flue dust. Most roasters are now insulated with rock wool orinfusorial earth, to conserve heat.

The real changes in copper smelting have been made in the actual smelting operations. Two lines of procedure are now being followed: (I) roasting and smelting, (2) direct smelting.

During the first 2 0 years of the cen- tury roasting and reverberatory smelting supplanted blast-furnace smelting. Two methods of reverberatory smelting have

been used. First, deep-bath center-drop smelting was the vogue, but as the charge changed from a coarse, granular product to the fine, high-grade concentrate, dust losses increased and high iron content caused rapid fluxing of furnace brickwork, it soon became apparent that changes would have to be made to offset these unfavorable factors. The answer was found in side feeding, which not only reduced dusting but also protected furnace side walls.

nilring these changes many efforkwere - - - __-_____ made to increase the size of reverberatory

-

furnaces to get larger tonnages, but little progress was made until it was found that these furnaces could not be made to smelt more material unless more fuel could be satisfactorily burned. This meant that exit areas for gases had to be made larger. The stress for copper and increased tonnage came as a result of the first world war and with this demand came the enlarged fur- nace uptakes. During the early days of the twentieth century, length of furnace was thought to be the best way to get more furnace area and larger tonnages, but soon this was found not to be so; closer studies proved that increased width gave much more satisfactory results. Furnaces are now being built just as wide as buckstay support will permit. One Canadian smelter using suspended arch construction has gone to a width of 35 ft. in the smelting zone. Furnace length is now governed by charg- ing conditions, most plants endeavoring to keep unsmelted charge away from the skimming bay.

Little or no progress was made in rever- beratory smelting during the early 1920's. This lull was caused by the Carson case, which involved most of the large copper- smelting plants. Just as soon as this was settled, two new smelting ideas were brought forth, the first being direct smelt- ing and the second gun-feed deep-bath smelting.

. . 1 . R. A. WAGSTAFF 3

to offset the increased costs of fuel and con- Direct Smelting version of a lower grade matte into copper.

Direct smelting of copper ores and con- Today many plants in Africa and 'the centrates was introduced in Mexico in an United States have adopted this type of

'Y LM ulnun

y.u I" r1.7 I*".. *I, DUCT

FIG. I.-GAS FLOWSHEET, MCGILL SMELTER, NEVADA DIVISION OF KENNECOTT COPPER CORPO- RATION.

effort to get away from heavy flue dust smelting. Most of them are new or have and slag losses and to realize the use been rebuilt during the last 10 years. Three excess fuel by waste-heat recovery. This factors govern the of this type of method of smelting called for the elimina-

smelting: tion of all roasting equipment and it was hoped to avoid the necessity of Cottrell I. Less capital required for first con-

plant installation and thus save enough struction costs.

4 CHANGES AND IMPROVEMENTS IN MODERN COPPER SULTING

2. Requirements for power make pos- sible the utilization of all excess fuel.

3. Sulphur content of charge allows the formation of an economical grade of matte.

The McGill smelting plant of the Nevada Mines Division, Kennecott Copper Corpo- ration, seems to have been able to push direct smelting farther than any other organization. By changing furnace design, this plant has been able to increase the fueling rate from 125 tons of coal per day to 230 tons and smelt as high as 986 tons of solid charge per furnace day. w aste-neat boilers are larger than formerly, and thus recover jo to 55 per cent of the heat released from the fuel burned. While fuel required per ton of charge has been high (6.376 million B.t.u., per ton of charge) the increase in tonnage no doubt has paid for itself.

Charging

Since all plants are not blessed with high- copper and low-sulphur charges, roasting and smelting operations still have a place in copper smelting. Two types of charging are being followed: (I) side-charging, (2) gun-feed, deep-bath smelting.

