Show A NEW PROCESS FOR FOK TREATING COPPER ORES BY FREDERICK LAIST B S the following is a brief summary of the 1 laist process owned by the am american ex traction company of salt lake city i 1 dissolving copper from ore in dilute sulphuric acid 2 precipitation of copper from its solution as copper sulphide by hydrogen sulphide gas accompanied by regeneration of acid 3 manufacture of the hydrogen sulphide gas a reduction of gypsum to calcium sulphide by coal b decomposition of calcium sulphide by carbon dioxide and water to hydrogen sulphide and calcium carbonate 4 convers conversion ion of the copper precipitate i into copper bullion discussion of reactions the process is based on the following reactions 1 cu plus equals cu soa plus plu 2 cuson plus hs equals plus cus cual 3 plush plus ac equals cas plus 4 cas plus plus equals plus hs 5 cus plus 20 equals cu plus indirectly the first reaction shows what takes place when copper carbonate or oxide is dissolved in sulphuric acid and is a well known straightforward reaction reaction 2 shows that when the salui t tion of copper sulphate made by reaction 1 is treated with hydrogen sulphide the copper is precipitated as copper sulphide C m ar U COPPER PLANT Q and at the same time an exactly equivalent quantity of sulphuric acid is regenerated in the solution in other words all the acid which had been used for dissolving copper is here recovered this reaction too is well known and is practiced continually in the atory in the quantitative and qualitative estimation of copper the third thir d reaction is the first step in making the hydrogen sulphide gas calcium sulphate e gypsum is reduced by carbon coal this reduction takes place quantitatively according to all good text books at ata a bright red heat about 1800 de grees F the reaction is accompanied by both the formation of carbon dioxide and carbon monoxide but the carbon monoxide preponderates ates largely in the reaction I 1 have assumed as is done in most text books that carbon monoxide only is formesi for medi now from the reaction the following is seen one molecule or parts of calcium sulphate sulp hati or parts of of gypsum require 4 atoms or 48 parts parta of carbon for dom complete reduction and yield 72 parts or one mole molecule dule of calcium sulphide from the fourth reaction we see that when calcium sulphide is treated with water and nd carbon dioxide it decomposes and calcium carbonate and hydrogen sulphide are formed it is practically the decomposition of a weak sulphide by a weak acid carbonic acid the reaction is mentioned in many books and is stated to go on easily and satisfactorily recently it has been applied forthe for the decomposition on an enormous scale in england of soda waste gastev which is large ly calcium sulphide A very high degree of efficiency is realized 95 per cent of the carbon dioxide being abb orbed and more than 98 per cent of the calcium sulphide cas recovered as hydrogen sulphide the method of proceed ure is fully discussed in wagners magners Wag ners chemical technology translated by crookes p 8 9 also in junges lunges Lun al kalie manufacture etc etc observing them the reactions once more we see that one molecule of cas gives one molecule of li hydrogen drogen sulphide i 1 e 72 apts of ca S gives 34 apts of hydrogen sulphide returning to reactions 2 we see that that one molecule of hydrogen sulphide or 34 apts precipitates one molecule of copper sulphide or 95 apts or one atom of copper 63 apts and at the same time regenerates an amount of acid sufficient to dissolve this amount of copper again it is clear then that one molecule calcium sulphate or apts of gypsum will furnish enough hydrogen sulphide to precipitate 63 apts of copper further for the reduction of this gypsum is required 48 apts of carbon it is customary graham otto vol 2 p to use 3 apts coal to 7 apts auly drous calcium sulphate that is to about 8 ftp gypsum this slight excess of coal is necessary to effect a quantitative reduction hence about 4 lbs lb of the mixture of coal and gypsum are used in precipitating 1 pound of copper or about 2 23 2 3 pounds gypsum or 1 li pounds calcium sulphide precipitate 1 pound copper the fifth reaction scarcely needs further elucidation it takes place in two stages first oxidation of part of the copper sulphide cus plus 30 90 equals cu 0 plus then cus plus equals cu plus then inte ration of sulphide and oxide with formation of metallic copper and elimination of sulphur as sulphur dioxide the reaction is of importance to us only in as much as it furnishes another and abundant source of acid As seen from reaction 2 all acid used for dissolving copper is recovered in the act of precipitation but most ores contain substances besides copper which consume acid