Grinding

Autogeneous Grinding VS Steel Grinding Media in Flotation

In grinding, the use of steel media in a rod/ball mill circuit clearly diminishes the subsequent froth flotation performance over purely autogenous grinding. The use of recycle water in grinding also clearly diminishes the subsequent froth flotation performance associated with fresh tap water. Some of these flotation deficiencies are recoverable by using stronger collectors at higher dosages. On the positive side, a new collector chemistry concept based on using the inherent electrochemical transformations initiated by grinding is showing good promise at the plant scale. However, the impact of organics from water soluble collectors can be significant in reducing the pyrite selectivity possible with the electrochemical approach

The Influence of Steel Grinding Media Versus Autogeneous Grinding on the Efficiency of Sulfide Mineral Flotation: For several years, this author was involved with a flexible size reduction/classification/froth flotation pilot plant that possessed the capability of either an autogenous mill or a ball/rod mill combination. The mill sizes were selected so as to offer the potential of producing a fairly balanced throughput rate to size reduction (approximately ¾ Mt per hour) giving a similar one-point size distribution match of the feed to the flotation rougher bank. The associated flotation process consisted of ….Read more

How to Reduce Ball Mill Overload Risk

Two different circumstances must be addressed for the reduction or elimination of overload risk: a) in existing operations, or b) in the design of new grinding circuits. The methods necessary are in the first case an adjustment of operating conditions; and in the second an adjustment of mill shapes.

Existing Operations: from Equation (1) for mill power with a system of fixed dimensions, power input, and ore properties, Equation (8) follows:

Fc (1 – Lf) Lf = C1………………………………………………………………(8)

The axial velocity equation at overload (Table 3) can be changed to:

Vax = C2/[0.50 – 0.66 Lf]………………………………………………………(9)

dimensionality in ball mill dynamics

With C2 determined by use of the Lf value at overload. As shown in Table 4, it is then possible to assume values for Vax/Vax to represent degrees of axial velocity reduction, along with lower values for Lf, and calculate the values of Fc necessary to hold power constant. The mill operating data in Table 2 show that at the already low Lf value for Lornex, little if any practical overload risk reduction is likely by this means. On the other hand, Sierrita’s Lf value of 0.48 suggests that major reductions in risk ….Read more

Ball Mill Circuit Classification System Efficiency

Fines may be defined as any material smaller than a specified product size, and “coarse” as any material that is larger. The target 80 percent passing size of the grinding circuit product is a convenient reference point, as it is often used to express the grind size objective. This size is close to the modal value of typical size distributions of ground material, and also roughly approximates the classifier cut size.

The effectiveness of the fines removal system depends on the transportation of material to the classifier. It also depends on the ability of the classification equipment to remove the fines that are presented to it from the circuit, and to return the coarse material to the mill. Immediate and perfect sorting action would result in a circuit product size distribution dependent only on the material fracture characteristics (Austin et al, 1984), and avail the maximum possible amount of mill volume (and applied power) for the breakage of coarse particles. The fraction of fines in the mill hold-up (the “fines inventory”) may therefore be taken as a direct measure of the lack of the system’s fines removal effectiveness. Its complement, the “coarse solids inventory” represents the fraction of the mill volume ….Read more

SAG Mill Components

Once the basic operating conditions have been defined for the mill designer then, and only then, can he begin taking into account shell and head stresses and trunnion bearing sizes required to carry the load and to give the required openings for feeding and discharging the mills.

Mill heads are key structural components of any grinding mill. They must carry the load of the shell assembly, liners, media and material in process, and transmit it evenly to the trunnion bearings to assure smooth operation.Image result for SAG Mill Components

Manufacturers in the “big mill business” have long ago recognized that simple extrapolation of existing designs from smaller mills is poor engineering at best.

The stress analysis of a mill head is difficult because of cyclic stresses, complex shape, clamping flange to the mill shell, change in shape from its conical portion to the trunnion portion, the load imposed on the trunnion and the “ovality” of the trunnion. There are many variables (perhaps as many as 20) which affect good head design, such as the conical angle, trunnion length critical thickness, and critical radius just to mention a few.

