Some mineral forms such as PCC are formed from a precipitation reaction and therefore need to be recovered from the precipitate liquor - mainly water - if required dry. A plant might be expected to produce both a paper coating slurry (where it is a satellite facility next to a paper mill),and a dry product for longer distance transportation. This is either because water makes slurry too costly to ship because of weight, or the application requires drying and de-agglomeration to maximise the specific surface area of the PCC crystals. The latter is best achieved by simultaneous high energy de-agglomeration (milling) and drying.

Some minerals need to be beneficiated to enhance or liberate their key properties. This often necessitates the removal of certain mineral species or impurities that impact on the properties of the required product. The main purpose of this beneficiation is to improve colour and reduce levels of silica and other compounds which can cause expensive wear rates on downstream equipment.

This is most pronounced in the dryers and mills used to finish the product, as well as extruders where the finished mineral goes on to be used as a filler in plastic extrusions. Beneficiation is carried out in various wet stages, sometimes involving reactive chemicals requiring a dilution and washing stage, but in any case ending up as a slurry which may need to be dried.

Whilst some of these minerals are then further  processed  and  sold

as slurried end-products, there is also an increasing requirement for dry product forms.

Some key minerals, which occur naturally as wet deposits, may be processed direct from the earth, such as ball clays, bentonite, kaolin and kaolinitic clays etc. Where purity, colour or abrasive impurities are not an issue, direct processing by drying and milling, often simultaneously, yields a saleable product for more basic applications.

To avoid back mixing, typically required to make the clay feedable, these "fresh from the ground" clays are often spread out to partially dry in the air if conditions are suitable, before finish drying, milling and classification.

Other mineral slurries are produced as a by-product, eg. From the quarrying of marble where blocks of marble are cut either by water jet or by water lubricated saws. The resultant fine suspension of marble particles can easily be simultaneously dried and de-agglomerated in a high energy mill-drying system. It is either back-mixed to produce a wet crumble or, if pumpable, sprayed directly into the mill-dryer.

Mill-dryers are available with integral classifiers to control the particle size of the dried powder leaving the dryer. There is a limit to the effectiveness of such integral classifiers whereby an independent unit may prove to be more efficient.

A key reason for the requirement of drying of minerals is that often, in order to achieve ultra-fine PSD's, it is wet grinding that offers the most cost effective process route, leaving the mineral as a suspension in water. Where possible, if the mineral does not require any beneficiation, this processing can be at relatively high solids content (>70%), typically assisted by chemical dispersing aids.