Show dolomite as a substitute for ume lime in recovery of gold and silver by cyanide process by E S leavera leavea C W davis davisa and J A woolf dolomite has recently been sold as a substitute for lime in cyaniding cyan iding gold and silver ores the calcine being cheaper per ton than lime it has been usually believed that the magnesia content of the commercial lime has no practical value in the cyanide process processo 4 julian and smart state that magnesia may be used in place of lime but a larger quantity is required in order to understand the facts recorded in the following pages it should be remembered that magnesia is practically insoluble as compared to lime lime in the cyanide process in cyanide work lime is used for two purposes 1 to prevent loss of cyanide and 2 to cause precipitation of undesirable substances and flocculation and settling of ore slime the primary object is to increase the extraction of precious metals lime prevents loss of cyanide in several ways it neutralizes tra lizes acidity thus tending to prevent the formation of and reacts with any formed especially during precipitation of gold and silver preventing its escape lime also combines with carbonic acid and other substances which would otherwise consume cyanide and replaces cyanide in compounds which have already formed liberating cyanide to usefulness A large excess of lime however has been found to increase the cyanide consumption ign over that resulting when less lime is used lime precipitates some substances which would interfere in the cyanide process causes coagulation and promotes the settling of slimes if the precious metals are combined with or mechanically held within certain base minerals not attacked by a cyanide solution the presence of lime may cause a reaction with avith mineral and thus permit the dissolution of the gold and silver A high alkalinity due to excess lime in solution however usually decreases the rate of solution of gold and sil ver r this in the case of soluble sulphides sulp hides may be due to the reducing educing action and to the removing of oxygen that is necessary for dissolution of the precious metals dolomite in preventing cyanide loss for neutralizing acidity theoretically should be weight ight for weight more efficient than cao in the ratio of 28 to 20 in a practical test magnesia was found to be three kinles mes more effective than lime for neutralizing mine waters far or use in in cyaniding cyan iding where a solution containing 0 bo was passed through aa to 14 inches aches in i diameter at the rate of 35 liters per hour the suiting solution was slightly alkaline 7 reacts with CO coz and with SO boz giving a slightly Il saline kaline solution since readily reacts with to form Mg Z CN a good for gold and silver an excess metallurgist metal atals 5 ex 1 and superintendent rare and precious of rill lines nes in station st atlon reno keno nov nev department of the interior bureau assistant cooperation with the university of nev nevada ada junior chemist bureau of mines L 4 metallurgist bureau of mines 1 it i degraw clener clenne 1111 ll 11 i J 1 13 the cyanide handbook ak 2nd and ed cd 1915 P 7 eng en li erbert X A in the cyanide process ana W ml abert chemicals used esti est italio tion n jour T vol 87 97 1914 p bahnen luther W rapid of cyanide pr access Tr available calcium oxide in lime used in the trans A I 1 N 1 E vol 42 P ajulian if J edja wing iding gold and silver res lid d forbes and smart ar edgar cyan cyaniding 1907 p 34 a pa ile li liw W J laboratory tests on the use of coarse and 8 vol vola 1808 ane for cyaniding Cyan iding ine tour jour chem met soc south africa P D 6 60 0 clena ciena ile practice 19 1913 1 p I 1 J E L barren vAr ren 11 1 1 W cyanide A work cited P 1 13 1 3 1 byor and min jour J 0 u VOL 1919 pm m S cited ct P of cyanide cit magnesia should be suitable for preventing Pv enting loss of free our laboratory tests show that magnesia added to tb dilute solutions of ferrous and ferric sulphates readily reacts causing precipitation of iron salts the resultant solution is is slightly alkaline this indicates that magnesia is as efficient as lime in in combining with many substances that consume cyanide in practice the iron salts should be oxidized to forms that would not react with cyanide although magnesia may be efficient in preventing loss of cyanide it may react to form soluble magnesium salts and in the absence of lime a saturated solution of magnesium salts may result if the solution be used repeatedly for treating new batches of ore A few tests showed that a saturated solution of magnesium sulphate caused a cyanide loss as high as 18 pounds per ton in clean solutions and this loss would of course be excessive in practice if an excess of cao either as lime or as dolomite be added to a solution containing magnesium sulphate magnesium hydroxide and calcium sulphate are formed these salts being practically insoluble are eliminated from solution A series of experiments using