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TEXTBOOK ERRORS:' GUEST COLUMN XI:
The Production of Mercury
E. A. PERETTI University of Notre Dame, Notre Dame, Indiana
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extraction of mercury from HgS is carried out commercially by either ( a ) furnacing unconcentrated ore in the presence of oxygen or (b) retorting of highgrade ore or concentrates. Many textbooks2 do not mention the retorting procedure, probably because not as much mercury is produced in this fashion as by furnacing. However, the application of this procedure is increasing because of certain advantages. In furnacing, mercury ores containing about 0.5% mercury with over 99% valueless material have to be heated to about 650°C. with the production of large quantities of gas having a low concentration of mercury. This process requires a relatively extensive condensing system to recover the mercury. On the other hand, if retorting is practiced, a much smaller quantity of mercury-rich concentrate is heated with the production of a gas very rich in mercury, whose condensation can be carried out very simply and inexpensively. For example, one plantS reports treating by flotation an ore containing 0.67 mercury to produce a concentrate assaying 46.21y0 mercury with a metal recovery of 95.4%. The weight of concentrate made from 100 tons of ore was 1.39 tons, thus requiring - the heating- in retorts of less than l l / .z %.. of the original ore. The retorting procedure depends upon the fact that HgS sublimes a t relatively low temperatures with decom~ositioninto mercury and sulfur vaDor. The decomposition can be represented by:
than one a t m ~ s p h e r e . ~This ~vould he sufficient to maintain a continuous distillation if the retort were vented a t a pressure of one atmosphere. However, the rate of the back reaction is very great, and condensation of the distilled vapors results in the reformation of HgS. In order to prevent this, lime is added to the retort where it combines and forms solid compounds with the sulfur vapor as it is being generated. This reduces the partial pressure of sulfur vapor so that in order to satisfy the equilibrium relationship of equation ( 1 ) the mercury partial pressure can increase markedly. The total pressure can greatly exceed one atmosphere, thus facilitating continuous distillation. Condensation of the gas stream results in the formation of mercury with only small quantities of HgS. The reaction with lime is frequently written in textbooks: There seems to be no evidence that the reaction proceeds according to this equation. It is highly probable, in view of the ease of sublimation and decomposition of HgS, that equation (2) does not represent the mechanism by which the reduction is achieved. Simpler reactions with greater probability of occurrence can be postulated in which solid-gas reactants are involved. However, it can be shown that if the stable solid products resulting from KELLEY,K. K., U. S. Bui-&u of Mines Bulletin No. 406, 50 (1937).
with K, = Pas
J P ~
If no other gas is present, the total pressure will he equal to the sum of the pressures of the mercury and sulfur. At industrial retorting temperatures of about 620°C. the total pressure exerted by reaction (1) is greater Suggestions of material suitable for this column are eagerly sought and will be acknowledged. They should be sent with as many details as possible to Karol J . Mysels, Department of Chemistry, University of Southern California, Los Angeles 7, California. Contributors of discussions in a form suitable for publication directly will be rtcknowledged as guest authors. Since the purpose of this column is to prevent the s p r a d and continuation of errors and not the evaluation of individual texts, the source of the errors discussed will not be cited. The error must occur in a t least two independent standard books to be presented. Denver Equipment Company, Bulletin No. M4B24 (1942).
VOLUME 34, NO. 3, MARCH, 1957
The Caloium-Orygan-Sulfu. Diagram
the reaction or series of reactions are CaS and CaSOn, the ratio of 3 moles of CaS to one mole of CaSOP mill result as long as CaO is also present in the retort. This can be seen by examining the figure--the ternary phase diagram for the system, calcium-oxygen-sulfur. We would be concerned here specifically with the subternary CaO-CaS-CaS04. If we consider reaction (1) as taking place in a retort containing CaO, the Hg (g) can be viewed as an inert diluent, and the reaction between the sulfur vapor and CaO will result in solid products represented by triangle acd. The over-all composition of the solids in the system changes along db in the direction of the arrows, as retorting progresses. I t is evident that
a t any point along db such as y the ratio of CaS to CaSOI mill be constant. Line db int,ersects ac at the point vhere the total solid contains 33L/3mole yo each of calcium, sulfur, and oxygen, or 3 CaS to 1 CaS04. In order to test the validity of the foregoing, several sample mixt,ures of CaO and S were evacuated in glass tubes and heated for 3 days a t 510°C. These were examined with X-rays and the diffraction patterns compared with those produced by knowu mixtures of CaO with 3 moles of CaS to 1 of CaS04. The patterns shown on the films were identical. One can conclude from this that reactiou (2) accurately represents the stoichiometry in the retortiug of cinnabar.
JOURNAL OF CHEMICAL EDUCATION
