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Sun, 30 Nov 1997 15:23:31 +1300 |
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A colleague (Phil Moore) and myself are presently writing an analytical
paper on in-ground gold ore roasting kilns, which in New Zealand are found
on only one goldfield - the Hauraki field in the North Island. Their
presence on the Hauraki field is a response to the hard refractory gold ores
found there. Until the introduction of ore-roasting, many of the Hauraki
mining companies were getting returns from their stamp mills of less than
50% of assay values. This situation prevailed despite considerable
experimentation by the Hauraki companies, using just about every
chemical/mechanical process that had been developed. Roasting (or burning)
achieved at least 3 things- it weakened the hard quartz ore and considerably
reduced wear and tear on crushing machinery, it burnt off nasty sulphides,
and importantly to some extent it 'tendered to free' the gold particles in
the ore so the recovery rates were much higher by means of mercury
amalgamation. Fortunately, perhaps, the period when in-ground gold ore
roasting was popular was brief (it required 1 ton of firewood for every ton
of ore roasted). During the 1890s N.Z. gold mining companies quickly
adopted the cyanide process, first commercially used (in the world) at the
Crown battery (on the Hauraki field) in 1889. The introduction of the
cyanide process with its superior capability to recover fine gold and
silver, lead virtually overnight to the obsolence of the gold ore roasting pits.
The Hauraki kilns were excavated and used from c1885 until c1900 (seldom
more than 5 years at most places as far as we can ascertain). Typically
they consist of 2 to 12 "U" shaped pits, 7 to 9 metres metres deep and of 8
to 9m diameter. Depending on the lithology, they were sometimes brick lined.
They were top loaded (very similar to lime burning kilns) with alternate
layers of firewood and ore. The resultant ore and ash was removed via a
chute in the bottom of each kiln accessed by a tunnel usually serving
several kilns in a group. As far as we can ascertain the ash was
floated/washed off, and the roasted ore was then treated in the usual ways
via primary crushing (jaw crushers or similar), secondary crushing (stamp
mills), and tertiary crushing (berdans/ball mills etc) prior to
concentrating and amalgamating
To ascertain the rarity/frequency of in-ground gold ore roasting kilns on a
world scale (and also in support of their registration) I would like to ask
members of the Hist Arch and Ind Arch groups if they are aware of examples
of this technology on historic goldfields beyond New Zealand?? At this stage
I am unaware of the existence of in-ground gold ore roasting kilns elsewhere
(which does not mean there aren't any!) but my impression (again at this
stage) is that they appear to be relatively uncommon. I am aware the
roasting of ores/limestone etc has a long history, but with regard to the
responses I'd like I am particularly interested in the presence/absence of
in-ground gold ore roasting kilns like those described here, and whether
they pre or post date the introduction of the cyanide process.
Thanks in anticipation
Neville Ritchie, Ph.D.
Conservancy Archaeologist
Dept of Conservation
Private Bag 3072
Hamilton
NEW ZEALAND
work email address: [log in to unmask]
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