Prehomogenization/Preblending is a unit operation to increase consistency of raw materials and solid fuel to ensure product quality and operational stability. Although modern technology and improved methodologies has greatly influenced to bring down inconsistency in raw materials through mine planning greatly facilitated by the introduction of Global Positioning System (GPS) . However, the remaining material variations can be reduced more economically by preblending via staking and reclaiming. Two piles are normally operated with one being built while the other is reclaimed, Length to width ratio of stock pile should be at least 5:1. The preblend pile is usually the principal buffer storage between quarry/crushing and raw milling, it should be minimally sized to maintain mill operation during the longest expected interruption in the supply of raw materials from quarry or crusher, which may be weekend shut-down of the quarry, or the time required for maintenance of crushing equipment. Total capacity of 7-10 days consumption is considered normal. Preblend piles need to be covered, both to prevent rainfall on the fine and, therefore, absorbent material, and to contain fugitive dust.
Chemical analysis of the crushed material as fed to stacking pile storage is inevitable. This may either be done manually by periodic sampling and analysis (Wet method or XRF), or by neutron activation analysis which can continuously analyse the material flow falling though a chute or, better, traveling on a belt conveyor.
Knowing the quantity to be piled and the feeding and reclaiming rates, time period for building up piles and number of layers in forming full pile can be worked out. Degree of preblending that can be achieved can also be influenced. Blending effect as a function of the number of layers. Preblending effectiveness (the ratio of estimated standard deviations [s] for feed and product) is approximately related to the number of layers [N] by;
Blending ratio = s (feed) / s (product) = (N/2) ^1/2
Typically, a pile is built of 100-400 layers resulting a blending ratio of 3-6 for raw data and 6-15 if the variations within each reclaimed slice (which should be eliminated by normal handling) are ignored (Labahn). As the number of layers increases, the standard deviation in the reclaimed material will decrease as depicted in graph.
Types of Stock Piles
There are two types of stock piles-
- Linear stock pile, arranged side by side or in line.
- Circular stock pile.
In case of linear piles when one pile is built up, material is extracted from the second pile. Thus two piles supply material to the plant alternately. Expansion is easily possible in linear preblends. In case of a circular stock pile, pile is formed to occupy three fourths of the circle. Pie is built up from one end and extracted from the other end. Circular preblends are of significantly lower construction cost than longitudinal beds, but preclude subsequent expansion.
Receives crushed material from the crusher. It consists of a movable carriage which moves on rails along the planned piles. Stacker belt goes over this movable carriage and drops material it is carrying on a cross belt which reaches up to the center of the stock pile and which can move upward and downwards. Material is dropped from this belt, as the main belt moves rate along the pile. When cross belt reaches the end of travel, carriage starts moving in reverse and drops another layer on the pile. Pile thus gets built up in layers with hourly material variation distributed along the pile length to accomplish blending of material.
Ways of Building Pile
There are many ways in which layers can be spread and piles built up; the most common being 'windrows' and' chevron'
Chevron - stacking in layers along a single axis with the feed conveyor
sweeping backwards and forwards along the length of the pile. This is commonly used stacking method.
Windrow - stacking in longitudinal strips side by side and then in successive layers; this avoids the segregation which characterizes chevron stacking. The windrow system requires a more complex and expensive stacking belt arrangement
Note: For building up piles of crushed coal different methods are used so as to form trapezoidal piles because piles are to be restricted to a height of 3 metres.
Reclaimer consists of a hoe or a rake which extricates crushed material in layers of small thicknesses. Extricated material falls on conveyor running across the width of the pile and drops it on a belt to take it to respected feeding hoppers of grinding section. Thus the hoe and conveyor also move on rails for entire length of the pile. Speed of travel of carriage can be varied as required. Angle of hoe may also be adjustable to suit angle of repose of the material reclaimed from the pile.
Ways of Reclaiming
End Reclaim – Reclaimer scrapes an entire end face with a transverse scraper at floor level moving material to a discharge conveyor
Side Reclaim - A boom mounted scraper working end to end along the side of the pile. This gives less effective blending and reclaiming rate is not constant.