Gold Detecting and Prospecting Forum
Would you like to react to this message? Create an account in a few clicks or log in to continue.

RETORTING AND MELTING GOLD.

Go down

RETORTING AND MELTING GOLD. Empty RETORTING AND MELTING GOLD.

Post  Guest Wed Jun 15, 2011 1:04 pm

RETORTING AND MELTING.

The kind of a retort we require in our mill-plant,
depends entirely on the amount of amalgam we
have to distil at each clean-up. There are two
varieties in use, the stationary ones, always used in
silver mills, but only employed in gold mills when the
amount of amalgam runs over 1,500 ounces; and portable
retorts in sizes holding from 30 ounces and upwards.
If the number of stamps exceeds 40, and the
ore is of good grade, we will require a stationary retort,
but its use is of much less importance in general
mill work than the portable ones.
Where retorting of large quantities is required, we
must have a building entirely separate from the mill.
This building will contain one room for the retorting
and melting furnaces, and one for the assaying and
chemical department, with a small room in which the
delicate balances, assay accounts and such things which
require care, are kept. The retort is pit in the centre
of the room and the outlet from the fire box extends
to a chimney placed to one side. Into this chimney,
from the other side come the flues from the two melting
furnaces and the muffle furnace. All around the furnaces
the floor is covered with iron plates at least four
feet wide, the remainder of the flooring being brick
set very closely together. The retort is placed horizontally
and is closed by a cap covering the whole interior
diameter, while the outlet for the escape of the
quicksilver, is a small orifice at the top of the rear
end, from which it is conducted by a water-jacketed
pipe to a receiver, which is an iron tank filled with
water. The balls of amalgam are placed loosely in trays
which just fit the contour of the bottom of the retort.
These trays are first thoroughly coated with chalk to
prevent the gold sticking to the iron when softened by
the heat. When all the trays are pushed in, the cap
is put on and keyed very tightly. Now a tight joint
cannot be made with iron against iron, and no gaskets
will stand the heat, so we must find a joint unaffected
by the heat or the action of the quicksilver. There are
several substances at our disposal, and the choice depends
only on the presence of one or the other and its
relative cost. The luting material can be pure clay,
either white fire clay or colored, as the heat is not sufficiently
high to melt or even thoroughly bake it. The
most common material employed is sifted wood ashes,
either mixed with water alone or with the addition
of salt. Either material is first screened through the
40 mesh sieve, and all lumps in the clay or cinders in
the ashes thrown away. A small portion is mixed and
puddled with a little water, until it can be moulded in
any form without breaking, and yet carries no surplus
water. As the puddling is carried on, either water
or ashes are added till the right quantity of the proper
consistency is obtained. At first a novice is apt to add
too much water, the paste seeming to be too dry, but
on working it with a knife, it grows softer and more
liquid, so that only after considerable puddling and
slight additions of ashes is it possible to get it thick
and at the same time soft. When ready the cover is
laid in a horizontal position and the lute put all around
the edge in the recess made for it. It is put on quite
thick, carefully smoothed with the knife till the sur
face is perfectly homogeneous, when the cover is placed
exactly in its place against the retort and tightly keyed.
The surplus lute is squeezed out all around the cover.
This is scraped off, and the junction between cover
and retort smoothed with the knife.
A small fire of shavings and wood or bark, is now
started and the retort gradually warmed, care being
used to keep a small fire till all the metal is thoroughly
heated. This is to prevent the deflagration of the amalgam,
as well as any danger of cracking the retort itself.
The heat is gradually raised till the bottom of the retort
is a dull red. Then the quicksilver will begin
to vaporize. No fuel but wood must be used, and there
Is no advantage in trying to hurry the distillation. It
will take just so long, as there is only a fixed area
for its escape in the outlet pipe, and if this is crowded
by the pressure of the quicksilver gas in the retort,
there is danger of some escaping before the condensation
takes place, thus endangering the health of the
operator and also of breaking the joint where the condensing
pipe is fastened to the retort itself. After the
quicksilver begins to collect in the tank, the heat is
slightly raised and the retort kept at a cherry red till
the amount of quicksilver coming from the pipe slackens
up. The pipe is now gently tapped with a hammer
occasionally and the fire kept up till no more quicksilver,
not even a drop, can be collected on holding
the hand under the pipe and tapping it smartly. When
this is found to be the case, the fire is allowed to burn