Side-charging reverberatory furnaces are used by all Canadian plants, Anaconda and some plants in Arizona. Many im- provements have been made in construc- tion, exit areas for gases are much larger and combustion areas in the smelting zone have been increased, to allow for greater consumption of fuel. Ribbed arch con- struction has been perfected to a point where furnace life is now a matter of months instead of days. During recent years, in Canadian plants, suspended arch construction has permitted the use of magnesite brick and has been very effective where the charge has been of a basic nature, but as the cost of magnesite brick is about four times that of silica brick, very few of the United States smelters have found it economical.

Deep-bath Smelling

Deep-bath smelting has attracted many operators, but, owing to the method of feeding, has given considerable trouble, both in metal losses and in erosion of brick- work caused by fluxing action of the dust in charging. To overcome these difficulties, the Garfield plant of the American Smelting and Refining Co. introduced the gun feed about 1928 in conjunction with deep-bath smelting. This method of charging showed very goo6 possiliii~ies auri iuciay is use;

on I 2 furnaces in five different plants. The main features of this type of charging are: (I) better control of furnace-charging conditions, (2) less flue-dust loss (40 per cent), (3) better combustion areas in smaller furnaces, (4) better heat release from matte bath, ( 5 ) larger matte reservoir, (6) cleaner slags.

During the past few years some very interesting papers have been written on the various plants. A comparison of some of the data gives a picture of what these

TABLE I.-Comparison o j Data, Year 1940

plants are doing (Table I). Recent data from these plants shows that even better results have been obtained during 1941.

These large increases in tonnage smelted per furnace day have made the problem of furnace life very important. This problem is being met in different ways by the differ- ent plants; some of the better known schemes are as follows: (I) silica slurry, (2) suspended arch construction in con-

Type Smelting

Direct side feed. Calcine side feed Calcine gun feed

deep bath.. . .

Name of Plant

McGill Noranda

Clarkdale

:,"ld Charge Smelted per Fce.

Day ----

986 1491

I450

y:t B.t.u.

&:d Ton

6.376 2 . 7 9 0

a. 730

H2O Evapo- rated Per

!!:!$': Lb,

NO $%re

424

R. A. WAGSTAFF 5

junction with magnesite brick, (3) slotted longer without a major arch repair. Clark- arch construction, (4) ribbed arch con- dale reports a furnace that has been in the struction, (5) water-cooled copper sidewall stack since June 27, 1939, having smelted jackets (deep-bath smelting). more than 1,300,000 tons of charge. These

he method used seems to be more or less a plant problem. Both silica-slurry patching and suspended magnesite arches have given satisfactory results. Where they have been used, more than a million tons of charge have been smelted before a major repair job was required. This type of furnace repair is being done a t a cost of from four to six cents per ton of solid charge smelted.

Silica-slurry patching has been in use about four years. Considerable improve- ment in method of application and make-up of slurry has been made in recent months. About 3 per cent bentonite is found to be very beneficial and helpful in making a mixtuie that will adhere to brickwork. Some very remarkable furnace campaigns have been made in recent years. A furnace a t the Hayden plant has now passed 1000- day life and is expected to last 60 days

figures seem incredible when compared with former figures of 90 to IOO days and 75,000 to ~oo,ooo tons of charge per furnace campaign.

Some experimental work has been done with insulating materials. The greatest success has been noticed in plants where furnace tonnages have been low,. These plants claim a saving of 10 per cent in fuel.

The use of water-cooled jackets along the bath line of deep-bath smelting furnaces is becoming standard practice. Every known type of refractory acid, basic and neutral, has been tried, with little or no success. When furnace tonnages are stepped up above 1200 tons per furnace day, the cost of side-wall repairs becomes a very important item. I n most plants copper jackets with 135-in. water coils have proved most satisfactory.

