Comminution in the mineral processing plant or “mill” takes place as a sequence of crushing and grinding processes. Crushing reduced grinding which can be carried out until the mineral and gangue are substantially produced as separate particles [4] .

Crushing is the first mechanical stage in the process of comminution in which the main objective is the liberation of the valuable minerals from the gangue. Crushing is accomplished by compression of the ore against rigidly constrained motion path. This is contrasted with grinding which is accomplished by abrasion and impact of the ore by the free motion of unconnected media such as rods, balls or pebbles [5] .

Grinding is the last stage in the process of comminution; in this stage the particles are reduced in size by a combination of impact and abrasion, either dry or in suspension in water. It is performed in rotating cylindrical steel vessel which contains a charge of loose crushing bodies called “grinding medium” which is allowed to freely move inside the mill, thus breaking the ore into particles. According to the ways by which motion is imparted to the charge, grinding mills are generally classified into two types: tumbling mills and stirred mills [6] .

Most minerals are finely disseminated and intimately associated with gangue; they must be deliberately unlocked or liberated before separation can be undertaken. Therefore, the study is geared toward establishing the liberation or release of the ore from the associated gangue mineral at the coarsest possible particle size. If such an aim is achieved, then not only is energy saved by the reduction of the amount of fines produced, but any subsequent separation stages become easier and cheaper to operate.

Particle size analysis of the various products of a concentrator is a fundamental part of laboratory testing procedure. It is of great importance in determining the quality of grinding and in establishing the degree of libation of the values from the gangue at various particle sizes.

In the separation stage, size analysis of the product used to determine the optimum six of the feed process for maximum efficiency and to determine the size range at which any losses are occurring in the plant, so that they may be reduced. It is essential, therefore that methods of size analysis must be accurate and reliable, since important changes in plant operation may be made on small amount of materials used in the size test. It is also necessary that adequate care should be taken to ensure that sample is true representative of bulk materials in size analysis; same care is required for assay.

Sieve analysis is one of the oldest methods of size analysis and is accomplished by passing a known weight of sample materials successively through fine sieve sizes, and then weighing the quantity of a particular size collected on each sieve size to determine the percentage by weight. Fraction sieving is carried out with wet or dry materials and the sieve is usually agitated to expose all the particles to openings.

Sieving when applied to irregular shape particle is complicated by the fact that a particle with a size near that of the nominal aperture of the test sieve may past only when presented in a favorable position. As there is an inevitable variation in the size of sieve apertures, due to irregularity of weaving, prolonged sieving will cause the larger apertures to exert an unduly large effect on the sieve analysis. Given time, every particle small enough could find its ways through such a few holes. The procedure is also complicated in many cases by the presentation of near-size particle which causes blinding, or obstruction of the sieve apertures, and reduces the effective area of the sieving medium. Blinding is most serious with test sieve of very small aperture size.

The process of sieving may be divided into two stages: first, the elimination of particle considerable smaller than the screen apertures, which should occur fairly rapidly; and the second, the separation of the so-called “near-size” particle which is a gradual process rarely reaching final completion. Both stages require the sieve to be manipulated in such a way that all particles have opportunities for passing the apertures, so that any blinding an aperture may be removed from it. Ideally, each small particle size should be presented individually to an aperture, but for largest sizes this is usually not practicable.

The effectiveness of a sieving test depends on the amount of materials put on the sieve (the “charge”) and the type of movement imparted on the sieve; a comprehensive account of sampling techniques for sieving is given in BS 1017 [7] .

Basically, if the charge is too large, the bed of materials will be too deep to allow each one a chance to meet an aperture in the most favorable position for sieving in a responsible time. The charger therefore is limited by the requirement that the end of the sieving should be appropriate to the aperture size. On the other hand, the sample must contain enough particles to be representative of the bulk, so a minimum size of sample will have to be subdivided into a number of chargers if the requirement for preventing over-loading of the sieves is to be satisfied.

This is the second fundamental operation in mineral processing concerned with the separation of valuable minerals from the gangue. Once the comminution process is completed, the succeeding operations in mineral processing are taken over by what is known as separation. Regardless of the method or methods used, the aim is always the same to take a natural aggregate of minerals (an ore) and separate it into two or more mineral products. In general, the products of separation are: 1) the concentrate which contains the valuable minerals; and 2) the tailings which contain primarily materials of little or no value. It may be borne in mind that minerals have been liberated, either by grinding or by chemical means, and must usually be “sized” prior to their separation from each other because the efficiencies of most separation methods are improved when closely sized fractions are used.

A common feature of all separation processes is that they are imperfect―some of the materials of no value contaminate the concentrate to some extent, and some of the minerals of value are always present in the tailings in small amounts [6] .

The research study will establish the particle size at which the various grains in an ore happened to be free of each other, thus setting the stage for effective and efficient separation process as well as beneficiating the ore by appropriate method of concentration.

