Pit to Plant - Current Trends

Pit to Plant - Current Trends A.D. Fernie

Abstract - Growing interest in pit crushing and conveying is evident from the recent many articles, technical papers, and new equipment development.

Application of a pit crushing and conveying system is complex. Many factors, such as mine design, production scheduling, as well as initial and ongoing capital and operating costs must be thoroughly and realistically examined.

This paper will address the fundamentals to be considered and review several approaches to developing a total system for a new and existing mine situution. The planning function will be outlined and operating and design criteria defined to interface the mining operation and the pit crushing conveying system.

Introduction

In-pit crushing and conveying (ICC) is not the universal panacea for reducing operating costs of open-pit mining operations. However, economic forces have caused a move towards ICC due to the rapid increase in labor, fuel, and maintenance costs.

As all open-pit mines are different, and since planning a multi-system method of ore and waste extraction is far more complex than following the traditional truck/shovel method with its built in flexibility, an independent evaluation of several mining schemes is required to select the most economical operation.

Mine Planning

There are two planning scenarios for ICC to consider in today's changing mining scene. First, new mines must develop basic concepts and long-range plans. Second, an operating mine must develop concepts that can be worked into the existing pit without interference with daily production.

Both planning functions may require evolving several different schemes to find the most practical and cost-effective method of transporting the ore from pit to plant.

There are a number of basic fundamentals that must be addressed in the development of an ICC system, whether it is planned for a new mine or an existing operation. For example :

Can a permanent high wall be established? ; Is it mandatory that the pit develop as a cone -

no fixed wall? ; Can the pit be mined with push backs, alternating

from side to side? ; Can a permanent ramp be established part of the

way into the pit, or to the pit bottom? ; and Is the surface topography and plant site or waste

dump such that a system of ore passes and a conveyor ramp to an adit would be practical?

In planning the pit, it must be remembered that access or service ramps need not be as wide as haulage ramps and do not have to follow ore production. The pit ramp exit is not as critical for a service ramp as for a haulage road, but should provide ready access to the pit shop.

In developing an ICC system, considerably more detailed long-term planning is required. It is possible

that more initial drilling might have to be done to establish confidence in being able to reach the ore with a conveyor system and moveable crusher, say 20 to 30 years hence. This does not mean loss of flexibility. I t is to prove that it is possible.

It is possible that phased development will become part of the overall scheme. Also for a new mine, it is likely that a pit rim crusher (fixed or moveable) will be required for a period of time, as the economics of ICC hinge on replacing the high lift and long truck haulage ramp.

To establish a fixed ramp for some length into the pit, it may be necessary to remove additional waste during pre-production or during the first few years of mining. This additional cost may be offset in later years by providing extended pit life with mineable ore at depth by the use of ICC.

One notable difference in ore/waste haulage by conveyor versus truck is the much narrower conveyor ramp at 20% to 25% slope, rather than the 7% to 8% limit placed on truck ramps. This results in a much shorter overall distance from the shovel at the face to the pit rim. In some cases, the service ramp for pit equipment access and conveyor ramps together are the same cost as the much wider main haulage ramp.

An important part of the concept planning process is applying financial analyses for various schemes projected for at least 20 years. Comparative system and financial analyses will indicate the concept most suited to the mine. Further detailed planning on the selected scheme can then be done to prove the feasibility and viability through the various phases of the operation.

One sensitive economic factor in financial projec- tions relates to petroleum fuel cost compared to electric power cost. The higher the price of electricity, the less cost advantage there is to ICC. This can influence a decision i f or when to proceed with a system.

Taking the complexity of the ICC planning function into account, it is not practical to develop a pit and then try to fit an ICC into it. Planning must be an integrated process.

Materials Handling

Large volume movement of ore/waste by truck from an open pit is usually classified as cyclic or non- continuous, as compared to continuous systems such as a bucket wheel excavator combined with moveable conveyors.

The combination of a truck/shovel operation with ICC brings the materials handling system very close to continuous. Elimination of long uphill haul distances, travel time, delays on the ramp, and truck queuing at the crusher and shovel means that with short-haul, fast-cycle truck haulage and a feeder ahead of the

A.D. Fernie is with Wright Engineers Ltd., Vancouver, British Columbia. SME preprint 83-417, SME-AIME Fall Meeting, Salt Lake City, UT, October 1983. Manuscript October 1983. Discussion of this paper must be submitted, in duplicate, prior to March 31,1985.

