1

To describe in detail the various tests made on this material would serve no useful purpose. The following brief summary will suffice:

The ore was roasted after being ground to many different sizes, varying from 8-mesh to 100-mesh material. The quantity of fuel used varied from 1 to 5 per cent, and the amount of salt from 3 to 10 per cent of the weight of the ore. The blast pressure was varied in order that temperatures of 600° to 900° C. could be obtained. The roasting period was varied from 15 minutes to 3 hours. In no test did it seem possible to roast more than 50 per cent of the zinc to a form soluble in acid brine. The chemical and physical properties of zinc sulphide are such that it usually roasts very slowly, and the brief time that the mineral is heated in an oxidizing atmosphere in the blast roasters used does permit complete roasting. With the shallow bed on the down-draft roaster, which gives a 15-minute roast, usually not more than 20 per cent of the zinc is oxidized or chloridized, and in many of the tests the total amount of zinc removed during such treatment was only about 5 per cent.

OXIDE ROASTING.

By oxide roasting is meant the roasting of zinc-sulphide ores to as low a percentage of sulphur as possible. As zinc sulphide is one of the most refractory of the major-metal sulphides to roast, advancement in the art of roasting these ores is far from satisfactory. However, no doubt considerable improvements will be made in mechanical contrivances for obtaining continuous roasting of such ores. The chief objection to existing types of furnaces is that when the roast is carried to the point of satisfactory elimination of sulphur the costs of building and operation are very high. For zinc-retort smelting it is important that no sulphur be left in the calcine. The statement has often been made that every pound of sulphur left in the calcine, restrains 2 pounds of zinc from distilling in the retorts. For leaching the zinc calcine with sulphuric acid, complete desulphurization of the ore is not required but complete oxidation of the sulphides of zinc is necessary. In other words, the amount of zinc converted to sulphate of zinc is immaterial, as long as all of the sulphide of zinc is roasted. Usually the lower the temperature of roasting the greater is the percentage of zinc sulphate formed, but also the greater is the liability of leaving some unroasted zinc sulphide unless the time of roasting is extended over a long period. The higher the temperature of roasting the higher is the percentage of oxide of zinc formed, because zinc sulphate decomposes thermally at temperatures above 680° C. The desirability of making zinc sulphate will depend on the acid requirements of the ore during leaching. An electrolytic zinc process returns the used acid for leaching and only enough new acid need be added to bring the solution up to the desired strength and make up for mechanical losses.

SULPHATE ROASTING.

The principles of sulphate roasting of zinc sulphide in the ordinary reverberatory type of roaster are fairly well known, but the sulphating efficiency of blast roasters has not been generally tested. As chloridizing roasting had failed in the tests just described, sulphating roasting did not seem to be a very promising field for experiment. However, a few tests were made in the Holt-Dern laboratory roaster, running slowly and with a low blast. The low blast permitted a low temperature and a long roasting period, conditions which are necessary when attempting to form zinc sulphate. The quantity of water-soluble zinc after such a roast was nearly always small and rarely exceeded 10 per cent of the total zinc content. Hence the blast roaster is not adapted to such work.

In a series of oxidizing roasts of zinc-sulphide ores, made in a small gas-fired reverberatory roaster, many of the roasts were leached with water. The conditions of formation of zinc sulphate in a muffle are discussed in connection with that series of tests.

At Great Falls, Mont., the Anaconda-Wedge roaster is giving a 22-hour roast to complex sulphide ore, the temperature being about 600° C., and the ore being rabbled four times a minute. About 30 to 50 per cent of the zinc is sulphated. Marmatite ores give better results in sulphate roasting than "free" ores, possibly because of the catalytic effects of iron oxide in direct contact with the zinc oxide. The low temperature used is to prevent the breaking up of zinc sulphate and also the formation of ferrites of zinc.

An extensive series of roasting tests of various complex-sulphide ores containing zinc was made at varying temperatures and for different lengths of time to determine the best conditions for roasting such ores. At first a gas muffle furnace was used, the ore being placed on the bottom of the muffle. A platinum and platinumrhodium thermocouple protected by alundum tubing was used to measure the temperatures in the furnace. It was soon found that roasting in such a furnace was of no value, as the ore next the bottom of the muffle became much hotter than that farther from the bottom. Hence the use of roasting dishes became necessary, and a gas reverberatory roasting furnace 2 feet by 3 feet in size, was used, to parallel more nearly the conditions in a commercial

roaster.

