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Stamp Mill

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Stamp mills were the first mass-produced machine used to crush and grind gold and other minerals.
A stamp was a type of metal arm raised and lowered in a similar fashion to a battering arm. Stamps were the first mass-produced machine used for crushing and grinding ore, as well as for preparing pulp and processing oil-seed.[1] Several stamps were often lined up in stamp mills where gold-bearing ore, gathered from hard rock gold mining, was broken up and crushed. Stamp mills were commonly classified as 5-, 10-, or 20-stamp mills based on the number of stamps used in the mill.[2] Stamp mills were heavy, awkward, and noisy machines but their ability to crush hard materials, including ore, into a fine powdery substance suitable for gravity concentration stretched the longevity of their use more than 400 years.[3] 

[edit] History

As demand for minerals grew, it became mandatory to devise a machine that could crush abrasive ores into smaller pieces at a rate higher than what could be achieved through manual power. This need led to the development of some of the earliest stamp mills.

The first incarnation of a stamp mill probably dates back to 300 A.D. in China when water-driven trip hammers were used in the crushing of ore pebbles. Around 900 A.D., dry stamp mills driven by manual power were used to make iron. The same technology was in use to some degree by the 12th century, to liberate materials from ore through gravity concentration. Early dry stamp mills were used for grinding components used to make black powder used in explosives and functioned very similarly to a mechanized mortar and pestle.[4]

By the 1500s the Renaissance generated sufficient prosperity to increase the demand for metals—particularly silver and copper. This called for a new type of machine that could grind at a higher capacity than any manually-operated machine at the time was capable of. The use of water and wind power already used in mills for grinding flour was applied to the crushing of minerals.
Early stamp mills used water as the primary source of power for crushing ore.

In 1512 a man by the name of Sigismund von Maltitz introduced the first wet stamp mill.[5] Soon, almost all ores were being ground down using wet stamp mills, and made into a type of slurry. Powered by water and featuring iron-shod stamps, stamp mills evolved into high capacity ore-processing systems. In Europe, for example, they were used for crushing ore in silver and copper mining.

Stamp mills powered by water wheels dominated fine crushing and grinding from the 1500s until 1900 when their use eventually faded out. Sometimes they were situated close to a natural or artificial water supply, particularly larger-sized ore processing operations.

Stamp mills became the only machine capable of crushing ore into a powder finer than 0.4 inches (1 mm) suitable for gravity concentration. Gravity concentration became important in releasing gold, lead, and copper minerals trapped in rather coarse-granular ore bodies being mined during this period. The machines were known for their heavy, cumbersome awkwardness and difficulty to maintain. Each stamp could weigh as much as 2,000 pounds (907 kg).[6] They were also incredibly noisy and produced vibrations close to the area of installment and operation. Some likened the noise produced by a stamp mill to that of a “stampede of horses galloping across the land.”[7]


In the latter part of the 19th century, stamp mills reached the peak of their production capacity and became commonplace in gold mining regions, especially in the United States and Australia, where their use was widespread during the boom years of the California Gold Rush and the Australian Gold Rush. Despite their limitations, they remained the predominant crushing and grinding machines from 1500 to 1900. Eventually, more efficient crushers and the development of ball mills that were easier to manage and proved less expensive superseded them. By 1915 the installation of stamp mills had become quite rare, though their use may have continued to some degree in more remote locales.[8]

[edit] Process

The use of a stamp mill required a miner to deliver the ore to the mill and load it into an ore car. A tram-track, with the assistance of a winch, was used to haul the heavy ore car up to the mill. The contents of the car would then be unloaded onto a screen called a grizzly. A grizzly plate featured a metal plate with separators placed two inches (5 cm) apart, equal to the spacing of the claws on a grizzly bear, hence the name. Smaller material would pass through the screen and down into an ore chute while larger material would pass into the crusher where it was pounded and ground into a workable size before being disposed of down the ore chute.

After crushing, ore the consistency and size of gravel would slide down onto metal plates situated under the stamp battery. Water would then slide down on it. The battery typically consisted of a set of five stamps or more. This set of stamps was lifted in sequence by a powered cam then dropped down by gravity. The ore got pulverized into a fine powder the consistency of sand.

The slurry mixture would then be distributed evenly over a recovery table, a copper-sheeted table coated with mercury; the gold in the sand would stick to the mercury.
The crushed sand from the table was loaded back into the stamp mill, pulverized further and once again formed into a slurry distributed evenly across the mercury coated recovery table. Once the table was full or the ore run finished, the gold and mercury mixture would be removed and separated by straining it through a chamois. This process was referred to as amalgamation. Further refining was done through a retort process and the leftover gold would be smelted down afterwards into blocks of bullion.[9]

[edit] References

  1. Stamp Mill. Nation Master Encyclopedia. 2008-12-15.
  2. Stamp Mill. Gold Mining. Digital Desert. 2008-12-15.
  3. Lynch, Alban J and Rowland A. Chester. The History of Grinding. SME 2005. pg 53 – 56.
  4. Lynch, Alban J and Rowland A. Chester. The History of Grinding. SME 2005. pg 53 – 56.
  5. Lynch, Alban J and Rowland A. Chester. The History of Grinding. SME 2005. pg 53 – 56.
  6. Stamp Mills. Mining History Illustrations. MS Book and Mineral Company. 2008-12-15.
  7. How Gold Mining in North Georiga Was Affected by the Civil War. Gold Rush Gallery. 2008-12-15.
  8. Lynch, Alban J and Rowland A. Chester. The History of Grinding. SME 2005. pg 53 – 56.
  9. Stamp Mill. Gold Mining. Digital Desert. 2008-12-15.