MINING ENGINEERING JANUARY 1985 49

crusher, continuous movement of material from the pit can be achieved.

It is acknowledged that the in-pit crusher can have 20% to 30% greater use than the surface crusher, thus achieving greater tonnage with a smaller unit (1.4 m or 54 in. gyratory instead of 1.5 m or 60 in. ).

Conveyors do not run out of fuel, develop flat tires, get stuck with a box that will not lift, or have to wait for the snow plow. Conveyors have a history of high availability when operated and maintained as a transportation system. This means providing a weekly scheduled preventative maintenance program, dili- gently followed, and preferably computer monitored.

Location of ICC equipment from benches being blasted is of concern. Today's blasting equipment and techniques keep fly rock to a minimum, allowing mining and materials handling to work closely together.

It is wise to keep the main haulage conveyors more than a safe distance from blasting. However, the crusher and in-pit conveyors can be within 100 to 120 m (330 to 390 ft) with relative safety. The most vulnerable parts of the crusher and conveyors are the drives. They can be shielded to a degree, thus reducing the risk of being struck.

System downtime for moving the crusher is of concern. Most large open-pit operations that are adding ICC have at least one fixed crusher on surface. It is often possible to find some "make-up" ore in the limbs of the pit that can be kept in-hand and hauled by truck to the surface crusher during the pit crusher move. Each mine will be different, but a way to keep the mill going for several days can usually be found.

Two important things to remember are to minimize the number of moves, and if possible, to schedule the moves at a time when the plant stockpile is full and when the mill is scheduled for a maintenance shutdown.

A criticism of ICC is loss of flexibility in an ore body with varying types and grades. Location of the pit crusher under these circumstances becomes critical as blending in the pit to one feed point becomes difficult. In some cases, it may be more economical to have two pit crushers in different locations feeding a main conveyor haulage system.

An ore pass system with two raises to separate locations in the pit is another alternative for dealing with the blending problem.

There are several basic configurations for pit crushing/conveying. All have been successfully applied: Fig. 1, underground ramp system; Fig. 2 ( a ) (b) and (c) , conveyor ramp system; Fig. 3, active ore pass, underground crusher and conveyor ramp; and Fig. 4, mobile crusher, cross-pit conveyor and high- wall bridge conveyor to pit rim.

Design Criteria

A comprehensive design criteria is usually defined after a pre-feasibility study. The study usually defines the production rate based on ore grades, reserves, and indicated metallurgical recoveries.

Design criteria for a mining materials handling system must be clearly defined to correctly select, size, and determine all components that influence capital and operating costs. This paper does not attempt to give a complete design criteria outline, but deals more specifically with factors that influence in-pit crushers and large capacity high tension conveyors.

Many of the factors set out a basis for equipment tender documents and construction/installation contracts.

CONVEVOR RAMP SECTION

Fig. 1 - Underground conveyor ramp system

50 JANUARY 1985 MINING ENGINEERING

General

Geographic location, elevation, and topography; Access, transportation, roads, rail, etc.; Climatic conditions: temperature range, precipi-

tation, rain, snow, wind velocity, and seismic data; Operating data: mine ore/waste production;

system use in hours per year deducting statutory holidays, unscheduled delays, fog, whiteout days,

snow days, p.m. shifts, and equipment moves; operating hours per day actual, deducting lunch breaks, shift changes, and travel time; system mechanical/electrical availability; unscheduled shutdown; and system start-up/shutdown;

Annual production ore and waste in tons; Daily production to meet plant rate in tons; Average production/hour ; and Design rate +20% over average rate, peak rate

(short term) +20% over design rate.