6 C H A N G E S A N D I M P R O V E M E N T S I N M O D E R N C O P P E R S M E L T I N G

Ever since reverberatory furnaces have been used as smelting units, one of the FUEL

important problems for the copper metal- One of the main problems in good smelt- lurgist has been a permanent furnace ing practice has been the efficient burning

FIG. 3.-REVERBERATORY FURNACE OF HUDSON BAY MINING AND SYELTING COMPANY. SHO\\'ING SUSPENDED ARCH.

bottom. Early practice was to put in a silica bottom and then carefully smelt this in place, with the hope that the highly infusible mass would serve as an impervious layer to the metal bath. Usually, owing to expansion and inability to make a homo- geneous mass, these bottoms floated out after a short time. I n recent years other schemes have been tried; the latest is the use of magnetite sand instead of silica. This material has a much higher specific gravity and n lower coefficient of expansion. Just what the metal-absorption properties will be is an unproved factor, but we do know that they will be no worse than a silica bottom that has been floated out.

of fuel. Ores today are being smelted by oil, coal and gas. Most plants are familiar with oil or coal as a fuel, but not until about 10 years ago was natural gas used as a fuel.

Combustion problems have been fairly well worked out for both oil and coal. Good burner equipment is now being used with efficient control equipment. Automatic con- trol of draft, air and fuel supply have been so well synchronized that i t is now almost impossible to get furnace combustion out of line.

Owing to the newness of natural gas, there will remain a number of problems t o be solved. Combustion areas a n d . proper

R. A. WAGSTAFF 7

atomization of gas and air are still fields for research. Results to date have shown that natural gas as a fuel is 8 to 10 per cent less efficient than either oil or coal. One of the main drawbacks has been the inability to obtain a comparative tonnage per furnace day.

Most plants.at first burned natural gas with the aspirating single-spud type of burner, which is very noisy and hard to regulate. High gas velocity also causes heavy turbulence, which makes for rapid deterioration of furnace brickwork. A great many plants have now installed multijet burners, which are less noisy and allow the gas to enter the furnace a t much lower velocities. Furnace life has been lengthened by this type of burner, with no loss of efficiency. Experiments are now being carried on with forced-draft burners, in the hope of having a more flexible burner, which, in the long run, will give larger tonnages and better fuel ratios.

To date no real tests have been made on the preheating of air for combustion in reverberatory furnaces for copper smelting. Two plants are now getting ready to do some real experimental work; namely, the McGill smelter and the Chino smelter.

The McGill plant will try preheated air with coal and the Chino plant will try preheated air with natural gas. Both plants intend to use extraneous heat, with the idea of raising the furnace temperature within the smelting zone and thereby getting a faster smelting rate. It will be interesting to note the effect of these high temperatures on the furnace brickwork.

Modern copper metallurgy does not con- sider type slags and mattes; each plant is governed by the ore to be smelted. This is especially true of smelters connected with large single mining units. Ideal con- ditions for good metallurgical results are a slag containing 37 to 38 per cent Si02,

39 to 40 per cent FeO, 4 to 6 per cent CaO, and 4 to 6 per cent AlzOa; matte to be 34 to 36 per cent Cu. Where gold and silver values are high, considerable metal can be saved by judicial use of lime, this factor to be controlled by the economics of individual cases. At plants where exces- sive silica or alumina is present, the charge has been made less refractory by the use of lime and usually a decided increase in metal-bearing tonnage has been noted.

The proper ratio of base-metal to precious-metal content is of the utmost importance to good metal recoveries. No amount of fluxing can overcome such a deficiency.

When blast furnaces were operated it was found necessary to properly flux all converter slag produced, to avoid high metal losses. When universal reverberatory operation was adopted, it was soon found practical to use the converter slag in a molten condition, thus avoiding the expen- sive handling and resmelting costs of this by-product. For some time it was thought that metal losses were increased, but when proper fluxing conditions were obtained, it was found that metal losses were actually reduced.