In order to have a true fraction representation of lead ore from the bulk, sample were collected from different points at different depth ranges using the random sampling techniques within the study area.

The equipment used in this research work is listed below:

1. Denver crushing machine

2. Denver pulverizing machine

3. Denver sieve shaking machine

4. Denver froth flotation machine

The samples collected in lumps size were broken manually with sledge hammer to provide a required size ac-

ceptable to laboratory jaw crusher. The samples were crushed and pulverized, then coned and quartered to yield a representative sample

200 g of the prepared sample was subjected to sieve analysis. The arrangement of the sieves were done using a sieve scale in which the ratio of the aperture widths of adjacent sieves is the square root of two (√2 = 1.414). Sieve sizes from 250 µm to 63 µm were arranged in a stack with the coarsest sieve on the top and the finest at the bottom. A tight fitting pan was placed below the bottom sieve to receive the final undersize and a lid was placed on top of the coarsest sieve to prevent escape of the sample. The arranged sieves were placed in a sieve shaker which vibrates the materials vertically. The duration of the screening was controlled to 30minutes by an automatic timer.

During the shaking, the undersize material falls through successive sieves until it is retained on a sieve having apertures which were slightly smaller than the diameter of the particle. After a successful operation each size fraction retained on each sieve was collected weighed and value recorded.

Mini Pal was used which is a compact energy dispersive x-ray spectrometer designed for the elemental analysis of a wide range of samples. The system is controlled by a PC running the dedicated mini analytical software.

The mini Pal 4 version in use if PW x-ray spectrometer, which is an energy dispersive microprocessor controlled analytical instrument designed for the detection and measurement of element in a sample (solid, powders and liquids), from sodium to uranium.

The pellet is loaded in the sample chamber of the spectrometer and voltage (30 KV maximum) and a current (1 MA maximum) is applied to produce the x-rays to excite the sample for a present time (10 mins. in this case). The spectrum from the sample is now analyzed to determine the concentration of the element in the sample.

The following equipment and materials were used:

- Laboratory flotation machine

- Flotation cell

- Beakers

- Scraper

- Collector; sodium ethyl xanthate

- Frother; pine oil

- PH regulator; sodium carbonate(soda ash)

- PH meter

a. 100 g of the sample was obtained from the sieve size of 63 µm being the liberation size.

b. The sample was empty into the flotation cell and 2 cm³ of water added below the cell tip.

c. Impeller was start up for agitation with no air passing through the cell. Impeller speed was about 2000 rpm, having about 1cm clearance from the bottom of the cell.

d. The PH was checked and adjusted to 9.5.

e. 3 drops (0.0015 kg/tonne) of collector was then added.

f. Conditioning (without air) was done for about 10 minutes after collector was added.

g. 3 drops (0.0015 kg/tonne) of frother was added before the expiration of conditioning time.

h. Air was allowed to pass through the pulp at a reasonable rate.

i. The mineralized froth was skimmed until barren froth persists.

j. The froth and tailings were filtered, washed and dried then weighed.

It was observed from the sieve test results presented (Table 1) that 27.26 g of the total weight (200 g) was retained on the 250 µm sieve size, 20.18 g on 180 µm, 35.26 g on 125 µm, 31.686 g on 90 µm and 34.64 g on 63 µm. From the table, the mesh of grind was calculated to be 189 µm.

The chemical analysis of the sample was conducted on all the five sieve sizes (Table 2). The result in its elemental form indicated that 250 µm contain 9.8% Pb, 180 µm contains 10.55% Pb, 125 µm contains 6.99% Pb, 90 µm contains 14.5% Pb while 63 µm contain 15.1% Pb.

The result has confirmed 63 µm to be liberation size of Enyigba lead ore having the highest deportment of lead in its elemental form.

The froth flotation results shows (in Table 3) two products; tailings with weight of 15.7 g and assay value of 30.7% Pb while concentrate with weight of 81.8 g, assay value 69.8% Pb respectively. The result revealed that the ore has been enriched from 15.1% to 69.8% Pb, also with 82 percent by weight lead concentrate obtained, it has been confirmed to be of economic grade having surpassed 60% wt Pb element in an ore [12] .

From the foregoing, the research work established the mesh of grind to be 189 µm for Enyigba lead ore and libe-

ration size to be 63 µm sieve size having the highest recovery of lead in its elemental form. Also further work was done on the separation using 63 µm sieve size (liberation size) by froth flotation technique and the ore was enriched from 15.1% to 69.8% Pb assessed by using x-ray fluorescence (XRF). And 82% lead concentrate by weight was obtained. It is hoped that this will enhance its development to meet both local and international demand for potential investment opportunities.

Further work needs to be done especially to determine the reserve estimates of the deposits and the possibility of metallurgical process of smelting of the concentrates obtained from the beneficiation.