CQlVLIOll lu

CoNVEIOll Ql R A W

ORE CllVSHER FIXED OR MOVABLE

Fig. 2a - Conveyor ramp system, phase I

Fig. 2b - Conveyor ramp and in-pit movable conveyors, phase II


_---. ORE CONVEVOR TO

ORE CONVEIOR

Fig. 2c - Conveyor ramp and in-pit movableconveyors, phase I l l

Material Cltaracteristics

Ore type, waste rock type, and overburden description;

Specific gravity of ore/waste; Bulk density of crushed ore/waste; Run-of-mine lump size; Characteristics of blasted ore/waste: slabs,

blocks, high fines; Size analysis of run-of-mine broken ore/waste; Size analysis of crushed ore/waste; Moisture conditions in pit influencing ore/waste; Ore/waste hardness and abrasion; Angle of repose crushed ore/waste; and Angle of withdrawal, crushed ore/waste.

From the foregoing design criteria, ICC system concepts can be evolved. Following a screening study, the most acceptable scheme can be chosen for further study. The next stage involves equipment selection to set into the concept.

Crushing Plunts

Crusher fixed, moveable, mobile (walking or crawler mechanism) :

Type - jaw, impact, roll, gyratory, feeder breaker;

Size - related to pit equipment; Setting - related to plant requirement; Feed arrangement: double truck dump, apron or

belt feeder; and Discharge arrangement: feeder, "take away"

conveyor.

Two major components must be considered in the design and selection of high tension conveyors, particularly for high lift, long center pit conveyors. The most costly part of the system for both capital and operating are the belts. The second is the initial cost of the drives. Horsepower and belt tension must be optimized to reduce the number of belt tension ratings

to one or two, as a spare belt is a high-cost inventory item. The use of multiple drives reduces the spares inventory and also allows continued operation of a conveyor at reduced tonnage should a drive fail. When lengthening or shortening conveyors within the pit, drive units may be added or reduced accordingly.

Conveyor design must fully address temperature criteria, as well as characteristics of the material to be handled. This paper is not intended to treat the subject of conveyor design, but the success of an in-pit conveyorsystem hinges on many component selection features. These are highlighted for consideration. Conveyor Selection:

Belt Width - To avoid spillage from a belt carrying coarse crushed ore/waste, particular attention must be paid to the load points and belt edge distance to the load on the belt.

Based on 305 mm (12 in.), maximum lump size (which could have a long dimension of 457 mm or 18 in. ) , a minimum of 203 mm (8 in. ) edge distance a t the load point should be allowed. For coarse ore belts, it is desirable to keep the cross-sectional load to less than 80% of the Conveyor Equipment Manufacturers Asso- ciation (C'EMA) standards when carrying the design tonnage. This means that flowsheet tonnage will be 65% to 70% of CEMA, depending on control of feed to the conveyor system. Peak tonnage can be as high as 85% of CEMA. The cross-sectional load should then be calculated using a 20° surcharge angle using 35 O equal length idler rolls. As the material is conveyed over a long distance, the dynamic settling of the load can often reduce the surcharge angle to 15'. This reduces the edrre distance to 198 mm (8 in.), which is adequate to c o k i n the load.

Speed - For coarse ore conveyors 4 to 5 m/s (1.2 to 1.5 fps) . For overburden conveyors 5 to 8 m/s (1.5 to 2.4 fps).

Incline Angle - A 25% ramp is considered maximum for maintenance access.

Decline Angle - A 20% ramp is considered maximum for containing material on the conveyor under braking conditions.


Fig. 3 - Active ore pass underground crusher conveyor ramp system

Fig. 4 - Mobile crusher, cross-pit conveyor, waste stacker

Conveyor Components:

Belt Carry side cover thickness 14 mm (0.5 in.) mini-

mum. Back cover 6 mm (0.2 in.) minimum; Temperatures below -3S°C ( -31aF) have a

dramatic effect on rubber flex resulting in high friction factors ;

Safety factor of the belt should be 6.7 minimum for all running conditions. For acceleration and braking the safety factor must not go lower than 3.0;

Cord pressure on pulleys for steelcord belts can be about 16 kg/cm2 (5 psi).

Drives - Controlled torque and acceleration/braking is a must.

Pulleys and Shafts - For high tension conveyors, the pulley, hub and shaft combination must be carefully designed as an integral unit to transfer large dynamic forces to and from the belt. Metallurgy and treatment of steel used for pulleys, particularly if cast end disks are used, should be carefully specified.

All welds should be subjected to thorough examina- tion. Pulleys should be stress relieved after welding and machining.

