DESCRIPTION OF THE FRANCKE "TINA" OR VAT PROCESS FOR THE AMALGAMATION OF
SILVER ORES.
[Footnote: Paper read before the Institution of Mechanical Engineers at
the Cardiff meeting.--_Engineering_.]
By Mr. EDGAR P. RATHBONE, of London.
In the year 1882, while on a visit to some of the great silver mines in
Bolivia, an opportunity was afforded the writer of inspecting a new and
successful process for the treatment of silver ores, the invention of Herr
Francke, a German gentleman long resident in Bolivia, whose acquaintance
the writer had also the pleasure of making. After many years of tedious
working devoted to experiments bearing on the metallurgical treatment of
rich but refractory silver ores, the inventor has successfully introduced
the process of which it is proposed in this paper to give a description,
and which has, by its satisfactory working, entirely eclipsed all other
plans hitherto tried in Bolivia, Peru, and Chili. The Francke "tina"
process is based on the same metallurgical principles as the system
described by Alonzo Barba in 1640, and also on those introduced into the
States in more recent times under the name of the Washoe process.[1]
[Footnote 1: Transactions of the American Institute of Mining Engineers,
vol. ii., p. 159.]
It was only after a long and careful study of these two processes, and by
making close observations and experiments on other plans, which had up to
that time been tried with more or less success in Bolivia, Peru, and
Chili--such as the Mexican amalgamation process, technically known as the
"patio" process; the improved Freiberg barrel amalgamation process; as
used at Copiapo; and the "Kronke" process--that Herr Francke eventually
succeeded in devising his new process, and by its means treating
economically the rich but refractory silver ores, such as those found at
the celebrated Huanchaca and Guadalupe mines in Potosi, Bolivia. In this
description of the process the writer will endeavor to enter into every
possible detail having a practical bearing on the final results; and with
this view he commences with the actual separation of the ores at the
mines.
_Ore Dressing, etc._--This consists simply in the separation of the ore by
hand at the mines into different qualities, by women and boys with small
hammers, the process being that known as "cobbing" in Cornwall. The object
of this separation is twofold: first to separate the rich parts from the
poor as they come together in the same lump of ore, otherwise rich pieces
might go undetected; and, secondly, to reduce the whole body of ore coming
from the mine to such convenient size as permits of its being fed directly
into the stamps battery. The reason for this separation not being effected
by those mechanical appliances so common in most ore dressing
establishments, such as stone breakers or crushing rolls, is simply
because the ores are so rich in silver, and frequently of such a brittle
nature, that any undue pulverization would certainly result in a great
loss of silver, as a large amount would be carried away in the form of
fine dust. So much attention is indeed required in this department that it
is found requisite to institute strict superintendence in the sorting or
cobbing sheds, in order to prevent as far as practicable any improper
diminution of the ores. According to the above method, the ores coming
from the mine are classified into the four following divisions:
1. Very rich ore, averaging about six per cent. of silver, or containing
say 2,000 ounces of silver to the ton (of 2,000 lb.).
2. Rich ore, averaging about one per cent. of silver, or say from 300 to
400 ounces of silver to the ton.
3. Ordinary ore, averaging about ½ per cent. of silver, or say from 150
oz. to 200 oz. of silver to the ton.