principally lime acid lost in this way is not recovered in the precipitating tanks but is recovered by allowing the acid solution itself to flow down conden condensers sors up which sulphur dioxide mixed with air oxygen from the copper sulphide furnace are passed the hot air in conjunction with iron salts in the solution oxidize a large part of the sulphurous acid to sulphuric acid technical operation of process the accompany accompanying aing cut taken from blue prints shows two sections through the plant the larger is a side elevation of the main plant showing mill leaching department and precipitating department the smaller is a side elevation of the sulphide plant in which the gypsum is reduced to calcium sulphide the mill and leaching departments are very similar to the corresponding department in a cyanide plant and very little need be said regarding them an aa seen from the drawings the ore from the mine mind is dumped into the upper bin and passes from here over a grizzly thi through lough a crusher into the second bin which has a capacity of about tons from here the ore is fed automatically as usual to the first set of rolls whence it t is elevated screened re rolled and screened again finally dropping from the second screen into the third bin the fineness to which the ore is crushed depends on the character of the ore but ordinarily it will be about 15 to 20 mesh it is now ready for leaching the liquor which we use for leaching the ore is made up of water containing about 5 per cent sulphuric acid together with varying percentages of iron sulphate and common salt this forms a very powerful solvent for the oxides carbonates bo nates and silicates of copper and even attacks the sulphides sulp hides to some extent particularly ticul arly when air is injected into the solution silver in all its forms is attacked and dissolved owing to the presence in the solution of the chlorides of copper iron and sodium gold is easily dissolved by addition to the solution of any oxidizing agent such as bleaching powder manganese dioxide etc to this powerful solvent and to the fact that it can be maintained at very small cost is owing a large part of the success of this process the leaching may be carried out either in wooden percolation vats such as are used so extensively in cyaniding cyan iding or else the barrel agitation system may be used which is the practice today in the modern gold chlorination mill the latter plan is shown in the cut the ore from the hopper is charged into the revolving tank and is then agitated with the proper amount of solvent As a rule the values are in solution within an hour the entire charge is then dumped into the filter beneath or if the tank contains an internal filter filtration and washing may take place within the solution running either first through a clarifying filter or directly into the solution tank the tailings are discharged directly into the tailings car by a few revolutions of the tank and hauled away to the dumps when the percolation system is used the practice of dissolving and washing is nearly the same as in cyaniding cyan iding the tailings are best sluiced away to the dump where water is plentiful the values are now all in the solution which is stored in solution tanks of ample capacity so that a slight stoppage in the milling or leaching departments would not interfere with the precipitation we are ready now for precipitating the values As already explained this is done by forcing hydrogen sulphide gas through the solution this gas precipitates not only the copper but the silver and gold as well so that all these metals are obtained in one operation the form of plant which is used with great success for effecting this precipitation is shown in end view beneath the solution tank it consists of a series of four closed lead lined or wooden tanks connected together by pipes and valves so BO that the gas entering no 1 will pass successively through the solutions in no 2 no 3 and no 4 and thence escape into the air the absorption of the by hydrogen drogen sulphide in the gas Is usually completed in the first two tanks and the precipitation takes place about as fast as the gas can be pumped in As can readily be seen tank no I 1 will be completely precipitated before the others when this has taken place the valves connecting it with the series are closed and others are opened 0 bened so that the flow of the gas may not be interrupted the liquor which now contains the black precipitate of copper sulphide together with the regenerated acid is allowed to run into the sump tank beneath the tank is recharged with fresh solution and made no 4 the former no 2 no now noi i becoming no 1 the precipitation thus goes on continuously and without interruption the