Granulometric composition of the alluvial gold features and shape of its grains are the main technological parameters, which determine (in the normal conduct of processes - gravity separation) the level of possible extraction. Despite this, in practice still there is no uniform scientifically based classification scale of placer gold by size.

Proposed several classifications of gold by size, but, none of them in domestic practice, widespread distribution has not received. Especially contradictory statements about the behavior of fine gold, as the fine gold of various researchers include gold of different fineness.

In some classifications gold by size (and in the technical literature) meets the concept of "floating gold". In our opinion, this term should not be used to characterize the gold by size, as it characterizes not the size, and method of transportation of grains in the flow. In the threads, can move large grains, that is determined by the condition of their surface (hydrophobicity of tiny bubbles of air, contamination by oils and t. D.). There is a "floating gold" of size up to 1 mm, having the form of thin plates.

The most frequently semiliterate gold channel placer deposits of large rivers and marine placers, Sands which contain organic inclusions of animal and industrial origin. The most common is to classify gold by size (table. 12.3).

Table 12.3

Recommended Irgiredmet classification of gold by size

Size, mm	Size the openings of the bottom sieve, mm	Area Hole mm2	The logarithm of the limiting grain size, microns	Class fineness of gold
+3(3,15) -3(3,15)+1 -1+0,315 -0,315+0,1 -0,1+0,03 (0,0315) -0,03	3(3,15) 0,315 0,1 -	0,1 0,01	4-3,5 3,5-3 3-2,5 2,5-2 2-1,5 1,5	Very large Large Average Small Very small Dispersed

The proposed classification of the placer gold fineness connected with its technological properties. So, gold larger 3 (3,15) mm is removed completely even if the enrichment of the sand in the simplest ways. The gold particle size 1 mm is completely removed by usadkoj and more than 95% gateways small filling. The grain size of 0,315 mm is the boundary for the effective extraction of gold gateways. The extraction is equal to 80%. The gold particle size 0,1 mm is removed by usadkoj on 50% and practically is not extracted in the enrichment on the gateways. Gold smaller 0,03 mm are not captured by known methods mechanical beneficiation.

In accordance with the classification of gold it is practical to classify and placers (rice. 12.4.).

In industrial placers the principal gold, as a rule, has a size of 0.1—4 mm (table. 12.3). However, in recent years involved in the exploitation of deposits and with smaller gold (table. 12.4).

Data of sieve analysis have not always adequately characterize the grain size of gold. So in addition to sotovomu analysis sometimes weigh individual gold particles.

Rice. 12.4. Classification of placers depending on the fineness of gold

• larger gold;
• with an average gold;
• fine gold;
• with very small gold

T. G. Fomenko brought experimental dependence of the rate of free fall of gold particles in water by mass. The rate of fall of grains in water (one of the main characteristics on concentration), defined depending on the weight of the gold particles closer to the actual, than defined taking into account only the grain size.

Table 12.4

Gold distribution by class size, % (according to Irgiredmet and VNII-1)

District	Classes size, mm	The average particle size, mm
-2+1	-1+0,5	-0,5+0,25	-0,25
Northeast ……… Lena............………… Yakutia .........……….. Northern Yenisei…… The same…………………….. Southern Yenisei....... Cupid..................... Yakutia.........………… The same............……….. The Northern Urals...…… The same…………………….. South Ural…………. Transbaikalia......……….	46.0 17.5 38,3 9,1	27,0 46,7 43,1 54,6 8.1 3.4 4.7 35,6 12.3 11.7 17.5 7.6 13.3	24,0 28,5 10,4 27.6 84.3 55,1 74,8 48,6 57,3 57,2 78,3 76,7 63,5	3.0 7.3 8.2 17.8 7.6 41.5 20,5 6,7 30.4 31.1 4.2 15,7 23,2	1,6 1,1
1,5 0,9 0,6 0,5 0,5 1.0 0.5 0.5 0.8 0.6 0.6

Table 12.5.

Granulometric composition of individual gold placer deposits, %

District	Classes size, mm	The average particle size,
	-2+1	-1+0,5	-0,5+0,25	-0,25	mm
Cupid	6.6	6.4	61,4	25.6
Yakutia (the ancient placer)	3.0	19.3	36.8	40,9	0,25
Western Siberia		32,5	44.5	23,0
Far East			5.1	94,9	0,15
Southern Yenisei	3.0	10,3	32.8	53.9	0,25
Yakutia		9,8	32,5	57.7	0,24