out, or is drawn and the retort left to cool. This takes
several hours on account of the mass of metal which has
been heated so thoroughly.
For this reason, we try to start the fire in the after
noon, and finish the retorting during the night. As
soon as it has become thoroughly heated, it is safe to
leave in the care of the watchman as no one is able to
open the retort or abstract the metal as long as it remains
so hot. It is rarely cool enough to open with
safety till 7 a. m., the next day, and it should not be
touched till the arrival of the one in authority. When
ready, the cap is removed and the trays taken out and
placed on the iron plates to cool still more. If the retorting
has not been done too rapidly and with too
high a heat, hard to get with wood fuel, the gold should
still be in its pristine balls, now, however, porous and
spongy. In the bottom of each tray we always find
some of the gold joined together, being partially melted
from its proximity to the red-hot iron; but it can be
easily broken with a blow of the hammer into convenient
weights to handle for the subsequent melting.
At most mines, the amount of amalgam at each cleanup
is small in actual bulk, and can be handled with the
portable retorts. Each mill should have two on hand,
of different sizes, the largest according to the rapacity
of the mill and the richness of the ore, capable of holding
in one charge the total of the clean-up. A retort
must never be filled more than three-quarters of its
capacity. The danger from over-loading is not appreciated
by the average person till a serious accident
occurs. Under the influence of the heat the whole mass
swells very much, and we have seen, from putting too
much amalgam in, first, the complete choking of the
outlet pipe by amalgam, and then when the pressure
became too great the sudden forcing of several pounds
of amalgam with quicksilver vapour, into the receiving
tank. In some cases if the retort was old or the cap
improperly fastened, a very serious and dangerous accident
would happen from the explosion of the retort
itself.
The portable retorts are in two shapes, those with
a flat cover, and those with a curved cover and known
as the Nevada retort. The latter style is very much
preferable, as it gives, under all circumstances, a reserve
space for the quicksilver vapour and allows a
little more latitude in filling the retort, besides giving
a much stronger keyway and general fastening.
Before putting in the amalgam, every part of the retort
must be examined. First see that the pipe is tightly
screwed into the cover, and is perfectly clear, tapping
it with a hammer and blowing through it. With a
new one, be sure and coat it thoroughly, because, if
the gold adheres to the iron in any one place, in subsequent
cooking with every care taken, it is generally
attached to the same place. If the retort has been
in use, remove all adhering particles of gold with a
chisel and coat most thoroughly. We have a choice
of two coatings, one, which we consider the best, is of
soft chalk, rubbed on dry from the chunk or powdered,
mixed into a paste with water, and plastered smoothly
all over the inside; the other, a thin paste of sifted
wood ashes and water, which is thoroughly daubed all
over the interior. If a paste is used, the retort should
be dried before putting in the amalgam. The quicksilver
condensing pipe should always be water-jacketed, and
can be bought already prepared in this way. This jacket
consists of an iron or tin pipe two to four inches in
diameter, and made water tight at each end, through
which the condensing pipe passes. At the lower end,
the furthest from the retort, there is put in a part of
a hose coupling to which the hose conveying the cold
water can be screwed. At the upper end is a small
outlet, to which a piece of quarter inch pipe is screwed.
This pipe is bent and carried down on top of the condenser,
discharging into the kettle which receives the
metallic quicksilver. If not bought with the retort,
any tinsmith or capable engineer can make a tin cylinder
of the required size, solder in a hose coupling at
the lower end, and solder the whole tightly to the pipe
at each end, The constant flow of water will prevent
any melting of the solder.
Where a water-jacket is used, the end of the outlet
pipe is placed a little below the surface of the water
in the receiving kettle, an inch being ample. Sometimes
it is necessary to use a retort, having only the naked
pipe over which water is poured from a dipper. In this
case care must be taken that the orifice of the condensing
pipe is partly out of the water, and this is very
rarely looked into by mill men. The danger of a diminution
in the heat consists in the creation of a partial
vacuum in the retort, which then sucks up water from
the kettle if the pipe is wholly immersed. This water
is converted into steam and causes an explosion of great
danger. With part of the orifice out of the water, only
air is sucked up, which causes no sudden increase in the
internal pressure; but we are confronted with another
danger, that of salivation from the escaping quicksilver
gas, unless we close the orifice in some way. Now we