The reason for studying roasts of this kind was that considerable trouble was then being experienced in the experimental electrolytic zinc plants at Anaconda, Mont.; Trail, B. C.; Murray, Utah; Welland, Ontario, and other places, in getting a calcine that was completely roasted. It seemed to be the common experience that in such calcines 60 to 70 per cent of the zinc would be acid-soluble;

the remainder was suspected to be largely sulphide of zinc, although ferrite of zinc was also known to be present. Chemical analyses showed sulphur left in the calcine. The question for determination was whether it would be possible, even by roasting in shallow dishes in a furnace of controlled temperature and with considerable rabbling, to calcine these ores in a short length of time, as can usually be done with copper ores in furnaces of this type. For instance, the Wedge zinc roaster can usually give the ore not more than 8 hours' roast, whereas the old Hegeler furnaces have been used for as much as a 60-hour roast. The more recent AnacondaWedge furnace gives a 20 to 22 hour roast.

A further point of interest was to determine the relative rates of roasting of lead and zinc sulphides in a mixed ore, as lead leaching was being tested at the same time in this laboratory. By treating the calcine with hot water the zinc sulphate formed could be leached and determined. In a similar way the total oxidized zinc could be determined by treatment with a solution of acetic acid (sulphuric acid tends to dissolve zinc sulphide slowly). The solution of acetic acid also leaches much of the oxidized lead. A solution of ammonium acetate dissolves lead sulphate, as does a saturated solution of sodium chloride. Finally, for practical purposes, a sulphuric acid leach to recover the total soluble (oxidized) zinc, followed by a brine leach to get the oxidized lead, was made.

ROASTING OF A LOW-IRON ORE FROM DALY-JUDGE MINE.

DESCRIPTION OF ORE AND OF METHOD USED.

The first ore to be roasted, known as sample 1, was from the DalyJudge mine, of Park City, Utah. This sample represents a great deal of the Park City ore; it contained 16.88 per cent Zn, 14.87 per cent Pb, 2.25 per cent Fe, 10 per cent CaO, 30.8 per cent insoluble, and 12.7 per cent S. The sample was crushed to pass a Tyler standard. 10-mesh screen and then gave the following screen analysis:

Results of screen analysis of Daly-Judge No. 1 ore crushed to pass a 10-mesh screen.

The curves in figure 2 show the percentages of the zinc and of the lead that were oxidized during the roasting of portions of this sample for different lengths of time at different temperatures. The roasts

were made in duplicate and the average of the two results taken. Oxidized zinc and lead were determined as previously described. The portions roasted weighed 300 grams each and were placed on clay roasting dishes 6 inches in diameter. At the end of each hour they were withdrawn, stirred, sampled, and returned to the furnace. The depth of the bed of ore being roasted was about 1 inch.

SULTS OF TEST.

The results show that 450° C. is too low a temperature for roasting either the sphalerite or the galena. At 550° C. little roasting takes place; at 675° C. roasting proceeds much more rapidly so that as much as 70 per cent of the zinc can be roasted in 10 hours, the greatest length of time that material can be held in the usual Wedge or McDougall type of roaster. Probably, more continuous rabbling would permit faster roasting at this temperature.

TOTAL LEAD OXIDIZED, PER CENT

TOTAL ZINC OXIDIZED, PER CENT

100

90

900°C

The

0

curve for the recovery

of zinc at 900° C. is dotted to distinguish

it from the

675°C

550°C

-450°C

0 1 2 3 4 5 6 7 8 9 10 11

TIME, HOURS

FIGURE 2.-Curves showing effect of temperature on speed of roasting, Daly-Judge No. 1 sample, 10-mesh size.

curves for the lower temperatures; for this curve a 10 per cent solution of sulphuric acid was used to determine the oxidized zinc in place of a 40 per cent solution of acetic acid. The sulphuric acid solution seems to dissolve some zinc sulphide as well as the oxidized zinc. Perhaps the most striking feature brought out in figure 2 is that the speed of roasting both sphalerite and galena depends largely on the temperature; also, these two minerals seem to roast at about the

same rate of speed. The drop toward the end of the curve for zinc recovery at 900° C. is probably due to the formation of zinc ferrites or similar compounds, which are known to form at such a temperature. In fact, in roasting an ore containing much iron, past work has shown repeatedly that 750° C. is about the highest temperature

FIGURE 3.-Curves showing effects of leaching with different solvents, Daly-Judge No. 1 sample, 80mesh size. Zinc: Solid-line curve, sulphuric acid solution; dotted curves, acetic acid solution. Lead: Solid-line curve, acidified ammonium acetate solution; dotted curves, acidified brine.

that can safely be used, in order to avoid the formation of ferrites. As much of the zinc is roasted to oxide and as ferrite forms on prolonged roasting, it might be possible to obtain quick roasting at high temperatures, provided the roast was discontinued promptly enough. This, of course, could apply only to ores low in iron.