Specific tasks of microscopical investigations

The systematic treatment of an unknown ore is impossible without accompanying mineralogical, particularly ore microscopical investigations. Of course microscopy must not be limited to crude ores or mill feeds but accompany the whole treatment process by studying all its intermediate and final products. Together with metallurgical bench scale tests ore microscopy forms an integral part of the predictive metallurgy, which tries to foresee the metallurgical behaviour of an ore and the quality of its concentrates. Thus microscopy plays a decisive role in generating parameters that are needed to decide the feasibility of a mining project and to select an appropriate ore dressing process. Thereby microscopy bases essentially on the determination of the mineral inventory and the mineral textures and lockings of an ore and its treatment products.

The determination of the mineralogical composition or mineral inventory is certainly the first thing to do when investigating an unknown ore. It allows to establish important parameters that have an impact on the metallurgical treatment, which a chemical analysis alone cannot provide. On the contrary, a mineral inventory is indispensable to understand and explain the chemical analysis. While a chemical analysis does not tell how the analysed elements occur in an ore, microscopy shows the minerals that are formed by these elements. It shows whether deleterious elements such as As, Bi, and Hg can be eliminated by metallurgical measures or if they will report to the concentrates attracting smelter penalties. The penalties are deductions from the original concentrate value which may represent a substantial reduction in the revenues of a mining operation.

The importance of a proper mineral inventory can best be explained by a few examples: It is obvious that alteration phenomena affect the surface and therefore the specific behaviour of minerals during the recovery process. It is clear that sulphide ores require other treatment techniques than oxide ores. And it is also understood that the metal recovery from complex polymetallic ores differs substantially from that of simple monomineralic ones.

It makes a difference whether the copper carrier of an ore is chalcopyrite or if it is bornite. Although both minerals are treated by the same process the copper grade of a chalcopyrite concentrate hardly exceeds 30% whereas with bornite one easily obtains a concentrate containing over 40% copper. Relative to the copper contents the sulphur contents are higher or lower, a fact that is important for smelters that have to control their sulphur gas emissions

There are minerals that affect a beneficiation process: Certain types of pyrrhotite as well as melnicovite pyrite tend to decay rapidly into sulphates after being exposed to air. The generation of large surfaces during the comminution of the ore often accelerates this oxidation process dramatically with the effect of a steep drop in the pH value of the ore pulp that requires immediate action.

Certain elements reduce the quality of the mill products if they cannot be eliminated during flotation: Iron contents in zinc ores e.g. may originate from independent minerals such as pyrite, pyrrhotite, and magnetite or may be an integral part of the sphalerite lattice. In latter case they cannot be eliminated by means of flotation and will reduce the value of the corresponding zinc concentrate. The same is true for arsenic in copper ores; it may be bound to arsenopyrite that can be depressed during copper flotation or it may occur together with copper in the same mineral e.g. as enargite or tennantite.

The determination of mineral textures and lockings is the second important task of ore microscopy. It indicates the liberation size of a mineral which is the key parameter in ore grinding. Ore grinding which is expensive, is best performed according to the principle as fine as necessary and as coarse as possible, and it is the ore microscopy that provides the required supervision. Once an optimum liberation for mineral separation is obtained the ore must not be ground any further. The quantitative microscopical assessment of mineral lockings is the only appropriate tool to evaluate directly the efficiency and economy of a grinding process and to warrant its permanent control and optimisation.

The following examples illustrate the metallurgical impact of mineral textures and lockings: A well known and frequent phenomenon in ore microscopy are tiny chalcopyrite inclusions in sphalerite forming an emulsion-like texture. In metallurgy this texture is called “chalcopyrite disease” and it cannot be cured because the chalcopyrite inclusions are resistant to grinding. It makes the sphalerite inclined to float with the copper minerals and it is responsible for copper losses in the zinc concentrate.

Another example that shows the limits of grinding are accessory bismuth minerals that often occur in the form of milling resistant inclusions within galena or chalcopyrite leading to painful smelter penalties for the affected concentrates.

Whenever colloform mineral textures appear we have to envisage problems due to poorly crystallised sulphides that tend to decay during the recovery process contaminating the ore pulp with all sorts of metal ions that lead to an erratic flotation behaviour.

Summing up the facts and examples just given it can be stated that ore microscopy is an indispensable and effective tool for the evaluation of an ore and its milling products. It can make very concrete predictions on the metallurgical behaviour of minerals and on the quality of the planned products. It can give specific recommendations to the metallurgist enabling him to elaborate an optimum treatment process for a given material. It helps to avoid unpleasant surprises by identifying metallurgical problems at an early project stage, and in extreme cases ore microscopy can even decide if an ore is amenable to beneficiation or not.