Energy efficiency in grinding (Molycop-Peru)

MEDIA TYPE EFFECT ON GRINDING EFFICIENCY

Levi GUZMÁN

Senior Applications Engineers

Moly-Cop Adesur S.A., PERU

Email: lguzman@adesur.com.pe

Key words: Grinding, comminution, Mineral processing, media type,

ABSTRACT

The consumption of energy in the grinding process is significant in both the amount used and the cost involved. Both imply that it is important to maximize the throughput for a given grinding task; which in turn implies it is important to maximize mill power draw, which is related to the efficiency whereupon this power is used.

In order to optimize the process it is first necessary to know the effects of the operative parameters on the ore grindability because it is the grinding efficiency that is to be evaluated; that is to say, the efficient use of the energy from the metallurgical point of view in conventional ball grinding, recognizing that such concepts and criteria also apply to other types of applications such as semiautogenous grinding (SAG) and vertical mills.

It was demonstrated that it is possible to optimize the grinding process by means of the correct selection of grinding media that allows maximizing the effectiveness (power draw) and the power efficiency of the process (correct use).  For example, simulations demonstrate that using forged steel grinding balls (high density) [compared to cast steel (lower density) balls and to high chromium white cast iron (lowest density) balls] increases throughput by 2.2-4.4% and reduces the specific energy consumption by 2-3% (at constant feed size and product size).

INTRODUCTION

It has been estimated that the amount of energy used in the comminution process is equivalent to 3% of the world-wide energy (Pease, 2007), and is the reason optimization of energy consumption in the comminution process has been and still is one of the main objectives of various researchers and operators.

At the moment the conventional grinding technologies of SAG and Ball Mills, are power inefficient.  They use from 3 - 5% (Fuersteneau, 2003) of the total of the energy consumed. Recently some researchers indicate that the maximum grinding efficiency is limited to about 20% (Arentzen, Bhappu, 2008).

On the other hand, it can be argued that the grinding efficiency is much more important than the grinding media cost, since the benefits of obtaining a greater capacity of treatment are several times greater than the magnitude of the grinding media cost. It is in this sense the effects of grinding media density and optimal ball size play a major role in the optimization of the grinding stage.

Other authors benchmark the power efficiency according to the configuration of the circuit; nevertheless there is not sufficient evidence to demonstrate statistically that a given circuit design is consistently superior from the point of view of energy efficiency (Morrell, 2009).

In addition, the belief exists that the classic Inverse configuration is intrinsically more productive than the alternative Direct configuration (Sepúlveda, 2008b), this under the context of the so-called “Fourth Law” of the Grinding/Classification, which affirms that “to obtain an efficiency of optimal energy in the grinding total process, the content of fine particles in the mill charge must be as low as it is possible for a given grinding task”. Practically speaking, the Inverse configuration will only be advantageous when the flow of fresh feed contains more than 30% of ore particles finer than the specified objective P80 size for the operation.

Recognize there is a potential to increase the apparent efficiency of the energy use by significantly increasing the circulating load and increasing the throughput (depending on each particular application). With a high circulating load, the balls in the mill act preferably on heavier particles that still need to be fractured, avoiding at the same time the overgrinding of the finest particles (Sepúlveda, 2008a). There is also a similar maximum potential from improving the classification efficiency (Morrell, 2008).

The present work purpose is to determine the effects and relationships among energy – grindability – grinding media type in grinding optimization stage.

THEORETHICAL BACKGROUND

ENERGY SPECIFIC CONSUMPTION

From the first studies on comminution of ores in the middle of the last century, it has been recognized that a major role is played by “energy specific consumption” in determining the parameters of the process results. In other words, the amount of mechanical energy applied to each unit mass of particles to a great extent determines the fineness of resulting fragments.  That is to say, the energy specific consumption is not more than the net consumption of energy (kWh) by each ton of processed fresh feed. As an illustration, note the experimental information shown in Figure 1 (Siddique, 1977) which was obtained using batch tests of dry grinding with mills of 10, 15 and 30 inches of diameter. From these results, Figure 1 demonstrates the clear relationship that exists between the energy specific consumption and the resulting product fineness in each test.

[pic 1]

Figure 1: Product Energy – Size Relation

BOND’S LAW

Bond postulated in 1952 an empirical law that has been termed the third law of the comminution, which is denoted by the following expression:

[pic 2]                                (1)

Where Wi is the ore work index or Bond work index and depends as much on the material as on the equipment used for each specific application. Wi consequently represents the specific consumption required to fracture very heavy particles, P80 = 100 microns.

and,

E        =        Energy specific consumption, kWhr/ton

F80        =        Size passing 80% in feeding (microns)

P80        =        Size passing 80% in product (microns)

Wi        =        Bond work index, kWhr/ton

[pic 3]

Figure 2: Bond’s Law

The previous equation confirms the importance of the energy specific consumption as the determining parameter of the comminution process. In the previous expression, the parameters F80 and P80 represent the defined grinding task; that is to say, the objective is to transform particles of characteristic size F80 into particles of size smaller than P80. The Bond Index allows then, by means of equation 2 shown below, to determine the energy (kWhr) required to grind each unit (ton) of ore. This energy specific consumption is also determined by the relationship.

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