ALKALII SIJLFIDE LEACH- OF CINHABAR ORE A!TD ELECTROLYTIC DEPOSITION OF RCTJIY FROH LEACH SOLUTION by TH IL LIcLEOD A T:-]EsIs subnictet to the ORIGON STATT COLLEGE in partial fuilLment of the requirements for the degree of DOCT OR OF ?ILOSPiY June l39 A?PiO7D: Proft3 sor of Chemistry In Charge of Major -k Detartment of Chemistry Jead&I Chairman of School Graduate Conmiit,tee Chairman of State Collee raduate Council AcKNO;LEDGEiENT I wish to take this opportunity to ex'ress my gratitude to those who have helped to make this work possible. To Dr. Wci. E. C?ldwell, who directed this work, I wish to ex- press my deepest gratitude. 11e has given generously of his timo as a teacher and advisor and also as a friend who has contribi'±ed much to my experienc as a graduate student. Dr. E. C. ilbert and Professor (. W. Gleeson have also given much help with their suggestions and constructive criticisms. TABLE OF CCTTEIS Page ..... IIntroduction..... ......... ...................... J.. II Experimental Leaching of Cinnabar Ores ................... .7 . A. Particle Size & Leaching Efficiency ............. B. Strength of Leach Solution & Leaching Efficiency..lO C. DiscussionofLoachingResults ......... 12 D. LeachinginBallLrill... .......... .......l3 III Recovery of Mercury From Leach Solutions.. ........ ..... ..].5 A. Precipitation of erouro Sulfide by Means of CO2.15 B. Electroly±ic Deposition.. . .. . . ................. . . .16 C. Regeneration of Chemicals During Electrodeposition.17 IVEconomic aspects ..................... .. ............... A. Cost of Replacement Chemica1. .................... 20 B Power Costs.... ................................ ...22 C. Incidental Costs .................................. 22 VSummary. ............................................... ...24 I.. II. IDEX OF TAL3 AD CURVES Table Table ..9A Table III... ........................................... Table IV. .......................... . ...................... .9E Table V ................................................. ...9G Table VI ................................................. Table VII ................................................ .18A. 1s ur.î, e T .......... ... ............... .. ............ (r) Curve II. .................................................. 9D Curve Ill... .............................................. .9F Curve IV. ................................................. ALKALII SULFI LEACi OF CIITNABAR ORE and ELECTROLYTIC DEPOSITION OF MERCURY FRO:.I TLIE LEACi SOLUTION INTRODUCTION In his'corical vir.tings, dain back even before 400 B.C., 10 element mercury is referred o as iIydrargyrixn (1iqui silver) or as quicksilver. One of the earliest recorded reports of this element was made by Arisotle, 1hO called it fluidsilver. Other early authors to mention mercury were Agricola in Ms "De Re Metallica" and Deoscor- ides, who was the first to describe the distillaiion me+}od for recov- ery of mercury from cinnabar. Pliiï d Theophrastus experimented with this metal. In all the early aichemical writings the eal is referred to by the spnbol of the seedj messenger of the tods, the caduceus or wand. Ur.e of this symbol of ercury the messenger () shows a reco,nition of the speed at which the metal vanished when heated, and of its fluidity of motion when imder stress. Due to its physical and chemical properties the 'netalurgy of mercury is much simler than that of most 'netals. Thus, from the days of Deoscorides to the present time, 'nercury has been obtained from its ores b:r the vaporization and condensation method. The very simplicity of its metalurgy has caused a lack of careful s--.udy into the problems of the industry. In later years, however, there has n.- been a marked change in this attitude, and modern producers are keen- ].y aware of the problems that face them. In all industries the producer is chiefly interested in the most economical and most eomplete utilization of his raw materials. One of the big problems of the mercury producer is that of using low grade ores and getting almost complete removal of mercury contained in it. Other problems face the prospector who is developing a newly dis- covered mercury mineral deposit or the ovmer of a small mercury mine. The cost of installation of a furnace for treatment is usually more than a new and unproved property can stand. It is, then, desirable to establish or develop new ways of mercury recovery from low grade ores or at a property that does not yet merit or afford an expensive furnace installation. The essential features of these new develop- ments should be low initial cost of equipment, ease of installation, and simplicity of recovery of the mercury. The furnace plants are expensive and usually must be run several weeks in order to saturate them before metallic mercury can be obtained from the stills. The furnace must be run 24 hours per day with labor on a 3-4 shift basis. Small mines, with no proven reserve3, need some less costly method of recovering the mercury, so that with a amaller capital outlay they can be placed on a paying basis. Numerous schemes have been used to concentrate mercury oros; gravity