liquor from the sump tank is forced through the wood plate filter presses as shown the copper sulphide accumulating in the frames in the form of hard compact cakes while the acid liquor runs by gravity into the wooden acid tanks beneath from where it is returned as needed to the leaching department for treating fresh charges of ore the copper precipitate contains 66 per cent copper and 34 sulphur and is fed into the small upright roaster where by the combustion of its sulphur contents it is roasted until 23 2 3 or more of its sulphur is expelled this product on being melted gives a very pure copper its ita purity varying from 98 to fine depending on the care with which the melting was conducted we come now to that department in which the hydrogen sulphide gas is made As stated the generation of this gas involves two steps reduction of gypsum to calcium sulphide ad decomposition of the calcium sulphide to form hydrogen sulphide the small drawing represents a section through the sulphide house showing a side elevation of the plant for handling and reducing the gypsum coal and gypsum are stored sep separately aracely in the upper bins from where they pass through a crusher into a second bin from where they are fed into a ball mill which reduces them to about 30 mesh A belt conveyor carries them into their respective bins the bin to the left is for coal that to the right for gypsum each of these bins is supplied with an automatic feeder which drops the material in the correct proportions into a screw conveyor this serves to mix the materials and at the same time conveys them to a ball mill where the mixture is completed the powdered mass is now lifted automatically and continuously into the rear end of a revolving furnace it slowly works its way through this furnace finally passing through the hottest zone and the finished material falls through the spout into a cement trough from where it if is picked up and car t tied ried to the steel bins in which it is stored j for use from here it is taken to the bin and mixing tank shown in the main draw ing where it is stirred with enough water to form a thin mud and drawn off as needle ed into the decomposing tanks beneath these are a set of iron tanks exactly like tho the precipitation tanks the calcium sulphide mud is fed into these and is decomposed by carbon dioxide gases which are pumped through when the first tank becomes exhausted it is cut out emptied recharged and made the last of the series exactly as in the case of the precipitation tanks the carbon dioxide used for decomposing the sulphide is obtained from the reduction furnace and averages 25 per cent to 30 per cent carbon dioxide furnace gases of any kind may be used as a source of carbon dioxide the decomposition goes on continuously and the hydrogen sulphide which results passes right on without intermediate ter pumping through the copper so lution in the precipitating tanks in the drawing the sulphide tanks cannot be seen as they are directly back of the precipitating tanks this process is especially adapted for the treatment of low grade copper ores sulphide ores are given a rough dazing roast before leaching practically the only raw material needed is gypsum which is abundant and widely distributed in this western country its reduction may be carried on at the mine or the sulphide plant may be situated to best advantage with respect to gypsum and fuel su supply aply the finished I 1 sulphide is then shipped to the main plant at the mine since the amount of this sulphide necessary for producing one pound of copper is only about ia pounds the near proximity of gypsum beds to the mines is not an essential to the economical working of this process also one sulphide house may supply calcium sulphide to a number of plants situated at the mines among the advantages of this process may be mentioned the following 1 A solvent whose action is rapid reliable and powerful 2 A precipitant whose action is rapid reliable and complete 3 the hydrogen sulphide gas throws down no base material from the solution and thus gives a copper precipitate of very great purity 4 the solvent is being continually regenerated hence no expense is incurred for acid 5 it may be worked at the mine remote from railway transportation the cost of working a copper ore by this process depends but little on the grade of the ore a 2 per cent ore being nearly as cheap to work as a 5 per cent ore per pound of copper produced the principal difference being in the mining and crushing under average conditions the cos cost of 16 treating an oxidized ore would be less than 3 cents per pound of fine copper the gold and silver being clear profit treating a sulphide ore would be somewhat more expensive owing to the expense of roasting |