Grinding Mills

INTRODUCTION Grinding is the second step of mineral processing and the last stage of the comminution process. The product from a crushing unit is fed to a mill in order to decrease the particle size (sometimes even to 10 microns) for subsequent processing. The reduction ratio is usually large (8 to 25:1 sometimes 500:1). The purpose of grinding differs with the material being ground. For example in a processing plant, the primary purpose is to liberate individual minerals trapped in ores for a subsequent enrichment. In some non-metallic beneficiation, grinding is practiced to satisfy market requirements. In hydro-metallurgical work, exposure of valuable mineral to the leach solution is the main purpose of grinding. PRINCIPLES OF GRINDING:- Grinding is performed in a rotating cylindrical steel vessel which contains crushed ore with a grinding medium free to move inside the mill, lifted by the rotation of the drum, to break the ore to produce a specified product. The grinding medium can be the ore itself, natural or manufactured non-metallic media or manufactured steel e.g. steel rods, steel or iron balls. Grinding within a mill is influenced by the size, quantity, the type of motion and the spaces between the individual pieces of the medium in the mill. Grinding is a more random process and is subjected to the laws of probability. The degree of grinding of an ore particle depends on the probability of the ore entering a zone between the medium shapes. Figure 1: Mechanisms of breakage: (a) impact or compression (b) chipping (c) abrasion Grinding can be done by several mechanisms, including impact or compression, due to forces applied almost normally to the particle surface; chipping due to oblique forces; and abrasion due to forces acting parallel to the surfaces. These mechanisms distort the particles and change their shape beyond certain limits determined by their degree of elasticity, which causes them to break. (Wills, 2006) The motion in a grinding mill comprises two distinct varieties (Wills, 2006):- Rotation of the rods or balls around their own axes lying parallel to the mill axis. CASCADING (rolling down the surface of the load) or CATARACTING (parabolic free fall above the mass). Figure 2: Motion of charge in a tumbling mill MATHEMATICS OF GRINDING Critical Speed:- The "Critical Speed" for a grinding mill is defined as the rotational speed where centrifugal forces equal gravitational forces at the mill shell's inside surface. This is the rotational speed where balls will not fall away from the mill's shell. Mills are driven, in practice, at speeds of 50-90% of critical speed, the choice being influenced by economic considerations. Increase in speed increases capacity, but there is little increase in efficiency (i.e. kWh/t) above about 40-50% of the critical speed. Height attained: The maximum height up to which the particles go along the mill shell and then get thrown off and follow a parabolic path. Power Consumption: As it earlier explained, grinding is the most energy-intensive operation in mineral processing. The basic formula for this is the Bond formula: where P80 and F80 are the 80% passing sizes of product and feed in microns, and Wi is expressed as kWh/t. The calculated power requirement is adjusted by utilizing efficiency factors dependent on the size of mill, size and type of media, type of grinding circuit, etc., to give the operating power requirement. (Rowland and Kjos, 1978) MECHANICAL CONSTRUCTION Mill shells:- Mill shells are designed to sustain impact and heavy loading and are constructed from rolled mild steel plates welded together. Mill ends: The mill ends or trunnion heads may be of nodular or grey cast iron for diameters less than about 1m. Trunnions and bearings: The trunnions are made from cast iron or steel and are spigoted and bolted to the end plates, although in small mills they may be integral with the end plates. Drive Tumbling mills are most commonly rotated by a pinion meshing with a girth ring bolted to one end of the machine. Mill feeders:- The function of the feeder is to transport pulp from some receiving point outside the mill into the mill barrel smoothly and with sufficient driving force to overcome any tendency for the pulp to move in the opposite direction. Three types of feeder are in use in wet-grinding mills. Spout feeder Drum feeder Combination drum-scoop feeder Liners:- Liners are the materials, which are used on the inner surface of the grinding shell to provide the necessary strength and resistance. Shell lining acts as the final link in the transmission of energy to the tumbling load i.e. the charge. It usually differs in type according to the size of feed, whether the feed is coarse or fine. Liner materials: Abrasion Resistant (AR) Steel, Ultra High Molecular Weight Polyethylene Plastics, Urethane liners (polyurethane elastomer), High Alumina Ceramic liners, Rubber liners, Metal magnetic liners etc. Here are different types of liner plates:-

DIFFERENT TYPES OF MILLS

                                        Figure 4: Different types of grinding mills

According to the ways by which motion is imparted to the charge, grinding mills are generally classified into three types: tumbling mills, stirred mills, and vibrating mills.



