Gold Cyanidation by agitation; carbon-in-leach (CIL) process

    Cyanidation by agitation. Usually the carbon-in-leach (CIL) process must be done under the following conditions.
    • Leaching must be intense in order to facilitate the processes of mass transference and have an optimum concentration of free cyanide and oxygen for a good rate of gold leaching.
    • Absorption of gold leached on activated carbon requires no intense agitation for avoiding attrition of carbon, and a lower concentration of cyanide for favoring absorption.

    As consequence of these conditions, the carbon-in-pulp (CIP) process considers two circuits working independently, cyanidation and absorption. The alternative of employing a process including cyanidation and absorption simultaneously is an interesting option since let reduce the cost of investment.  This type of process is known as carbon-in-leaching (CIL) and was implemented for ores that have some organic material that can absorb the gold leached. These materials are carbonaceous species and can produce losses because they will go the tails. The next table shows an ore with this kind of material:

    Table 2. Effect of organic material in activated carbon performance.

    Time

    (h)

    Without A.C.

    With A.C.

    Au

    (g/t)

    Recovery (%)

    Au

    (g/t)

    Recovery (%)

    2

    4.55

    69.67

    4.33

    71.13

    4

    4.96

    66.93

    3.76

    74.93

    8

    5.12

    65.87

    3.21

    78.60

    12

    5.45

    63.67

    2.98

    80.80

    24

    5.78

    61.47

    2.56

    83.60

      A.C. = Activated Carbon

    From the information presented, is notorious the effect of organic material. Thus final recovery can be reduced in 8% which is a high value. This loss in a small operation of 15 g/t gold treating 100 ton per day represents a gold loss of more than 10 kilograms each day.

    CIL is a good alternative, but there are some disadvantages in this type of agitate d cyanidation processes, there are activated carbon losses by attrition during agitation and fine particles can go to the final tails. Usually, these losses can be 8 to 12% activated carbon employed in cyanidation. Many times the problem occurs due to a bad design. Table 3 shows a special case where one tank worked with higher speed than the other ones due to a bad design.

    Table 3. Effect of agitation speed on the gold recovery by a CIL process.

    Point

    Au, g/t

    Total Recovery, %

    Fresh feed

    16.3

    100.00

    Overflow

    9.34

    42.70

    Tank 1, tails

    4.23

    74.05

    Tank 2, tails

    2.56

    84.29

    Tank 3, tails

    1.67

    89.75

    Tank 4, tails

    1.01

    93.80

    Tank 5, tails

    1.35

    91.72

    Tank 1 to tank 4 have the same lineal speed, 1400 ft/min, but tank 5 has 1500 ft/min.

    Working with different speed in the last tank, carbon reduced its size by attrition and the gold absorbed in fine particles became part of the final tail. As consequence, recovery was reduced in 2%.

    When there are good amounts of oxidized iron minerals such as limonite and hematite, tanks can work with lower speeds and minimize the losses in big proportion. Also, there is a favorable aspect with this type of ores, gold can be leached during grinding in the range 30-45%. In other words, the first leaching tank is the ball mill and the agitated tanks work without much pressure. The high kinetic is due to the good grade of liberation obtained by the ore in the first moments of grinding. The finest particles improve the grade of leachibility of the ore.

    Time

    (h)