wish to impress this point very, very strongly on all
those intending to retort the amalgam without a water-jacket.
Even if they are fortunate, time after time,
in having no serious accident, they will always be more
or less salivated, and it takes a very slight exposure to
quicksilver fumes to find the teeth becoming loose and
other evidences of quicksilver poisoning. The way to
prevent it is this: The pipe is wrapped with burlaps or
gunny sacks, and a V of tin placed underneath, suspended
by wire, to carry the water poured over the
upper part of the pipe, away from the fire. This sacking
is extended several inches beyond the end of the
pipe and into the water of the receiving kettle. In this
case the pipe must be straight from the retort to the
kettle, with no bend, as shown in the sketch of the
water-jacketed pipe. The wet sacking prevents any
escape of the quicksilver fumes, and yet part of the
orifice of the pipe is out of water. Should a partial
vacuum be formed inside the retort, the only effect is
to suck the gunny sack against the mouth when the
air enters freely through the web.
The retort has been chalked, all parts examined, and
the amalgam in balls ready to be packed in it. This
packing depends on the subsequent handling of the retort-
metal. Where not convenient to melt into a bar at
the mine, it is just as well to ship the gold just as it
comes from the retort, but in that case we want to have
it as compact a mass as possible. If it is melted at
once it is better to keep it in its original balls from
which the quicksilver has been distilled. When the retort
metal is shipped, the balls are broken and the
amalgam packed with the iron pestle into one solid mass,
that around the sides particularly being smooth, so as
to leave no thin ridges to break away. When it is all
smoothed, a hole is bored in the centre to the bottom,
with an iron rod, to act as a vent for the escaping gases
and to prevent the entire mass from being raised by
the pressure underneath before it makes its own vent.
In this case also the heat during the last of the operation
is raised to a bright red so as to anneal and partially melt
the gold and so hold it together. When it is melted on
the spot, the balls are put in unbroken, and sometimes
each ball is wrapped in a piece of cloth or paper. The
wrapping is unnecessary and only a false idea of excessive
carefulness, which results in the distillation along
with the quicksilver of pyroligneous acid, and this very
quickly chokes the condensing pipe and flours the quicksilver,
hindering the operation considerably.
After all is ready, the lute of wood-ashes, with or
without salt, is mixed and put on the cover, not on the
retort; the cover is put in place and keyed tightly, and
all the oozing luting is scraped off and smoothed around
the joint.
The next question arises, how and where to retort.
Retort out of doors, or under an open shed, where there
is no danger of tire. Do not use the blacksmith forge,
because too much heat is unconsciously given with the
bellows, nor a furnace into which the retort is set, as the
heat is not under control, while it is hard to remove a
heavy and hot retort, and in both these cases no direct
heat can be applied to the bottom. Make a tripod with
a ring into which the retort just fits, and high enough
to allow a good fire place underneath. This can be
made by any blacksmith, and the only point to be observed
is that the legs shall be extra thick and strong,
to withstand any bending from the heat and weight.
In addition to the welding of the legs to the ring, put one
or two rivets In each leg, for additional security.
The tripod is set within distance of the water pipe and
the retort put in place, the hose connected and the receiving
kettle, partially filled with water, is put at a
slight inclination with the end of the condensing pipe
submerged in it an inch. A small fire of shavings and
a little oily waste is started directly under the retort.
This is gradually increased with either pine bark or
wood, but slowly until all the air is driven out, and a
little quicksilver begins to run. Then the fire is increased
and the retort kept at a cherry red till no more
quicksilver can be collected, after tapping the pipe.
When this is shown to be the case, the fire is withdrawn,
the pipe taken out of the kettle, and the cover of the
retort taken off. To prove that no more quicksilver is
left, take a cold shovel, wet it and hold over the open
retort, when it will be covered with a white film if any
of the quicksilver remains. Before the retort has a
chance to cool, dump out the retort metal into a pan
set on an iron plate or directly on the plate, and scrape
out any adhering gold. The gold must now be cool
enough to handle with the bare hands, before it is
weighed, or a correct estimate of its weight is not obtained.
It should be porous and bright, or the amalgam
has not been properly cleaned.
The question as to what fuel to use is of some moment.
Do not use charcoal or coke, even though on account of
the greater heat it would be surmised that time can be
gained. The top of the retort must be kept as hot as the
