Show cheor d z ng leache leaching ing at A t park V ark city C ity by THEODORE P HOLT the mines operating co s plant at park city utah was designed to treat the low grade fillings in the old of the ontario mine these fillings carry 6 to 14 oz of silver I 1 to 2 ib copper to oz of gold and a small percentage of lead and zinc the treatment consists of mixing the crushed ore with coal dust and salt and then roasting in a new type of furnace by combustion of the contained fuel the roasted ore is leached beached with an acid salt solution to dissolve the silver gold copper and lead at present these metals are precipitated together on scrap iron and the product sold to a refinery the only new feature of importance in this scheme of treatment is the roasting process which makes possible the chloria izing of ores without any loss of the valuable metals and at a very low cost development of the process the roasting process had its beginning in connection with research work at the utah state school of mines N C christensen jr who held one OF of the school of mines fellowships was engaged in some experimental work on blast roasting at the same time I 1 was doing some work on the insoluble gold in mercur base ores mr christensen roasted some of this base gold ore in his pot furnace and found that when v hen he mixed a sufficiently low percentage of fuel with the ore it did not sinter but roasted to a bleachable lea chable product we considered this a new application of blast roasting and proceeded to test out a large number of ores in several different types of small roasters asters ro which we constructed ted this experimental work continued well into the summer of 1911 in august 1911 the co consolidated soli dated mercur gold mines C co 0 installed a small roaster of our design which we proposed to operate on the countercurrent counter current principle feeding mixed ore continuously on top and at the same tim time 1 e drawing off the roasted ore from the bottom by proper adjustment we hoped to maintain a permanent roasting zone near the center of the column the ore moving downward as roasted however we experienced difficulty in getting the moving mass to roast properly and also other troubles of a mechanical nature developed mr christensen at this time disposed of his interest in the process to G H dern we continued experimental work on a supt bupt mines operating co park city utah paper to be read at salt lake meeting of american institute of mining engineers in august reprinted from the july bulletin of the institute number of machines of varying design up to the time construction work began on the mines operating co mill no satisfactory continuous roaster had been developed so we decided to install an intermittent roaster which our experience had demonstrated would give a satisfactory product for subsequent leaching operations the plant the mines operating co s plant is installed in the old concentrator ot of the ontario silver mining co an outline of the present scheme of treatment may be followed by reference to the accompanying flow sheet A second motion electric hoist operating 12 ton skips in balance delivers the ore from the haulage tunnel of the mine to a bin above the crusher from here is changing the eccentric throw this tape t of feeder works very well for the salt and a coal dust but is not very satisfactory for ore on account of rapid wear the mixed ore is crushed in two sets of 15 by 35 36 in rolls and is elevated to the mixed ore bins above the roasters asters ro there are eight shaft roasters asters ro of the intermittent type the roaster is 10 by 10 ft inside the walls the walls are reinforced in forced concrete 10 in thick about 2 ft above the bottom is a wood grating supporting a layer of coarsely crushed rock the space below this grate forms an all air chamber the grate supports the roasting charge and also distributes the air blast the method of operating the roaster is as follows A special starting mix of about one ton is prepared in front of the 4 M 4 zo mill of the mines operating company park city photo by will C higgins is fed by a stephens adamson apron feeder to a with 1 rin in openings the undersize of the passes through a revolving drier where the moisture is reduced to about 5 per cent the oversize of the falls upon a picking belt where mine wood and waste are removed it then passes through a no 5 gates gyratory crusher and joining the drier fines is elevated to the crushed ore storage bin this bin has a capacity of tons of ore sections of the same bin are respectively used for salt and coal storage from these bins the ore salt and coal are fed in the required proportions upon a belt conveyor and delivered to the rolls for final crushing the feeders by means of which the proportioning and mixing are done are 0 of the plunger type the feed being varied by roaster A layer of coal dust mixed with oil is then spread over the gravel floor of the roaster this is ignited and sufficient blast admitted to burn it rapidly to glowing coals the special mix is then shoveled evenly over the surface by the aid of the air blast the coals ignite the fuel in W when h e n the the ore which begins to roast starting layer has roasted through so that the ure fire appears on top the air is shut off for a few minutes while the first