|View single post by Salada|
|Posted: Sun Jun 28th, 2015 11:41 pm||
|In view of the overwhelming (?) response to my previous "Mine Mill Interior" photos
(thanks to Jose for counting all 5) I thought some of you may appreciate a little more info on the 'black art' of successfully stamping metal ores.
A set of stamps usually comprises 5 individual "heads" within a single frame, the whole also being known as a "battery". So a 50 battery mine stamp mill would comprise 50 x 5 individual stamp heads. An individual battery could stamp anywhere between 1 - 20 tons of mine run ore per day, depending on many other factors as outlined below.
The classic 1800's gold mining stamp is also known as a "percussive" stamp, of which there are two types :
Rotary Percussive or Californian & :
Square Percussive, more usually known as Cornish Stamps.
All the below photos are of the Californian or Rotary percussive type. Firstly a photo of the 'business end', the actual stamp heads :
Photo 1 :
This front view of the battery shows the 5 stamp heads (confusingly called "shoes" by Americans) inside the "mortar" (a.k.a. mortar box or stamp box). Actually there are only 4 heads, one is missing. Also missing is the sieve plate which normally fits into the inclined grooves left & right of the box, retained by the central lower lip. Stamp head weights can vary from about 80 lbs up to about 1,100 lbs per head. This is a relatively lightweight battery.
The stamps must never be allowed to run empty - the mortar box would be shattered in seconds. Someone has put a small piece of timber in place of the rock-ore.
Running up vertically above each stamphead are the "lift-poles" (or lifters in U.S. speak) which run through loose fitting guides in the main frame.
Photo 2 :
This shows the rear of the battery (together with Madame Salada - wondering where is the nearest shopping mall ?!).
Just above the twin cylinder engine can be seen the small feed hopper. Often the feed hopper spans the full width of the stamp box. This feed hopper will be directly fed from a much larger timber chute constructed on site.
Immediately below the upper cross timber/lift-pole guide can be seen the "collars" or bobbins attached to each lift-pole by which the camshaft cams lift each stamp head in turn.
Photo 3 :
This shows the curved lift cams (looking like curved horns) fastened to the drive shaft.
The lift on each cam isn't much, usually only about 7" - 8" but that is eneough to smash any normal ore-bearing rock.
Photo 4 :
This shows how the lift cams only operate on one side of the lift-pole collars. This imparts a partial rotation at each drop of the lift-pole - hence the name Rotary Percussive. The square or Cornish Stamp lifters are arranged centrally on each lift-pole so no rotation is imparted to the stamp head. Rotaries are better as stamp head wear is even all round & output is higher per hour. Cornish stampheads wear terribly on one side with attendant down time & reduced efficiency.
Photo 5 :
This front view also shows the offset arrangement between lift cams & lift-pole collars.
The stamp box sieve can also be seen in place - the light grey perforated sheet with output spout running down to the left. The horizontal pipe running across behind the lift-poles is the water feed for wet stamping.
Correct adjustment of the running of the stamps could be critical to the success or failure of a mine & so was usually entrusted to an experienced & skilled engineer.
Factors such as stamphead weight, sieve mesh size, the number of "drops" per minute of each head, ore-rock feed rate, ore particle size, the sulphide content of the ore & whether to dry or wet stamp could make all the difference to recovery rates.
Was it better to stamp all day to produce only 1 ton of very fine pulp through a 40 mesh or better to bash through 20 tons/day with an 8 mesh ?
With higher sulphide content gold ores was it better to run the pulp off onto the mercury amalgamation tables or do the amalgamation by adding mercury directly into the stamp box ? Both methods were used.
'Old timers' reckoned overall mill recovery rates were determined 'in the mortar' - probably correctly.
So how good were rotary percussive stamps ?
In their day nothing could beat a well managed Californian for stamping gold, tungsten, tin & some silver ores ( anything where the metal ore particles were finely dispersed through the lumps of ore) when values ran to many ounces or lbs per ton of mine run ore.
But percussive stamps are now totally obsolete due to reducing ore values & increasing costs. No stamp can cope with gold recovery rates that are often now measured in decimal points of a gram per ton. Modern low grade ore mining relies on strip (open-cast) mining, cone crushers, ball mills, cyanide extraction, froth flotation or electro-magnetic separators.
The stamp battery shown in Photos 1-3 is a fairly rare type - it is a rotary percussive tappet stamp, a patented variation claimed to improve output rates; the only one I have ever seen. But for practical purposes it is almost identical to the normal type of Californian.
All photos by Salada. Additional fashion by Madame Salada.