Heavy-duty diamond lagging for drive pulleys pays off, as pulley change outs for relagging are costly in


down time. Non-driving pulleys that are in contact with the

carry side of the belt should have plain lagging. All pulleys for belt speeds over 5 m/s (16 fps) should

be turned and dynamically balanced. Idlers - Belt tensions often are sufficiently high to

permit wide idler spacing with minimum sag. Idlers should be checked for load capacity, particularly the returns, as the high tension belt weight can be significant.

Idlers should be heavy-duty, high-quality rolls with through bored end discs. Minimum total indicated runout of 0.8 mm (0.03 in.) should be specified to prolong bearing life, particularly on high speed belts.

For belt tracking, two roll V-return idlers have been found desirable with the added benefit of having four bearings to carry the heavy return belt.

Special attention should be paid to idler spacing and blocking on convex and concave curves. Even though the radii appears to be large, the load imposed on the idlers, particularly on convex curves, is significant.

Impact idler life can be greatly extended by using resilient mountings. At load points having high impact, the entire support frame for all the impact idlers can be vibration isolated to the benefit of idler and belt life.

Structure - Conveyor drive heads, tails, and take-up modules should have supporting steel designed with allowance for fatigue stresses resulting from 100,000 load cycles.

Control - Control of a large crushing conveying system and the ability to quickly ascertain the cause of and respond to unscheduled shutdown is an area that must be considered.

First, define the extent and location of control and monitoring by producing an Instrument Control Dia- gram. From instrument data sheets, loop diagrams can be prepared.

The use of video display, programmable logic con- trollers, combined with a telemetering system to a central control panel allows complete monitoring and control of the system.

The foregoing discussion of design criteria and

component features covers a small part of the total. Areas such as mechanical, structural, electrical, instrumentation, dust control, fire detection, and suppression requires complete coverage to define the system.

Summary

In-pit crushing and conveying is in the development phase. New equipment and design innovation will come about in an effort to reduce the cost of mine materials handling for all open-pit operations.

Planning an in-pit crushing and conveying system is a complex procedure. I t requires careful consideration to confirm its practicability and economic benefit.

As the pit configuration is influenced by the materials handling system, mine planning incorporating pit crushing and conveying must be an integrated design. Applying crushing and conveying to a pit requires long-term planning and possibly more initial drilling as it is difficult to change basic concepts after 10 or 15 years of production.

In pits with varying ore types, weekly scheduling may be necessary to determine the practicality of feeding a blend to an in-pit crusher. This complication combined with grade cutoff and strip ratio becomes a vital part of planning crushing plant relocations.

The in-pit conveyor system must be designed for high use and availability. To this end, preparing a comprehensive design criteria is necessary. It plays an important role in estimating capital costs, project- ing operating costs, and sustaining capital require- ments.

All these factors become part of the total economic picture. They determine the success of the mining operation incorporating in-pit crushing and conveying. . References

Hays, R.M., 1983, "Mine Planning Considerations for In-Pit Crushing and Conveying," SME-AIME, Salt Lake City, UT, October.

Rock Stability Analysis by Acoustic Spectroscopy D.R. Hanson

Abstract - The acoustic vibrational spectra of impacted rock slabs were examined at the Denver Research Center, US Bureau of Mines, in an attempt to characterize the behavior of partially detached mine roof rocks. The ultimate goal of this study was to develop a technique or instrument that could provide a quick, accurate, quantifiable measure of mine roof stability. The power spectra of unstable rock slabs --

D.R. Hanson, member SME, is a mining engineer with the US Bureau of Mines, Denver Research Center, Denver, CO. SME nonmeeting paper 84-205. Manuscript January 1984. Discus- sion of this paper must be submitted, in duplicate, prior to March 31, 1985.

were found to contain more energy in the range of 200 to 1000 Hz than solid rock. Comparison of the power contained between 500 and 1000 Hz with thtt between 5000 and 5500 Hz provided a quantitative measure of rock stability.

A lightweight prototype field instrument was designed, constructed, and field tested. l%is battery- powered instrument computes the power contained in the two frequency bands, compares their magnitude, and displays a number related to the stability of the block tested. Field tests showed the imtrument provided reliable measurements of block stability even under conditions of noise that would have seriously degraded the accuracy of the standard methods of roof sounding.


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Pit to Plant - Current Trends