A field-proven computer program has made possible the performance of multiple calculations ….Read more

AG – SAG Autogenous & Semi-Autogenous Grinding Mill Testing

Ff the drill cores indicate that the ore could contain sufficient lumps in the sizes needed for ore media in autogenous and semi-autogenous grinding, then bulk samples (750 kg) of mine run or primary crushed ore, with an even distribution by quantity in the size fractions between 100 mm and 200 mm, should be obtained for media competency tests. The Allis-Chalmers media competency test, which consists of tumbling 50 pieces of ore in a drum 1.8 in diameter and 0.3 m long, has been described previously. Media competency tests should be run on each ore type in the deposit, which the preliminary mining plan shows could, for an extended time period, become a major portion of the mill feed. It is recommended that each type be tested separately. Testing of composites should be reserved for pilot plant testing, if pilot plant tests are to be run.primary crusher secondary ball mill

The basic data that can be obtained from the media competency test is:

  1. Suitability of the primary media sized ore to be grinding media.
  2. Bond impact work index.
  3. Work indices from Bond rod mill grindability test.
  4. Work indices from Bond ball mill grindability ….Read more

How to Size a Rod Mill Grinding Rod Diameter

grinding

Slurry Flow Rate Through a Mill

The factors upon which the rate of flow of the pulp through a mill depends appear not to have received extensive investigation. In an article by Anselm translated by Pearson a method for the calculation of the time of passage of cement through a ball mill was given. The basis of the method is that the volume rate of throughput is known and it is assumed that the surface of the powder is coincident with the surface of the ball charge. It follows that the average throughput velocity is the quantity of flow divided by the area open to flow. From this average velocity the time for a particle to traverse the mill may be computed. This treatment does not, however, indicate whether, for a given rate of flow, the surfaces of the powder mil the ball charge will coincide or not. For example, with discharge by peripheral ports, it would be expected that the surface of the powder charge would slope quite sharply from the entrance to the exit end. With a trunnion overflow mill, however, by the nature of the design, a steep slope is precluded and there would be but a small “head” to maintain flow.

The small amount ….Read more

Grinding Mill Surging

The phenomenon of “surging” in a mill is a subject upon which very little has been written; presumably because it is a condition which cannot be tolerated in mill operation and which must be eliminated by variation of some or many of the physical dimensions or characteristics of the mill or mill charge.  The phenomenon known as “surging” consists of a pendulum-like oscillation of the whole charge of the mill, within the mill shell. Thus for one part of the cyclical motion the charge is moving around the centre of the mill in the same direction as the mill shell, and during the second part of the motion, in the opposite direction. As a consequence of this oscillatory motion of the charge, the torque necessary to maintain the mill shell in steady motion fluctuates widely and, in certain circumstances, can make such demands upon the driving motor that the circuit breakers are thrown out. This throwing out of the circuit breakers is, however, a lesser evil since the danger is, in such a case, apparent and must be remedied. A more subtle danger is hidden in fluctuations of torque which are insufficient to bring the circuit protective devices into operation. In ….Read more

Effect of Grinding Media on Milling

In the previous chapter the influence of the various physical quantities defining the mill and mill charge has been studied in connection with the performance of a mill as a device for the creation of new surface in the powder. For some purposes, however, it is also desirable that the product should have a preferred form of size distribution curve. For example, in mineral dressing it is usually necessary that the material below about 200 mesh should be a minimum. To some extent such modifications in the size distribution characteristics can be brought about by changes of the mill dimensions. It must be emphasized, however, that such changes are of limited scope but, even so, are often worth-while.

Effect of Ball Diameter, or Rod Diameter Variation

The first relationship to be considered is the effect of the variation of the ball diameter, or rod diameter, upon the size distribution obtained. In Fig. 6.1 are shown typical size frequency curves obtained by ball milling; these curves being deduced from the data of Coghill and Devaney.

As would be expected, the grinding of a coarse feed with small balls does not give rise to the effective grinding of the coarse particles in the feed ….Read more

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