Since increases in fuel consumption have been the key to large tonnages, it has also been necessary to keep waste-heat boiler installation more or less in line. Today we find that waste-heat boiler installations are much more expensive than the furnace proper. In the past, waste-heat boilers were installed in units of 350 to 500 hp., producing steam of 150 to I& lb. pressure, whereas the latest installations call for boilers of 750 lb. pressure and 1100 hp., equipped with water walls, tracifiers, soot blowers, superheaters and water softeners. Water evaporation a t some of the plants is ~m,ooo lb. per hour per furnace and uses 50 to 55 per cent of the total heat developed.

Converting of copper mattes has under- other factors have considerable bearing on gone very little change in the last 2 0 years. the final outcome; namely, Two types of converters are now used; I. Temperature control by the use of namely, Great Falls type and the Peirce- proper grade of silica for the various grades

FIG. 4.-STREAMLINED EXIT FLUE AND WATER WALLS, NO. 4 REVERBERATORY FURNACE, HAYDEN PLANT, AMERICAN SXELTING AND REFINING COUPANY.

Smith. During recent years there has been aisteady drift to the use of the Peirce- Smith type. The size now commonly used is a 13 by go-ft. converter. These con- verters have given the best expression for tonnage and life. Most plants base their tonnage on 37 per cent copper matte a t 100 tons per day, increasing or decreasing this figure 5 tons for each per cent of copper above or below 37.

Considerable variation in life of tuyere lining is being experienced by the various smelters. The length of life varies with the success of the various smelter organizations in putting on magnetite linings; some plants get only 6000 to 8000 tons of copper and others get 50,000 to 60,000 tons per 18-in. repair of the magnesite-brick tuyere lining. While the proper application of the magnetite lining is the controlling factor in the long life of converter campaigns,

of matte. '

2 . Proper selection of size of each copper charge to be finished.

3. Sufficient expansion left in tuyere brick to prevent excessive spalling.

There have been some improvements in design in recent years of the 13-ft. Peirce- Smith converter. Shells are being butt- welded, making a stronger and smoother job. Trunnion suspension rings and driving gears have been placed on the outside edge of the shell, allowing more space for the tuyeres. The 13-ft. converter now has fifty-two 1%-in. tuyere pipes instead of forty-six. Improved hoods have eliminated the smoke trouble in the converter aisle. Many plants have installed speedier hoist- ing and faster traveling-crane service. All these changes have made for increased production through smoother operating conditions.

One of the big improvements noted in the use of the Peirce-Smith converter has

R. A. WAGSTAFF 9

been its ability to smelt larger quantities of by-products. This is especially true when the grade of matte is kept below 38 per cent copper. This fact has allowed most con- verter departments to take care of the major portion of by-products produced.

The problem of dust and fume losses has always been a subject of much concern to copper metallurgists. When true flue dust was the main source of loss, enlarged flue systems and dust chambers were used, but with the increased fineness of the charge and the gradual introduction of low-grade gold, silver and lead siliceous ores as a converter flux, the losses became excessive. To overcome these losses the Cottrell electrical precipitator was installed and has done a wonderful job, saving from go to 97 per cent of the metals formerly lost. These installations were first made on converter gases, but today there are installations operating on roaster and reverberatory gases.

During recent years other types of dust- recovery equipment have been used, especially where gold and silver values are low and no great amount of true fume is present. Cyclone separators of the multi-

clone type are being used with very good results, recoveries of 85 to go per cent of the true dust being reported.

Smoke and dust control is now a part of every plant. Dustless connections a t charging points in both the roaster and copper departments have been satisfac- torily worked out a t most of the plants. Matte and slag launders are now under suction, allowing little or no smoke to escape. Dust control in the crushing de- partments has been given much thought and conditions are being improved daily.

A resum6 of the changes and improve- ments made in copper smelting shows a steady and healthy advance. Costs and metal recoveries have been maintained in the face of a steady increase in labor and material costs and an increase in metal con- tent of charge. This has been done by in- creasing furnace tonnages and conserving metals. Improvements in furnace life and the ability to get better expressions from the fuel burned have also contributed a great deal to keep better than an even balance.