FLOTATION TO CONCENTRATE ZINC MINERALS
Flotation as a method to concentrate the valuable minerals in an ore was developed in the last part of the 19th century. The ore is crushed and ground in ball or rod mills in a suspension in water. Air is bubbled through this suspension held in a tank and, due to the previous addition of various chemicals, and by agitation, the valuable mineral particles become attached to the air bubbles. These bubbles rise to the surface forming a froth, which can be skimmed off. The waste particles are not attached to the air bubbles and they sink to the bottom of the tank. Frothing agents, which are complex organic chemicals, are added in very small quantities to produce a stable froth. Collecting agents are added to give the desired mineral the best affinity for the froth, generally by providing them with an aversion to water. Other chemicals are used to depress the collection of certain minerals or to remove the effects of depressants. Thus it is possible to achieve a reasonable separation of several minerals in the ore, for example zinc, lead and copper, into their respective mineral concentrates.
Typically a zinc concentrate, after flotation, will contain 50-60% zinc, and 30% sulphur if the principal mineral is sphalerite (ZnS). The remainder of the concentrate represents gangue and other minerals that it has not been possible to remove by flotation. There is, for example, never a complete separation of lead and copper from the zinc, even if the concentration plant has produced separate lead and copper concentrates. Other elements, some with value such as cadmium, silver, gold, indium, germanium and gallium, and some without significant realisable value such as arsenic, antimony, cobalt, nickel and mercury, tend also to be present in zinc concentrates. Because zinc minerals are found in natural deposits, and nature can be somewhat random in the way that it distributes the different minerals, each zinc mine produces a unique zinc concentrate that can be recognised from its composition, rather like a fingerprint.
Zinc concentrates are the raw material for zinc smelters to produce zinc metal. Because of the wide variation in composition, different smelters favour concentrates sourced from particular mines, according to the flowsheet used for zinc production and its ability to produce pure zinc and to separate the other components. Whilst some zinc smelters are limited to the use of specific, generally high-grade concentrates, many others prefer to blend a number of different materials together in order to achieve an acceptable mix that is not too high in any particular other metal or gangue component.