different amounts of magnesium sulphate together with an excess of either lime or dolomite showed no excessive sive cyanide loss thus confirming the use of cao as a corrective for presence of excessive magnesium sulphate in using dolomite to neutralize acidity most of the cao would react first the content would then react to neutralize further acidity the reactions may be expressed as follows ai C aoh OH 2 caso 20 hoso ngoh magso ah 0 the protective alkalinity would be low due to the slig slight ht solubility of magnesia in cyanide solutions however with certain ores low alkalinity is desirable elimination of magnesium sulphate if more dolomite be added to the solution a further reaction takes place caso this shows that the content of dolomite is effective in neutralizing acidity but is detrimental when the solution is to be used continuous continuously sly for treating fresh soluble magnesium salts to build batches of ore by causing in solution unless enough excess calcine is present for up salts in the soluble i its cao content to react with these latter case as much cao is required to precipitate magnesium hydroxide as would be necessary to neutral neutralize ize acidic acidity y in the first place A senieff derieg of tests was made in which a cyanide solution used several times to treat new batches of ore there was being added to each batch a quantity of magnesium sulphate ds ton of ore and an excess equivalent to 60 poun pounds per of dolomite the silver was demov removed ed from the zinc dust after each extraction cyanide was solutions by concentration was kept at the added in such quantities that concen ton of solution the cyanide of per a about out 4 pounds free from solutions were normal and the tion was consumption I 1 is dolomite is salts showing that axce excess SS magnesi magnesium effective tim in preventing c cyanide loss and in in preventing soluble solution lution is is building when a so up from salts magn magnesium estu cyanic ides and cont containing aaning used several times for treating ore soluble magnesium salts r protective alkalinity Alkali all our tests confirm usual practice in that the loss of cyanide is considerable when no al akline compound espres is pres ent but is greatly reduced by adding the proper quantity ot of protective agent the results of the experiments showing the relational relation of r cyanide consumption or loss to the kind of alkali uselin used in dacate that for each ore there is isa a quantity of lime for which the cyanide loss is at a minimum and that large deviations from this optimum in either direction will increase cyanide consumption in this favorable range dolomite or magnesia although their protective alkalinity slight solubility ubi lity is less have about the same value as lime for preventing cyanide loss where an insufficient amount of protective agent is added lime is perhaps slightly more ef than an equal weight of dolomite but if there is a considerable excess of alkali present the excess dolomite does not consume as much cyanide as an equal weight of lime this suggests that the excess cao which is relatively quite soluble may be detrimental while the dolomite having less cao than an equal weight of lime consumes less cyanide whenever a large excess of alkaline agent is used the it protective alkalinity is as great with dolomite as with lime since three is sufficient cao in the dolomite to give a saturated solution of calcium hydrate in tests where the ore had been treated with previous to cyanide extraction the dolomite seems to be superior to lime which in turn is more effective than tha n naoh in preventing cyanide loss dolomite in settling slime julian and smart have shown that it requires grams of magnesia to cause the same settling effect as grams of lime indicating that for certain ores lime ma may y be more economical than dolomite as a settling agent some of our experiments show that magnesia is satisfactory for settling slimes from cyanide solutions but that the foll lowing wash with water causes co conditions editions to change so that settling is very slow tests with other ores indicated that dolo dolomite mite has about the same coagulating and settling effect as lime except that in some instances the solid matter does not settle as compactly in a few tests the settling effect of c dolomite e was more lasting than that of lime but since impurities fines method of burning and method of hydrating have a influence great these tests are not conc conclusive lusi ive filtration it has been suggested that if dolomite be used instead of lime in the cyanide process magnesium salts would build up in solution and cause difficulty in filtration and in zinc precipitation we have shown previously that the presence of excess dolomite prevents the of accumulation soluble magnesium salts but that hydroxide is j magnesium formed as an insoluble bulky precipitate which might retard filtration in order to de determine lermine the effect of soluble ailts and the