charge ol of about five tons is dropped from the mixed bin gate into the roaster this is aprea out by hand and the air agal again n turned on until ft the roast is allowed to proceed choge aba cba S appears on the surface of the then a second charge of about ten tons is billal dropped on and four hours later a charge of about the same amount F brings the total depth of charge up to about se seven ven feet under normal conditions this will roast through in eight to twelve hours when the roasting zone has reached the surface at all points the air blast is shut off the discharge door is opened and a sluicing apron inserted to connect with the launders the hot roasted ore is often sluiced out with mill solution the sluicing nozzle is made of hard wood and has a lin l I in nozzle opening it operates under a head of forty five feet the nozzle is suspended in position in front of the discharge door the operator stands at electric hoist I 1 coarse ore bin SA sabin bin 4 iya trammel undersize t oversize 1 J drier ler gyratory crusher Fl elevator evator 4 4 Cru bin Salt iBin coal bin conveyor r f I 1 arc mmel I 1 sluicing sat sql storage st orage I 1 oversize un de rs alze 1 1 1 r condenser er 15 x 3 36 rolls Is L I 1 I 1 I 1 impact screen een ap L I 1 I 1 r elator 0 oversize undersize El elevator I 1 i I 1 rolls mixed mix ed storage saral c bins 1 laster 1 J accent Ec cent ri c screen boasters roasters Ro asters J oversize undersize dang leaching 1 tanks 4 1 l alit denit 1 4 1 silver r f S solution 0 i ut io n Pre elid boxes i baran barro n Solli tion saule ns SAut loll on lt 1 L flow sheet mines operating mill park CRY city utah one side to avoid the numerous steam explosions plo and directs the nozzle with a long rod under normal conditions it takes from one to three hours to sluice out a roaster charge of twenty four tons from the roasters asters ro the hot ore mixed with mill solution passes by means of launders to the leaching tanks the leaching tanks are 20 ft in diameter and 12 ft deep there are six of these each holding bolding ing tons of ore in the bottom of each tank is a 12 in round discharge hole for sluicing out the tailing this is closed by ia a turned wood plug having a 4 by ain stem extending to the top of the tank for convenience in sluicing out the discharge hole is surrounded by a box extending to the top of the tank and provided at intervals with 12 by 12 in sluice gates of the various filter bottoms tried one made of 4 in drain tile has given the best service four lines of tile are spaced across the bottom of the tank and cemented in position by a thin slab of concrete on each side this tile system connects at one side of the tank with a 3 in wood pipe solution line the solution lines from the leaching vats lead to a distributing box here the solution is directed either to the weak tank or the silver tank depending upon its metal content from the weak storage tank the solution n is pumped P back f for or sluicing purposes while from the silver tank it goes to the precipitating boxes box es the iron precipitation boxes are similar in design to the wood zinc boxes sometimes used in cyanide mills they are larger however and so constructed that the iron contact with the acid tie rods are not in soaked wood prom from the precipitation boxes the solution solutions 1 passes to the barren sump and is returned as a leaching leac hing solution to the vats having followed the ore treatment through in a general way we will now return to discuss more in detail certain features that are more or less new importance of proper mixing perhaps the most important step in the whole operation is the proper mixing of the ore preparatory to roasting roasting for requires a very close regu lation of temperature the best results on many ores are obtained between and C in any case to sinter parts of the ore is to render it unfit for subsequent via tion the three chief factors which determine the temperature are 1 the percentage of fuel in the mixture 2 the percentage of moisture in the mixture and 3 the amount of air blown through the charge per unit of time the first of these three is much the most important coal dust is used as fuel the present supply costs per ton fob cars at the coal mine since this class of material takes a cheap freight rate it is the most economical fuel to use under our conditions the amount required varies from 24 to 30 per cent of the weight of the ore being roasted the ore as it comes from the mine is very wet the drier is supposed to reduce the moisture to about 5 per cent but the variation is considerable we might suppose that an increase in moisture would lower the temperature of the roast it has exactly the opposite effect however and it is necessary to reduce the percentage of fuel as the moisture in the ore increases the mill was designed to treat tons per day it is handling considerably more than this the average for the month of march being tons this tonnage is mixed and rolled on one shift the mixing is in charge of a trustworthy man who checks up the feeders by weighing the output of each frequently the new holt dern roaster during 1913 a new continuous roaster was developed which overcomes the difficulties experienced with previous machines A full description of the roaster will not be attempted in the present