Manual for Training Artisanal and Small-Scale Gold Miners

GMP - Manual for Training Artisanal and Small-Scale Gold Miners
instead of a continuous production ball mill. In Indonesia, artisanal miners use
sets of 12 to 48 small unlined batch ball mills (∅48 x 60 cm, 2.5 to 4cm thick
walls) to grind primary gold ore. Each mill grinds 40 to 50 kg of material per
batch. The grinding time in Indonesia is often too long (3 hours) because
miners use excess water and the wrong grinding media (gravels and rods).
Similarly, small welded octagonal mills are used in Tanzania but miners do not
use water, because after grinding, they have to carry the ground ore in bags to
another group of workers who perform the concentration step—dry ore is much
easier to carry. Unfortunately, dry grinding is inefficient, takes more time and
spreads dust in the air (dust is a hazard to miners). On the other hand, miners do
not introduce mercury into dry grinding circuits as they commonly do in wet
batch ball mills in Indonesia. With more organization, these Tanzanian miners
could improve their milling process. Despite the low production rate of a single
small ball mill, using many small batch-ball mills instead of large ones is a
good concept for artisanal miners with limited capital. Miners and millers can
increase their milling capacity in a step-by-step approach by acquiring one mill
after another and connecting them with a belt drive. This is not the best solution
in terms of energy consumption, but definitely suits the limited financial
capacity of the miners, employs more people and is already a fully accepted
concept in many artisanal gold mining regions.
Fig. 3.17 – Series of small belt-driven
ball mills in Sulawesi, Indonesia
Fig. 3.18 - Diesel driven octagonal
welded dry ball mill in Tanzania
Small mills can be electrically or diesel driven. If expansion is required, a
larger mill or more small mills would be necessary. The specifications of an
Indonesian small ball mill with are given below:
• Size: ∅0.48 (1.6 ft) x 0.6 m (2ft) long (internal)
• Lining: unlined (25-40 mm thick steel shell and ends)
• Critical Speed: Nc = 42.3/D 0.5 (in m) = 61 rpm
27