OCTOBER, 1946 229 solution now enters th e first of four dispersing units, in series. These are towers packed w ith grids over which th e solution trickles down, counter-current to blower air which sweeps th e hydrocyanic acid gas up a n d out to the absorber. This is a d u ct or tunnel, 40 ft. long and 24 sq. ft. in cross-section, close-circuiting the disperser w ith th e blowing fan inlet.
The alkaline absorber solution flows gently along th e slightly-sloped tunnel floor and is picked up in its passage by a series of six rotor sprays which m aintain a m ist of absorbent, a protecting baffle preventing m ost of th is m ist from being draw n to th e blower. The absorbing agent is a weak lime slurry which trap s the CN as calcium cyanide.
Absorber units w ork in parallel. In this p lant 91|-% of th e cyanide available for regenera should investigate b o th aspects of th e silicosis hazard. A typical dust-counting program m e is concerned to find th e m axim um contam ina
tio n to which a worker is liable to be exposed an d is accordingly interested in th e collection of samples head-high and in such places as will ever contract pneumoconiosis through w orking in th e surface crusher stations, konim eter counts, and doubtless th e increased use of flotation in gold extraction w arran ts scrutiny of th e dust-producing conditions above th e cells. I t is n o t always realized
th a t b u rstin g bubbles can launch dust
solidated Mining an d Sm elting Co. of Canada.
The tin occurs as particles of cassiterite handled daily could be achieved in reasonable space. The B uckm an table, w hich has been running since 1941, was developed to satisfy th e requirem ent. I t consists of five alum inium tra y s, each 6 ft. square, supporting a rubberized baffled m a t across which th e mill tailings flow. Twelve such units, worked- b y a 1-h.p. m otor, are ru n and successively tilte d to 45° an d flushed, in a cycle tim ed for 190 seconds, th e feed, tilt, flush, etc., being m echanically synchronized. This quick cycle exploits th e fact th a t th e g a th e r three m en per shift suffice for p lan t operation.
The shaking tables, b y receiving a high-grade acid-proof m echanical flotation cells. The reacted pulp is brought to a p H of betw een 6 and 7 w ith lime (0-914 lb ./to n ), conditioned, and floated (rougher, cleaner, a n d recleaner) w ith th e occasional use of an am yl alcohol
1 E ng. M in . J o u rn ., M ay, 1946.
OCTOBER, 1946 231
Mining Camp in Venezuela
frother, th e copper float being tab led to separate out th e residual lead, which joins the flotation cell tailings to provide a high- grade lead concentrate.
Sulphur dioxide is generated by burning some 40 lb. of brim stone hourly, th e resu ltan t gas being absorbed by w atersprays to form a solution of H ,S 0 3 containing 0-3% of S 0 2- After flowing over iron scrap to produce some ferrous sulphite, which has been found an im p o rtan t condition for sm ooth process control, th e solution is ready for use. Starch is used as a pulp disperser to depress slime lead, which m ight otherwise be entrained into th e float. T he flotation feed is all
— 200 mesh and 80% — 325 mesh and cell density is controlled between 10% and 15%
solids. Preferential flotation of copper is achieved b y dry-fed lime bringing th e pulp to p H 6 or 7, when galena and sphalerite sta y down.
(18) Gold.
Amalgamation Reviewed
A t 20° C. gold can be dissolved in m ercury to 0-06% . This, together w ith such pheno
m ena as recovery of gold particles by dissolving th e m ercury in the am algam w ith H N 0 3, indicates th a t th e association of the tw o m etals is of th e n atu re of solid-in-liquid suspension ra th e r th a n tru e solution, in which th e physical id en tity of the gold is lost.
This is in spite of th e fact th a t compounds of th e tw o elem ents are known. As a working approxim ation it is practical to th in k of gold as being w etted by m ercury w ith a sufficiently deep sorption to create an unusually stable pulp. This can be “ filtered ”
through chamois leather to rem ove excess liquid and leave a plastic residue of m ercury- wet gold.
B roadly there are three m ethods of applying th e am algam ating technique—by contact during grinding, contact during flow m ovem ent, and pressure contact. All of these are v arian ts of th e sam e basic idea, which bears a considerable resem blance to th e flotation process. In b o th separation is effected by differentiating betw een an ore constituent which floats on the liquid m edium and one which is w etted by it and sinks. In both a certain am ount of chemical or physical p reparation is called for. In th e case of am algam ation th e id e n tity of th e solid is lost by deep sorption in an opaque liquid. T h at liquid—m ercury—
m ust, however, be chemically conditioned by preserving it in a suitable condition, free from substances which would “ sicken ” or contam inate it and depreciate its ab ility to w et an d contain th e gold. Since am algam a
tion selects th e heaviest ore constituent as a ,“ sink ” product and uses a heavy medium for th e liquid phase, th e process also contains points of analogy w ith sink-float separation, where a mobile m ercury b a th is m aintained, although th e deep sorption w etting of th e solid gold b y th e liquid m ercury, w ith subsequent edge adhesion into the characteristic plastic p u tty , produces a very different appearance in th e " sink ” fraction.
The oldest m ethod, which would seem to go back to the pre-Christian era, calls for grinding in th e presence of m ercury. P late am algam ation after grinding was widely used in this century u ntil th e cyanide process brought in o th er factors, so th a t to-day it
is little used. G rinding m ore or less con surface control w herever reaction depending on sorption is required. W hen a strak e co ncentrate is ground in a barrel, perhaps w ith control of th e pulp alk alin ity and chemical tre a tm e n t to discount th e con
ta m in atin g influence of ore constituents liable to upset th e m ercury, th e operator is actually period in barrel am algam ation which precedes th e ad d itio n of th e m ercury becomes a con
ditioning tim e, to be followed by a differential- liquid flotation. D uring this conditioning tim e surfaces are cleared an d prepared, m ercuricides are neutralized or flushed aw ay, an d th e gold is liberated from any encasem ent.
In addition to th e above m ethods pressure