| Show ae S alphur U D dioxide I 1 ox I 1 de method for determining I 1 n I 1 ng copper copp er M minerals in jr in 1 n partly oxidized dl zed ores BY CHARLUS CHARLES E BARNEVELD AND EDMUND S LEAVER milling practice at the porphyry copper mines of the southwest is in a transition stage because of the rapid changes following in the introduction of notation flotation among the several problems that are receiving earnest attention from the operating companies is the recovery of the copper minerals forming the complex ores of the oxidation zone of the deposits this problem is under investigation at the tucson experiment station of the federal bureau of mines A survey of the status of present laboratory practice brought out the need of a correct and rapid method for the selective determination of the quantity of copper in the sulphide form on the one hand band and of that in the form of combined oxides carbonates silicates and native or metallic copper on L le ie e other hand in partly oxidized ores and in mill products from these ores such a method is necessary for control determinations on flotation mills treating sulp ores and also on works using either acids or al balines as the active solvent for the copper in oxidized and silicate form for convenience in this report such copper is termed oxidized copper the in common use for selective determination of copper minerals are the sulphuric acid method and the ammonia method both are unsatisfactory sulphuric acid method the sulphuric acid method in which a 5 per cent solution of cold sulphuric acid is used to dissolve all the oxidized copper is unreliable for several reasons A certain amount of metallic iron ranging from one tenth to one half of one per cent is nearly always present in the sample introduced by the various crushing mill and pulverizing laboratory processes this metallic iron precipitates more or less metallic copper from ane copper sulphate solution formed by leaching biti dilute sulphuric acid thus giving correspondingly low oxidized copper returns another source of error in the same direction is the fact that dilute sulphuric acid dissolves only one half the copper present as cuprite cua 0 A serious source of error in the opposite direction making for high oxidized returns is the fact that co and are more or less soluble in dilute sulphuric acid see table 1 hence more or less copper in sulphide form is reported as oxidized copper it sometimes happens that the plus error due to solution of some of the copper copper in united states bureau of mines department of the interior washington D C technical paper sulphide sulp nide ilide form is largely compensated by the minus error due to partial solution of cuprite and to deposition of copper on metallic iron the majority of the ores tested by the writers show high oxidized and low sulphide returns and aneck samples show considerable sid erable variation the results of tests with and showing the solubility of the copper content in different reagents are presented in table 1 following table 1 results of leaching and co with 5 per cent solutions of different reagents the mixtures were agitated two hours on a bottle rolling machine five grams of pure mineral was used for each of the mesh sizes indicated was developed by H D hunt and R V thurston of the research laboratory of the i miami copper company this method was n brought to the writers attention after the 11 completion of their own work on the sulphur dioxide method described in this paper A description of hunt and thurstons Thurs tons which may be called the sodium method has been published in the september 1917 issue of the colorado school of mines magi azine the procedure is essentially as foli lows procedure place 2 grams of pulp in a beaker add 20 c c of a solution of caustic soda sodium and boil the mixture gently five to ten minutes with occasional shaking of the beaker to the hot mixture add 25 c c of a 20 per cent solution of ammonium sul proportion of copper content leached beached from with solution solution mesh ul g M H B a s U M art cr 0 g 3 ps 0 B it at ilal 5 0 0 0 p a 0 h C 0 0 a 0 r j pa p D Z is i s S B Z S S D 5 nt S 3 20 to 40 38 7 none 7 none 40 to 65 do do 65 to 1410 do 02 do to 1660 do do to 1710 do do to 2270 do 0 do 1 1 1 1 ammonia method the ammonia method in which a 5 per cent solution of either ammonia or of ammonia and ammonium carbonate is used to dissolve the oxidized copper is even more unreliable than ane sulphuric acid method the results in table 1 show that both co and when pulverized are decidedly soluble in solutions of ammonia and ammonium carbonate the riner liner the material is ground the greater is the degree of solubility on the other hand complete solution of the oxidized copper is rarely attained this defect is most noticeable in ores having a siliceous gangue and in ores carrying the solvent gela the silica and it appears that thal the formation of this gelatinous spongy mass prevents complete decomposition of some of the copper mineral in addition to holding dissolved copper during nitration filtration sodium method while the investigation at the tucson station was in progress an improved method phate heat for ten minutes filter wash several times with a hot solution of ammonium hydroxide and ammonium sulphate and finish the washing