paper A few of its general characteristics and the results obtained will be noted we installed a commercial size machine at our plant in december 1913 except tor for a few brief delays for changes it has been in continuous operation in the holt and dern roaster the collumn of ore moves down at intervals as the roasting zone travels upward the air blast travels in an opposite direction to the ore in this way the air passes passe s first up through the hot roasted ore and becomes highly heated it then passes through the roa roasting zone where active combustion of the fuel in the ore is taking place and finally through a layer of moist ore hence when it leaves the roaster it is f fairly cool and entirely free from dust our experience at this plant has demonstrated the following advantages in this roaster over the intermittent type already described comparative cost data per ton of ore roasted old holt dern roaster roaster labor power repairs estimated starting oil coal dust A series of extraction tests for a period of five weeks gave an average of per cent of the silver for the old roasters boasters ro asters and per cent for the new roaster this is on the coarse crushing we have found most economical in the old type of roaster following is a screen sizing test on the ore fed to both roasters asters ro per cent by weight 12 in n 10 14 in 22 18 in 33 39 1 14 in r 15 1 14 in 20 the ore treated is a dense one and the recovery would be materially increased by finer crushing with the old type of roaster this is not economical while in the new roaster 10 mesh and even finer product may be handled very well the salt used in both cases was equal to 75 per cent of the weight of ore roasted we are of the opinion that this can be economically reduced in the new roaster but this has not been determined when compared with the roasting furnaces commonly employed for preparing ore for leaching purposes the following advantages may be pointed out I 1 there is no perceptible volatilization loss of the valuable metals 2 there is no dust loss 3 an ideal product for percolation is furnished even when the raw ore contains much slime 4 only a low percentage of an inexpensive fuel is required 5 A high recovery without fine crushing is po possible isible 6 the roaster gases are concentrated cool and easily condensed for leaching purposes the leaching process the leaching process is ve very ry simple there is in fact but one mill solution which is designated as silver weak or barren depending on its value in silver the solution is made up merely of soluble salts from the roasted ore carrying in addition about four pounds of free acid the acid acid is in part supplied from the condensed roaster fumes the remainder being added as sulphuric acid at such times as the ore carr carries ies a consid considerable erab e perc percentage of pyrite no extra acid is required of the salts taken up by the solution from the roasted ore during leaching forms the greater part chlorides and sulphates of the various other metals are also present the sluicing out of the roasters boasters ro asters into the leaching vats is done with weak solution this dissolves the greater part of the silver and copper from the hot roasted ore while conveying it to the leaching vat the solution passes from the vat as the silver solution and most of it is pumped to the precipitation boxes when the leaching vat is full it is leveled off and leached beached with barren solution for twenty four hours it is then washed with water down to from 50 5 to 10 B specific gravity 1036 to 1075 and sluiced out the washing is regulated so as to keep the solution at the proper density experiments have shown this to be about 24 B specific gravity 12 the excess barren solution is run to waste from time to time the best results are obtained by leaching with warm solution during the summer months the heat from the roasted ore is sufficient to maintain a temperature of from 30 to 40 C but in cold weather steam is blown into the solutions this is best done in the precipitation boxes as maximum efficiency is obtained by heating the solutions at this point leaching costs the cost of leaching varies considerably the minimum was attained during the summer when the heating plant was closed down and the roasters asters ro were making the necessary acid on the other hand during the month of march when it was necessary to buy both acid and coal the leaching cost reached the high figure of precipitation the precipitation boxes are packed with scrap iron of every description thin sheet scrap old screens etc are desirable on account of the large surface presented the metals in solution are deposited on the iron in accordance with their position in the electromotive series that is the metals of greatest potential difference are deposited first this segregation of the metals however is not sufficiently clear cut to be of value in their separation A general cleanup of the precipitation boxes is made once each month an intermediate ter mediate is sometimes necessary on account of the boxes clogging the method of cleanup is rather crude due to the present layout the loosely adhering precipitate is brushed from the scrap iron put into filter boxes and washed if is then dried and sacked for shipment A partial analysis of the product is as |