bottom, and this can only be done with a fuel 'Which
gives strong flames. The best fuel is pine bark, but if
this is not easily available, dry wood, either pine or
other soft wood, will do as well. The wood is sawed
into two-foot lengths and split to small size. When the
fire is well started, some pieces are placed under the
retort between the legs of the tripod, and a complete
circle of the wood, placed on end, is stacked against
the ring of the tripod, which forms a chimney for
draught and a gradually increasing heat. Frequently
only one firing is necessary when placed in this way.
The retort metal, if shipped direct, is now wrapped
carefully in paper and then sewed and sealed in an
ore sack of canvas. If melted on the premises into a
bar, we can use the assay furnace, provided there is no
special furnace. The fuel used for melting is either
charcoal or coke, the latter good English and not gashouse
coke. The coke is very much to be preferred.
The black-lead crucible, for a long time before use,
should be put bottom up on the boiler or other hot place,
to get thoroughly dried out. It is then put into the furnace
where there is a hot fire dying out. Here it is
put bottom side upwards, and left to be annealed till
everything is cold. It is then ready for the melting of
the gold attended by a minimum of danger as regards
cracking. Before the retorting is completed a fire is
started in the furnace, and the crucible, resting on a
piece of brick, is placed in position. The fire is gradually
increased till the crucible is heated almost to a white
heat, and the supply of incandescent fuel just reaches
to the top of the crucible. The retort metal is now put
in from an iron scoop. This can be home made from
rolling a thin sheet of Russia iron so that it has a smaller
diameter at one end, or a scoop that can be purchased
ready made. The crucible is filled nearly to the top
with the retort metal, and a tablespoonful of a mixture,
containing ground borax and a little sand or ground
glass, is at once added. If all of the metal cannot be
added at once, the first charge is thoroughly melted
before any more is put in. On each subsequent addition
a little more of the fluxing material is used. When all the
metal is in quiet fusion, the supernatant slag is skimmed
off by means of gently passing a coiled rod of cold iron
over the surface, frequently chilling the slag
by pressing it on an iron plate, or slight immersion in
water. When the slag is removed, the graphite pot is
lifted from the fire and the contents, after thoroughly
stirring and mixing with a paddle made from old crucible
and heated in fire before use, are carefully poured into a
mould already prepared for them. This mould has been
first smoked evenly by inverting over a fire of cotton
waste, coal oil and pitch or rosin, and then heated as
hot as possible by exposure to the heat from the top
of the furnace. As soon as the gold has been poured
a pinch of candle scrapings is thrown on top and the
mould is covered by an iron plate or a piece of wood and
left to chill. When solid, but still red hot, the mould
is inverted and the gold brick at once immersed in a
cold pickle, consisting of water acidulated with sulphuric
acid. When cold enough to be handled with the
bare hands, it is removed and scrubbed with a brush.
Any projections are hammered flat, and the bar is ready
for chipping and weighing. Even if no assay is made
at the mine, always chip off two pieces from diagonally
opposite corners, mark them with date and number of
bar, and file away, as a precaution against any future
dispute. Stamp on bar number and weight in troy
ounces; but it is advisable to abstain from putting on
fineness of bullion or its value. In any and all circumstances
the buyer will have it remelted and assayed,
and it is better for the seller not to publish so widely
his own knowledge of its value.
Practically all gold bullion from placers and quartz
mines, saved by amalgamation, contains some silver, and
this has to be separated before the gold is ready for
coining or other purposes. This cannot be done at the
mine, and as some gold is lost to the seller in remelting,
it is of advantage to ship the bullion as retort metal,
save the expense of fuel and the loss in melting. Under
any circumstances a fixed charge has to be paid to the
refiners.
Louis Falkenau, State assayer of San Francisco, tells
us that if the slag dipped off is powdered and panned
and old crucibles treated in the same way, there is no
loss in melting. And also, if the bar has been properly
melted and mixed, the chips will ensure a control which
he considers important.

Taken from the book

HINTS -ON AMALGAMATION GENERAL CARE OF GOLD MILLS

By W. J. ADAMS, A. M., E. M.

Graduate of the School of Mines of Columbia University, New York.
A REFERENCE BOOK OF ACTUAL, GOLD-MILL PRACTICE, AS
DETERMINED BY AN EXPERIENCE OF TWENTY YEARS
WRITTEN IN LANGUAGE THAT CAN BE UNDERSTOOD BY ALL.

CHICAGO:
MODERN MACHINERY PUBLISHING COMPANY. 1899

Guest
Guest


Back to top Go down

Back to top

- Similar topics

 
Permissions in this forum:
You cannot reply to topics in this forum