t with hot hoi water neutralize the filtrate with sulphuric acid and add 2 c c of concentrated nitric acid the filtrate is now ready for elect electro rc J a the first treatment with of caustic soda and sodium complete 01 and rapid solution of the aup au K 1 tp p malachite and and barci parc lt il of the cuprite the rest of the cuprite morite is dissolved later the chrys alsa byc goes into solution care must be 1 jin t not to boil to dryness or to a small vo volu w the reagent must be of sufficient str jr n to ane copper hydroxide ali formed by the action of tile the solution on tl tb oxidized copper minerals in the sample the ammonium conium in sulphate solution has 1 t tv wo o fold function it converts the excess sr hy hunt IT 11 D and thurston R V the determination t ermi nation of oxidized copped copper in oreb colorado V school of mines mag vol 7 sept september elliber 1917 pp ap as I 1 drate te into sodium sulphate and it forms am hydroxide which in turn dissolves Lis solves remaining cu counte prito any ny i ons required the following 9 stock solutions are required sodium j lydgate sodium solution lution consisting A i grams of sodium hydrate drate ana 50 gr gios of sodium dissolved solved in 1000 c c of distilled water ami conium sul Phat f solution consisting of grams of amm ammo illum sulphate in 1000 c c of distilled adi baic c i and ammonia ammonium sulphate for washing prepared by adding c c of 0 ammonium hydroxide and grams of ci LL sulphate to 1000 c c of distilled water possible Pos sibe sources of error in testing out this in method ethod the writers i found that it gives fairly satisfactory re 1 suits on the low grade porphyry ores for which it was developed the method is f however subject to two well defined macs cu I 1 one of these arises from the fact that r is decidedly soluble in ammonia washing with hot solution of ammonia and I 1 ammonium sulphate unquestionably dis solves more or less copper in the form of from the mass on the filter paper several tests were made of 2 gram grain samples containing 3 to 4 per cent of oxidized material and varying proportions of mixtures containing 8 16 and 24 per cent copper as showed respectively s OM and per cent dissolved in washing and erroneously reported as excess oxidized copper another source of inaccuracy as regards ores containing results from the i difficulty of washing all the dissolved copper from the gelatinized gelatini zed spongy mass on the filter paper mixtures containing 21 2 to 3 per cent copper as showed losses ranging from to 01 per cent and mixtures containing 5 and 1 per cent in this form respectively snowed losses of and per cent these errors tend to balance each other for ores containing both chalco chalaco cite and in small amounts in analyses of ores containing less than 5 per P cent copper the resultant error see table 2 is not serious sulphur dioxide method the experiments wita the sulphur dioxide method were made with the following requirements quire ments in mind t 1 the method must be applicable to a wide de range of ores and must be tio thoroughly roughly reliable and rapid 2 copper sulphides sulp hides especially the more easily decomposed must not be affected by the reagent used 3 metallic iron must not affect the determination the method evolved is based on a large number of experiments the results of which show conclusively that cuprite malachite azurite and metallic copper when finely pulverized are readily and completely soluble in sulphur dioxide solution sulphurous acid copper sul aphides are not attacked no matter how finely pulverized the mineral may be nor how long the time of contact the two essential features of the method are ul 11 fine pulverization in order to completely free the particles of copper minerals from the gangue 2 the powdered mineral must her f al 0 A S c f C ZA yi afy 5 r figure I 1 be kept in suspension by shaking or rolling during the or solution period procedure the procedure in the sulphur dioxide method is as follows place 2 grams of pulp ground t round to a fineness of 0 f to mesh in a bottle add c c of a 3 per cent solution of sulphur dioxide seal the bottle and agitate by rolling one half to two hours filter wash the residue with sulphur dioxide solution add the th e washings to the filtrate which will contain contafi n in solution all oxides carbonates and cates of copper and all metallic copper add 5 to 10 c c of nitric acid doll boil down to 20 c c dilute with distilled water to c c and determine the copper by the electrolytic method in the usual way the residue from filtration contains the unaltered and copper sulphides sulp hides in the experimental work the copper present as sulphide was separately determined in order to check the determination of oxidized copper ordinarily this step would not be necessary in analyses of the low grade porphyry copper ores of the southwest the sul aphides may be readily decomposed and all the copper dissolved by as follows to the residue add 5 c c of sulphuric acid and 10 c c of nitric acid ana boil until dense white fumes appear add 5 c c of nitric acid and dilute with distilled water to c c determine the copper by the electrolytic method this method of determining ter mining copper in the residue is not suited for heavy sulphide ores containing interfering bases