Tumbling mills: In this mill, the mill shell is rotated and motion is imparted to the charge via the mill shell. The grinding medium may be steel rods, balls, or rock itself. Tumbling mills are typically employed in the mineral industry for coarse-grinding processes, in which particles between 5 and 250 mm are reduced in size to between 40 and 300 microns.

                                   Figure 5: Mechanism of grinding in tumbling mills

Different Types of Tumbling Mills

Rod Mills:- Mills loaded with rods as the grinding media are used for primary grinding of rocks and minerals. The rods fall from a height and roll down the mill so the rods impart an impact force as well as an abrasive action. The product size from a rod mill is much coarser. Hence a rod mill generally precedes a ball mill in a grinding circuit especially where a fine size product is required.

Ball Mills: -The final stages of comminution are performed in tumbling mills using steel balls as the grinding medium and so designated "ball mills." Since balls have a greater surface area per unit weight than rods, they are better suited for fine finishing.

Autogenous (AG) Mills:- Autogenous mills are so-called due to the self-grinding of the ore: a rotating drum throws larger rocks of ore in a cascading motion which causes impact breakage of larger rocks and compressive grinding of finer particles.
Semi-Autogenous (SAG) Mills:- SAG mill is similar to AG mills, but utilizes grinding balls to aid grinding just like ball mills. It uses a ball charge of approx. 6 - 15%. SAG mills are characterized by their large diameter and short length as compared to ball mills. SAG mills are primarily used in the gold, copper and platinum industries with applications also in the lead, zinc, silver, alumina and nickel industries.

Stirred mills:- The mill shell with either a horizontal or a vertical orientation is stationary and motion is imparted to the charge by the movement of an internal stirrer. Fine grinding media inside the mill are agitated or rotated by a stirrer, which typically comprises a central shaft to which is attached pins or discs of various designs. Stirred mills find application in fine (15-40 microns) and ultra-fine (<15 microns) grinding.

Most common stirred mills are:-

Tower mills (VertiMill):- In a tower mill, steel balls or pebbles are placed in a vertical grinding chamber in which an internal screw flight provides medium agitation. Product sizes may be 1-100 microns at capacities of 100t/h or more.

IsaMills (horizontal):- IsaMill uses small grinding media and high stirrer velocity to impart energy to the media, which increases the breakage rate of fine particles caused by attrition/abrasion at relatively low power consumption. It is claimed that the IsaMill can efficiently grind minerals to below 10 microns. The grinding media that can be used include granulated slag, river sand, or a sized portion of the ore itself.

Vibrating mills:- The motion is imparted to the charge by the vibratory motion imparted to the shell. The vibration can be generated by springs, electromagnets, air cushions or unbalanced mass. These mills also have application in fine (45 microns) grinding.

High-energy vibration mills can grind materials to surface areas of around 500m2/g, a degree of fineness, which is impossible in a conventional mill (Russell, 1989). Vibration mills are made with capacities up to 15t/h, although units of greater capacity than about 5t/h involve considerable engineering problems.

COST OF GRINDING


The main costs for grinding are energy, liners and grinding media. They are different for different mill types.

Figure 6: Cost of grinding in different mills

GRINDING CIRCUITS


The feed can be wet or dry, depending on the subsequent process and the nature of the product.

Circuits are divided into two broad classifications: open and closed.

Figure 7: Two different grinding circuits

  Rod mills are generally operated in an open circuit because of their grinding action, especially when preparing feed for ball mills. Ball mills, however, are virtually always operated in a closed circuit with some form of classifier. Various types of classifying device can be used to close the circuit, such as mechanical classifiers, hydro-cyclones, etc. The AG/SAG mills can be operated in open or closed circuits. However, even in an open circuit, a coarse classifier such as a trommel attached to the mill, or a vibrating screen can be used. The oversize material is recycled either externally or internally. In many recently designed plants, the traditional three stages crushing followed by rod and ball milling circuit has been replaced by the popular SABC circuit, denoting a circuit comprising SAG mill, followed by ball mill, then closed with cyclone.