Molybdenite Recovery by Flotation of Moly

Although the most common sulphide collectors such as xanthates are known to be absorbed on molybdenite surface, they are not necessary for molybdenite flotation. The type of reagents that are often employed in direct flotation of molybdenite or in molybdenite-copper separation include many types of hydrocarbons as fuel oil, stove oil, or special combinations of hydrocarbon compounds. For this, is easy concludes that molybdenite differs from other sulphides in that its surface is hydrophobic. The natural floatability of molybdenite is attributed to its crystal structure. Molybdenite surfaces formed by rupture of weak Van Der Waal’s bonds is hydrophobic and consequently, naturally floatable. Other aspects, perhaps no considered is that oxidation characteristics of molybdenite are different from those of several others sulphides and there is a possibility that this, may have an effect on the natural floatability of molybdenite.

Once obtained the depression of copper sulphides is possible observe a heavy froth which is named Heavy Porridge. This froth is characterized for small bubbles with high mineralization, dark blue color, and presents a movement very homogeneous to the launders. See Figure 3.

There is other type of froth named Light Porridge which is characterized for light and rounded bubbles. They present high mineralization and dark blue color. Their movement to the launder is slow and constant. See Figure 4.

Froth appearance is similar in the rougher bank, mainly in the first cells. Maybe, a practical way to know better the flotation performance is using a panning dish. This behaviour can be some times strange but the operator’s experience is the best judge.

Working with nitrogen, the gas flow in the rougher circuit must be low. An excessive value creates two things: overflow to the launders and more copper is going to the first cleaning stage. If the overflow is a serious problem, it is necessary reduce the aeration to the cells. A PLC system is very useful for smoothing aeration in each cell.

The froth depth must be high, per example 300 to 350 mm. These values can de modified (up or down) according to the molybdenum content in the copper concentrate.

Molybdenum-copper separation circuit must be fed according to the copper concentrate production so that the separation circuit can work without doing many changes in operative conditions that many times trend to create instability in the flotation performance. Per example, the storage tank that pumps slurry to the rougher circuit must maintain the same level all the time.

 

It is important the pH value which can be 10.0 to 10.3 in roughing flotation. However, work with lower values is not a restriction. Thus, values such as 9.6 to 9.8 trends to improve molybdenite flotation until certain limit. If the pH is modified, the NaHS addition to the conditioning tank has to be monitored in order to don’t have unexpected behaviour in the froth.

During roughing flotation the slurry must have 40 to 45% solids. Consequently the addition water has to be regulated according to the circulating load and the bulk copper concentrate that is being fed to the circuit. All the cells to be employed must be connected to an exhaust system and covered as much as can be possible for two reasons: first, environmental and healthy conditions, and second, NaHS trends to create hydrogen sulphide (H2S) gas which promotes copper depression. If there are not closed cells, the depression efficiency of NaHS is being bad employed during molybdenum – copper separation. See Figures 5. Hydrogen sulphide is formed during copper depression by hydrolysis of NaHS. Hydrogen sulphide can exist in aqueous form into the slurry or can take a gaseous form and be released to the atmosphere. For this reason must be connected the exhaust system to the flotation cells.

When the final copper concentrate has to be pumped for a long distance, it is very important work with the optimum dosage of NaHS because the Yield Stress is sensitive to the NaHS dosage. But, decrease the dosage so much can bring two consequences, first, flotation with bad performance; and second, the Yield Stress can take a value very low which is bad too. For these reasons is important consider that the NaHS addition to conditioning tank and rougher cells must be 70 to 80% of the total addition. The key point of addition is the conditioning tank. Per example one operation can have problems when the NaHS addition is 1.8 kg/tbulk, but not when the addition is 1.2 kg/tbulk.

Sometimes, operators can confuse molybdenite with galena because the color is very similar. One way to differentiate them is to the touch. Molybdenite is greasy and oily but no galena. This operation must be done with a panning dish when we are trying to qualify the grade of molybdenum concentrate.

The cleaning stages must be done preferably employing column cells. The cells must work under conditions no turbulent because the slurry into the cell will trend to be unstable and the overflow could be a serious problem which affects the grade of the molybdenum concentrate. Columns design is a parameter very important in the performance to be obtained in the cleaning circuit.

The washed water is very important during column operations because there are fine particles of gangue that can affect the grade to be obtained in the final concentrate. For instance, if the cleaning circuit has three stages, the columns must work with higher level in the first cleaning and increase the level in the second and third stage. The air flow must be high in the first cleaning stage and lower in the second and third cleaning stage. The washed water can increase its flow in each cleaning stage. It is necessary indicate that the cleaning circuit can have three or five stages. Its number depends on design criteria used during the metallurgical testwork. See Figure 6.

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