concentration being the first one tried. Due to the high density of cinnabar, simple devices readily produced high grade con- centrates. With development of flotation methods, this principle has also been applied. Flotation methods also produced high grade concentrates because the cinnabar responds readily to flotation 3 reagents. Löfh of these fundamental mhods, vrhle successful as a reans of rodcing a concentra+e, have the handicap of roducing a very finely divided ooncentrte which can not be r:adily handled in the stills and furnaces. In order to get away from these difficulties in treatment, leaching methods have been suggested. L ehing of or's to extract the desired metal content is not new; the cyanide leaching of gold ores is on of the most .dely used leaching processes. Technique de- veloped in gold cyaniding might be ampli ed to more or's. Various tymes of leach solutions have been f r ed ex'erinehtally mercury ores. Among the methods discovered by Sohnahle and Louis Ori (3) is at devised by Sievelcing (5,6) "hich in'olves treaten± with cuprous chloride in the presence of an alloy of copper a:-id zinc. The reaction place Cu2Cl2 ji Ig3 - CuCl2 / CuS y" Hg The mercury algamates with the alloy an is later distilled. The chemistry of this method seems vague since cuprous chloride is water inslulbe. it also seems to be an indirect method since retorting of the mass for c'Ercury recovery i s required anyway. Tie method proposed by R. [agner () uses as he leaching agent, solutions of concentrated Hydrochloric acid and Bromine -"nter fo obtain the mercury as a soluble halide. A third ''ethod that is mentiöned by 3chnable and Louis (3) uses alkali sulfide solutions for the leaching agent. The soluhulity of mercuric sulfide in alkali sulfide solutions has been known for more than a century. In 1799 Kirchoff (2) described a wet method fr the preparation of vormillion which was based on this fact. Several of t- classical methods for the quantitative estimation 4 of mercury depend upon the separation of mercury sulfide from other group II sulfides by its solution in a solvent of sodium sulfide. The solubility is due to the formation of soluble complex double salts; much in the same way soluble. gold cyanide complex is formed. Ground red cinnabar or pure black mercury powder when dissolved with sodium sulfide yields a water clear solution. The reactions which take place are as foIlow: Na2S HgS a HgSiNa2S ' 2 Na S / HgS HgSi2Na2S The two complex salts are soluble and stable in the presence of al- kali, but in acid solutions are decomposed giving insoluble mercuric sulfide. It is necessary, therefore, that the leaching sol'itions have an excess of free alkali present at all times. In 1917-1918 Walter W. Bradley (1) of the "State Mining Bureau of California" experimented with California cinnabar ores using an alkali sulfide leach. In his work he found that ocher and other iron compounds caused trouble in the leaching process by using up sulfide in forming colloidal suspensions of iron sulfide. In ores containing little iron this problem is not important. The recovery of the mercury from the leach solutions is another problem which needs considerable study. The mercuric sulfide can be precipitated by lowering the pH of the solution, but once again a high grade concentrate is obtained. However, such a concentrate could be placed in a retort and the mercury distilled therefrom. Such treatment with strong acid would entail complete loss of the leach chemicals and hence might not be practical. One method proposed and patented for lowering the pH of the sul- fide leach liquors uses sulfur dioxide sas. The gas is forced through the solution forming sulfurous acid and thus neutralizing the excess base by formation of Na2303. This causes the mercuric sulfide to be precipitated from solution, and it is separated by filtration for further treatment. Â method of mercury reaovery from lfide solutions with a partial regeneration of the chemicals has been used at the Buffalo Mines, Co- balt Ontario, and described by Thornhill (7) in 1915. The process de- veloped for the precipitation of mercury frem the caustic alkaline sulfide solutions consisted of treatment with netallic ahvninm. The equations for the precipitation and regeneration are; 3 HgSNa2S / 8 NaOH / 2 Al 3 Hg / 6 Na2S / NaAlO2/ 4 H20 Ca(OH)2 2 NaAlO 2 NaOI-I / Ca(A102)2 i' This process will regenerate the sodium sulfide but part of the alkali must be replaced. The use of aluminum is also apt to be extensive. However, scrap metal and turnings are comparatively inexpensive and can be employed. In the west, a long way from the source of the cheap waste metal, its cost might be excessive. Other metals have been suggested as replacement for aluminum in this process, among t1se mentioned are chromium and zin. The use of these metals would also be expensive and it is probable that cheaper methods of reoovery are possible. The eleotrodeposition of mercury from the leach solution has also been proposed bi.th as yet very little work has been done on this phase of the problem. Schnable and Louis (4) in their handbook sug-