and for such ores standard methods should be used preparation of solution although sulphur dioxide solution sulphurous acid may be readily purchased it is decidedly unstable hence the solution should be prepared in the laboratory as needed small quantities are easily made by adding moderately strong sulphuric acid to scrap copper tinned on one side the resulting sulphur dioxide gas is absorbed in water for continuous work it is better to purchase liquid sulphur dioxide in steel cylinders and drums which are obtainable in sizes ranging from six pound to pound capacity in the first experiments at the tucson station the sulphur dioxide gas was introduced trod directly into the bottle containing the water and pulp much loss of gas resulted and the procedure ure was ot otherwise berwise unsatisfactory later the simple apparatus shown in fig I 1 was evolved the absorption tower a 43 inches long made of three fourth inch to one inch glass tubing and filled with broken broken hard burned fireclay fire clay is set at an angle ang ae of 75 degrees between two glass bottles b and d of three to five gallons capacity the bottle b being placed about five feet above the other bottle the tower is open at the top and sealed at the bottom with a plug of se sealing aling wax e through which two small glass tubes extend the upper bottle b contains distilled water which is siphoned into the upper end of the absorption tower the flow being regulated by a stop cock f A cylinder c capacity 6 to 50 pounds containing liquid sulphur dioxide is connected to one of the g glass lass tubes extending into the absorption tower on opening the valve of this cylinder the liquid sulphur dioxide issuing from the valve is gasified by the reduction in pressure and passes into the tower where it is absorbed by the water from bot tle tie b converted into solution of the desired strength and caught in the stock stack bottle tl e d this apparatus gives entire satisfaction with little attention a 3 per cent solution of sulphur dioxide may be produced at the rate of 3 liters per hour the cylinder containing liquid sulphur dioxide indicated in the sketch may be replaced with a gas generator strength of solution and time of contact considerable variation as regards strength of solution and time of contact will be necessary in treating different ores from different localities in general a solution containing 3 per cent should be used with some ores much weaker solutions as low as per cent will do the work merely introducing the pulp into the solution shaking the bottle for a few minutes and letting it stand will not dissolve the copper constant agitation is essential for a small number of tests a bottle agitating machine will give ive satisfactory results for analytical work where large numbers of samples are run as in a mine laboratory a bottle rolling machine will be found more satisfactory not only for this purpose but for all solutions requiring constant agitation the time of contact necessary to completely dissolve the oxidized copper minerals was found to vary from one half to two hours most of the ores and products tested gave complete recovery in half an hour and the most re factory ores yielded in less than two hours testing strength of solution to determine the strength of the sulphur dioxide solution the following adaptation of a well known reaction is recommended it is based on ane fact that introducing either weak or concentrated sulphurous acid into a solution of iodine will result in the complete oxidation of the sulphur dioxide prepare an iodine solution by dissolving grams of potassium iodide in distilled water adding 84 grams pure re sublimed iodine and shaking until the iodine is completely dissolved the more concentrated the potassium iodide solution the morehead ily will the iodine dissolve bring the solution to proper strength by adding enouf enough h distilled water to make a volume of one liter then standardize the solution by the method using starch indicator the determination is made as follows to a measured quantity of standard iodine solution add slowly with constant stirring the proper volume of sulphur dioxide solution so regulate the volume of iodine solution used that the mixture always contains a decided excess of iodine over the quantity required to oxidize the sulphur dioxide being added an excess of sulphur dioxide causes the solution to clear and to lose its dark red color if an excess of sulphur dioxide is added the determination is spoiled and the test should be repeated with fresh sulphur dioxide solution and larger quantity of iodine solution thus there is a direct ratio between the strength and quantity of sulphur dioxide solution and the quantity of standard iodine solution in general for solutions containing I 1 to 3 per cent sulphur dioxide I 1 c c and for weaker solutions 20 c c of standard iodine solution should be used the mixture is then by the method to determine the quantity of iodine remaining in the mixture the difference between this quantity and the total quantity of iodine represents the iodine presented in table 2 the table shows that compared with the sulphur dioxide method the results for oxidized copper are high wit aitu the sulphuric acid method and low with the ammonia method the sodium method shows uniformly and slightly |
