Pyrite magnetic separation

Magnex A process for removing mineral matter from coal by first rendering it magnetic. The coal is treated with iron carbonyl vapor, which deposits a thin skin of magnetic material on the pyrite and other mineral matter, but not on the coal. Conventional magnetic separation is then used. Developed by Hazen Research in 1976. 

As regards cleaning, for example, a roast process can be applied to convert pyrite to a magnetic form, followed by its magnetic separation together with garnet and pyroxene. Another cleaning step in the presence of sulfide minerals would be a sulfide flotation. 

The high sulphur content of Eastern Canadian coals, as much as 8 , has led to studies aimed at reducing these sulphur levels (1). In general about two thirds of the sulphur is pyritic, often occuring as small inclusions, and therefore difficult to remove by conventional means. One method which holds promise is pyrolysis followed by magnetic separation. 

Pyrolysis involves the heating of coal in a non-oxidising environment producing char, liquid and gas. This publication deals with the reactions of pyrite in the coal, the partitioning of the pyritic sulphur during pyrolysis, and the effect this has on magnetic separation. 

The fate of pyrite in coal has been the subject of a number of publications, particularly in the area of liquefaction, where pyrite or its products are thought to play an important catalytic role (2). In a previous publication (3) it was reported that the decomposition of pyrite to pyrrhotite occurred in the temperature range 500-550°C for a run-of-mine(ROM) Prince coal. This was within the range of 440-580°C reported by other workers for the decomposition of pyrite in coal (4,5). The current work extends the previous study to three washed coals and includes some preliminary work on magnetic separation. 

Magnetic separation has been applied by other workers (6) who have shown that rejection of 95—99% of pyritic sulphur is possible, if advantage can be taken of the pyrite/pyrrhotite transformation. This latter process not only involves a loss of sulphur but concurrent 100- fold increase in magnetic susceptibility (7). It is this increase which makes magnetic separation attractive. 

Magnetic Separation. The future utility of pyrolysis as a process depends upon the successful use of the char. From the discussion on sulphur transfer, it is evident that as the result of pyrolysis some sulphur which is difficult to remove physically (well disseminated pyrite) is transformed to sulphur that is impossible to remove physically (organic sulphur). In addition it has been shown that this newly formed, organic sulphur is present in a very stable form. This would seem to be counter productive to any scheme which must include sulphur reduction as a primary goal. However, by carefully controlling the pyrolysis reaction conditions it should be possible to produce chars which are amenable to sulphur reduction by magnetic separation. 

Perhaps the most interesting studies are on the effeet of coal pretreatment, which enhances the magnetic properties of the mineral matter, on the subsequent magnetic separation. It has, for example, been shown that microwave treatment heats seleetively pyrite paitieles in coal without loss of coal volatiles, and converts pyrite to pyrrhotite, and as a result, faciUtates the subsequent magnetic separation. 

In the Magnex process, crushed coal is treated with vapors of iron carbonyl [Fe(CO)5 ], followed by the removal of pyrite and other high-ash impurities by dry magnetic separation. Iron carboiyl in this process decomposes on the sitrfaces of ash-forming minerals and forms strongly magnetic iron coatings the reaction with pyrite leads to the formation of pyrrhotite-like material. 

The removal of sulfur from high-sulfur coal is costly and incomplete. One method is to pulverize the coal to the consistency of talcum powder and remove the pyrite (FeSg) by magnetic separation. Reducing the sulfur content of fuel oil is also costly. It involves the formation of hydrogen... 

Key words Pyrite cinder. Roasting, Magnetic separation. Desulfurization... 

A double sulphide of copper and iron known as Chalmersite, CuFe2S3, occurs in the ore deposits of the Prince of Wales Sound, Alaska,2 and in small quantities in Brazil.3 It is massive, pale yellow in colour, and strongly magnetic, which latter property enables it to be readily separated from the copper pyrites with which it is intimately associated. Density 4 04 to 4 68. The crystals arc rhombic,... 

The preceding indicates that in order to optimise sulphur rejection, the conditions of pyrolysis should be controlled so that the magnetic susceptibility becomes sufficient for particle separation, but also so that the transformation of pyrite to pyrrhotite should be kept to a minimum. For example, it has been shown that a 1 conversion of pyrite to pyrrhotite leads to an increase in magnetic susceptibility of two orders of magnitude (13). In addition, grinding could be controlled so that the sulphur... 

---