In conventional closed circuit grinding practice including classification in one hydrocyclone, or in a group of hydrocyclones in parallel, classification takes place in a single stage. Under conditions where the fineness of the overflow product is expressed e.g. as 95 % – x microns, the normal sharpness of size separation (recovery) at size x into the fine product is of the order of 50% at a common circulating load of 200% “Classification is undoubtedly the least efficient unit process in our plants.”
Ever since the general introduction of the hydrocyclone into the concentrators of the world, extensive attempts have been made to improve its unsatisfactory performance, including injection of wash water above the spigot and reclassification of the underflow of the first cyclone in a two-stage unit. “Neither method has been sufficiently successful to warrant general adoption.”
In spite of its limitations, the hydrocyclone can no longer be expelled from the thousands of wet grinding circuits of the world. Accordingly, the natural primary step in an industrial two-stage hydraulic classification system will consist of the conventional cyclone separation. Two major advantages will result:
- the overflow pulp density will be substantially higher when classification takes place in a centrifugal field rather than in an open gravitational classifier;
- the hydrocyclone will reduce the size of the apparatus needed for the second stage.
The basic philosophy behind the present two-stage hydraulic classification process is to accept the hydrocyclone with its limitations, but for the primary classification step, only, its shortcomings should be corrected by reclassification of the impure sand product in a separate independent secondary step.
The essential requirements imposed on the second-stage classifier are:
- its normal feed material will consist of the impure cyclone sand product, reclassification of which by existing means has proved to be difficult, if not impossible
- its capacity must be great and sharpness of separation good
- it must be simple, fool-proof in operation, wear resistant and economical on energy
- it must be small enough to fit easily into the existing circuits.
Figure 1 shows the new hydraulic cone classifier developed in Finland as a result of over 15 years of active work. The cyclone sand product is introduced into the apparatus with plenty of additional water via feed tube. The separating fine product overflows rim 3 in- to launder 4; radial vanes 2 make the ascending flow substantially laminar. The grains not carried away in suspension proceed outward on discs 10 and 11 into cone 5 where they are kept in forced continuous motion to prevent accumalation of sand layers on the inside wall of the cone. For this purpose the classifier is equipped with vertical mechanism, rotated at a low speed, carrying two sets of radially projecting blades 13 and 15. A multitude of wash water jets are introduced from ring 6 via e.g. 100 holes across the settling sand layer against conical member 14. The fines removed with the jets obtain a flow component upward via central flow tube 12 and return back to the upper classification space proper. Washed and cleaned sands are discharged via apex opening 8.
The biggest cone classifier built so far has an effective diameter of 250 cm. Its feed capacity is 250 – 400 tons of solids per hour. The motor required to rotate the low speed mechanism is 11 kW, only.
Figure 2 presents the basic principle of the present two-stage hydraulic classification circuit. Essentially, it includes at least one grinding mill, at least one hydrocyclone operated as the primary classifier, normally one cone classifier operated as the secondary classifier, and normally on a centrifugal pump for pulp circulation. For the said two-stage classification system
- feed to the cone classifier is by gravity; no additional pumping step is required
- the additional space requirement is small
- the additional power needed is negligible
- the fine products from the two stages can be processed together or separately .
The two-stage classification system has the following advantages:
- greatly improved final sharpness of size separation. If sharpness of classification in the first stage cyclone is 50% and in the second-stage cone similarly 50% in respect of the remaining fines, the overall sharpness of classification becomes 75%.
- increased grinding capacity by replacing unnecessary, even detrimental circulation of fines by a higher feed rate to the (primary) mill.
- reduced energy consumption (kWh/t) due to higher feed and production rates.
- narrower particle size distribution and reduced proportion of extreme fines (slimes) in the final fine product
- reduced specific surface area on the product.
- improved overall performance in flotation, thickening, filtration and drying as a result of lower fines content.
In mineral processing the following three key questions exist regarding closed circuit grinding:
- How to produce (with a given mill or mills) the maximum tonnage of final fine material that meets a given size specification (such as 95 % – x µm)?
- How to produce the said fine product to have the lowest possible specific surface area (in other words the steepest possible size distribution curve and the minimum of extreme fines)?
- How to produce the said fine product at the lowest possible energy consumption?
The two-stage classification system as presented here seems to offer a simple and economical way toward a practical solution of the problematic questions presented above. The said system, furthermore, should be applicable to the majority of wet closed grinding circuits regardless of the scale of operations.