presence of magnesium m precipitated on the rate of filtration a buchner magnesium hydroxide funnel was fitted with heavy canvas disk i a as a filter medium and suction flask which in turn was connected attached to a and an aspirator the to a manometer vacuum was I 1 kept constant at about inches of for all mercury the tests it was found f 0 und that eliat time of filtration in the test where had been added to its saturation magnesium magnesia magne siu m sulphate point was w as about the as that of the sample to which no magnesium same been added these results indicate sulphate had that magnesium magnesia magne siu m salts should not cause difficulty in A precipitation when zinc dust was used to precipitate silver from pregnant solutions saturated with magnesium sulphate no trouble was encountered extraction it has been shown that for the ores tested when optimum amounts of alkaline agent are used dolomite is as effective as lime in preventing cyanide loss and causing settlement of slimes if dolomite has no more detrimental effect than lime on the plant processes a point which would have to be tested at the plant the suitability of the former as a substitute for lime in the treatment of ores can be determined from the value of the precious metals extracted and the cost of the alkaline agents used in the two cases gold extraction the results given in the accompanying table indicate that for the ores tested the extraction of gold was slightly better with magnesia than with either lime or dolomite mt e and that the 9 gold old recovery was the same for lime and dolomite table is omitted sliver silver extraction two series of tests were made to show the effect of magnesium sulphate on the extraction of silver from an ore in the first series the cyanide solution was saturated with magnesium sulphate and the same solution used to ex tract three batches of ore the silver being precipitated with zinc and fresh sodium cyanide added after each test the extraction of silver was very poor and the results rather erratic this was probably partly due to the fact that since the cyanide loss was great it was difficult to keep sufficient cyanide present to effect solution of the silver this shows that if a solution became saturated with magnesium sulphate negative results are to be expected in the second series a cyanide solution was used repeatedly peat edly to extract three batches of ore to each of which epsom salt equi equivalent valent to 60 pounds per ton has been added excess dolomite was used in each test and as in the previous series the silver was removed by b Y zinc dust and fresh cyanide added after each extraction A check was run in which no magnesium sulphate was used this series shows that the presence of excess calcified dolomite prevents cyanide loss from a solution originally containing conta containing inin g magnesium sulphate buts but that the extraction of oi silver from an ore by cold solutions is lowered by the presence of magnesium sulphate A summary of all our tests indicates that under proper prop operating operatic r conditions the recovery of silver is about the ir same wi with th dolomite as with lime summary the results of our experiments indicate the na following a in regular cyanide practice where cyanide of tion is to be used repeatedly for extracting fresh batches batan used ore the content of dolomite can not be DC advantageously to neutralize acidity used j to 1 where an excess of dolon dolomite lite is is u the prevent the building up of magnesium salts in solution content takes no part in the reaction ne 2 if just enough cal dined dolomite is used eventually to 1 ny tralice acidity magnesium salts will build up and eve s N form a saturated solution in which the cyanide iosa 1055 bior 1 is S be high unless an unusual amount of bleeder sow run to waste calcified joined t b in the cyanide treatment of certain ores ore s dolomite may be substituted forline for lime with profit in pre 1 dolomite is as effective as lira li e 3 venting cyanide loss its pre P lt 2 Cac lined dolomite nearly parallels limed in r and settling effect and causes no difficulty in filtration of the solution or in the precipitation of silver by zinc 3 dolomite equals lime in the quantity of precious metals recovered c in treating certain ores by the cyanide process the recovery of silver with the use of dolomite as an alkaline agent is not equal to that obtained with the use of lime d the use of content of dolomite would therefore be confined to neutralizing acidity and aiding settlement where the ore is crushed in water for amalgamation ga mation prior to cyanide treatment and in cyanide practice where the solution could be discarded or bled sufficiently to keep down excess of magnesium salts dol dolomite should not be purchased as a substitute for lime in the cyanide process without a thorough investigation vesti gation as to its action on the ore to be treated and its effect on all operations connected with plant practice |