Flotation was used in the recent working of Wheal Jane, initially to produce a
mixed zinc-copper concentrate but subsequently these were seperated.
Historically Wheal Jane was probably the largest zinc producer in the UK (mostly
from the 1970'S-1990's) but of course had negligable lead. Many other Cornish
copper mines produced small quantities of zinc ores, but I guess only where it
was easy to hand pick the ore or keep it seperate at the mining stage, as
generally it was a low value commodity. West chiverton was I think the most
important mine in the SW producing lead & zinc, don't know what evidence or
archive information there is regarding dressing processes there.
________________________________
From: Peter Jackson <[log in to unmask]>
To: [log in to unmask]
Sent: Tue, 5 July, 2011 12:24:03
Subject: Lead and Zinc ore dressing
Phil,
Been thinking about your questions:
Can anyone enlighten me on how dressing processes would have differed
between lead and zinc at mines, where both metals are known to have been
exploited?
ANSWER: Lead and zinc sulphide ores are generally found together. Dressing
used gravity processes ( until froth flotation was invented). The difference
in the specific gravity of the minerals meant that the same equipment could and
was used to separate the two ores, either from each other or from other
minerals. (There were difficulties in seperating the zinc ore from minerals of
similar specific gravity).
I'd be particularly interested to know if there are any specific
forms of archaeological evidence that might survive, exclusively associated
with zinc dressing at British mines. Any published examples, preferably in
England, would be particularly welcome.
The 19th Century dressing plant at Nenthead produced lead and zinc dressed
mineral, although it was built to process lead ore. This plant was replaced in
1910 by a gravity plant, built on the same site, designed to process zinc ores,
which also continued to also produce lead ore. This plant was closed down
around 1922 and mothballed. It was resurrected in 1936 to produce lead ore and
worked for about 18months before being mothballed again. In 1943, the building
was re-equipped as a froth flotation plant to dress zinc ores from the old dumps
and closed down again in 1945. The gravity equipment was largely removed in
1943. The same plant was then used from around 1950 to 1963 to process lead,
zinc and fluorite ores. The equipment was then removed and 50% of the building
demolished. The remaining 50 % is currently used as a bus depot and the
locations of zinc dressing gravity equipment ( 1910 phase) can be seen. No
archaeology has been done on this site.
Adjacent to the 1910 building was a magnetic separator plant. this was
apparently where the zinc concentrates were roasted and subject to a magnetic
process. The intention was to remove the siderite and pyrites which devalues
the dressed zinc concentrates and is difficult to separate via gravity means (
Similar SG). This building was demolished before it was recorded ( other than
photographs).
On early editions of the OSMaps for Nenthead is a "spelter works". There is
also a photograph. This site is approx 400 metres to the north of the Nenthead
dressing plant. From the size of the site and the photo, this looks to me like
a crushing mill and possible small roasting furnace for zinc ores. Building has
been demolished and site landscaped. No archaeology has been done on this site.
This plant was linked to Brownley Hill and Gudhamgill Mines.
Nentsberry Mine nearby also produced zinc ores and had a small dressing mill.
Again, demolished and unrecorded.
Threlkeld and Thornthwaite mines, near Keswick, also produced zinc ores. I
think that the dressing mills were demolished after closure and the sites
cleared.
Mill Close Mine produced zinc concentrates - can someone with more knowledge of
that site discuss the dressing plant? This may be the only English example where
froth flotation was used for zinc sulphide dressing, unless there was an example
in Cornwall?
West Shropshire mines produced zinc concentrates - ditto as above?
By the way, your question illuminates an enormous gap in our collective
knowledge of zinc mining in the UK.
Peter Jackson
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