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EVALUATION OF RAWMATERIALS

D W POOK

EVALUATION OF RAW MATERIALS

CONTENTS

1. INTRODUCTION

2. LITEIWTURE SEARCH

3. RECONNAISSANCE

4. GEOLOGICAL INVESTIGATION

4.1 Detailed Field Mapping and Sampling4.2 Drilling4.3 Core Logging4.4 Sample Preparation4.5 Planning of Drilling Programme

5. INTERPRETATION OF BOREHOLE RESULTS

6. RESERVES ASSESSMENT

1. INTRODUCTION

Imagine that a business consortium wishes to build a new cement works and they engage aconsultant geologist to advise them on raw material aspects. The geologist will employ some orall of the methods described below to produce a report, which should meet the following criteriaif a sound judgement is to be made in flu-theringthe project:

i) The proposed raw materials are, en masse, chemically suitable.

ii) Sufficient reserves are availableto meet the raw material requirements for at least theminimum work’s life specified by the investors.

iii) The reserves are economically extractable.

iv) The resewes, if quarried correctly, will provide an acceptably uniform feed for the life ofthe works.

The procedures described below are largelyreferred to investigations for limestone reserves; theyapply equally to finding and proving reserves of secondary materials.

2. LITERATURE SEARCH

Today there is published geological information about most countries, so it is usually possible togain at least an impression of the extent of limestone occurrences in any country. Where theseare few in number then locations for a fist visit may choose themselves. In cases where limestoneoutcrops are extensive, commercial factors such as infhstmcture and main markets may determinethe areas of interest.

Where countries have been geologically mapped there are often detailed descriptions of the rocktypes which can indicate suitability of the limestone for use in cement manufacture.Topographical plans and aerial photographs are also usefi.d, as can be information on any cementcompanies already operating.

Whatever inflorrnationis available, it usually enables you to target regions which you would wishto visit.

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3. RECONNAISSANCE

This could be, at first a brief visit to all sites of interest in order to confirm the correctness of thei.niiormationderived from the literature search. Some sites may have to be eliminated because ofnon-geological reasons, e.g. property development, National Park or military installation. If anumber of sites remai~ it may be necessary to put them in order of priority and concentratefin-ther efforts on the top 2 or 3 sites.

The aim of that tier effort is to assess rapidly the geology of an area in relation to its economicsignificance. This essentiallyinvolves the preparation of a basic geological map and initialsampling in order to determine whether or not the material being studied filfils, in general terms,the quaMative and quantitative requirements for the project.

The efficiency of the reconnaissance depends very much upon the skill of the geologist. It isessential to know which features are important, and those that will affect the long term value ofthe deposit. Accurate topographical maps are not required, and although in certain, areas, it maybe necessary to make rough topographical surveys, it is often found that existing maps suffice;enlarged road maps can be used as reconnaissance base maps where no others are available.Apart from collecting as much relevant geological information as possible, the geologist duringthe reconnaissance will also be considering how much fi.u-therwork will be required to obtain theinformation necessary to prove the deposit. This will enable him not only to report on thepotential economic value of the deposit, but also on the amount of development work whichwould be required.

In the reconnaissance survey the area will be traversed for rock exposures, in particular strewsand rivers will be followed and the soils, together with their contained rock fragments, will bestudied. The effects of the rocks on topography will be noted; in areas where the land-form is dueto sub-aerial weathering processes only, and such factors as glaciation have not been effective,the nature of the terrain is largely determined by the underlying rock. The vegetation is noted,since this is often dependent on the soil type. For example, unexposed lenses of limestone inschist have been successfully mapped in Portugal using the distribution of trees, the limestonesupporting oaks whereas the schist is covered by pines. In limestone country, unexposed orgrass-covered areas will be carefidly recorded as they may well be underlain by low-grade ormagnesian horizons. As well as the ground reconnaissance, an aerial survey by light aircraft orhelicopter may be made. This may well help the geologist to form a coherent picture out of themass of data he has collected on the ground.

Once the potential deposit under consideration has been generally mapped and some samplestaken for analysis,the geologist is in a position to prepare a reconnaissance report and map. Thiswould describe the areas most suitablefor fhrther detailed investigations, the potential qwdity andsize of the deposits and the possible ease or otherwise of extraction.

Assuming that the results of the reconnaissance survey are favorable and that the project is togo ahead, then a decision has to be made as to which specific area(s) are to be investigated inmuch fi.u-therdetail.

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4. GEOLOGICAL INVESTIGATION

4.1 Detailed Field ?vfi3DDhIfZand %rrmling

Having chosen a site which has the potential to provide the required limestone tonnage,then the geologist carries out surf ace mapping as in the reconnaissance survey but inmuch greater detaii. A decailea topographic map is required and is usually prepared byprofessional surveyors. All detail of the rock exposures is noted, such as the dip of thebeds, and where feasible some trenching or pitting can be carried out to expose therocks; systematic and well recorded sampling can be extremely useful. The geologistneeds to gain as much information as possible from the surface as this will determinehow much investigation is required by other methods such as drilling.

4.2 Drilling

When assessing a deposit, it is necessary to know the disposition and quality of thestrata at depth. Although this can be inferred from the surface information recordedduring detailed mapping, it is invariably necessary to confirm those inferences, usuallyby means of drilling although geophysics can provide an alternative method fordetermining the thickness of overburden.

Drilling boreholes to provide samples for identification and analysis of strata is usuallycarried out by one of three main methods.

Augering can be used on soft rocks such as clays and shales which would be quarried forsecondary materials. For satisfactory results a powered auger is required, which willeither bring the sample to the surface continuously or require the auger to be broughtback up the borehole periodically. On site logging of the sample as it is recovered ishighly desirable. Advantages are rapid progress from hole to hole, disadvantages canbe the presence of harder material such as boulders preventing effective penetration.

A second method is a percussive one, suitable only for hard rocks, where a chisel bit isdriven into the rock and the rock chips are cleared from the borehole and collected.Compressed air is used to operate a hammer device which both rotates and hits thechisel biG the hammer can be located on the rig and the bit is located on the end ofspecialised drill rods or it can be designed with an integral bit and located on the endof the drill rods in which case it progresses down the hole as it is drilled.

Whichever type of hammer is used, it is necessary to clear the rock chippings from thehole so that the chisel bit can operate efficiently, and compressed air is passed downthe drill pipe to blow the chippings back up the borehole to the surface where they canbe collected.

The third method is core drilling in which a purpose designed barrel, about 3 metres longand from 76 to 115mm in diameter, is used to obtain a cylindrical sample of rock calledcore.

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A diamond impregnated bit is fitted to the end of the barrel so that, when rotated, itwill cut out a ring or annular shape thus creating a cylinder of rock over which thebarrei will pass. Once the barrel is full the core can be retrieved by bringing the barrelback to the surface; a catch spring in the barrel facilitates both snapping off andretaining the core as the barrel is brought back up the hole.

Barrels are usually double ones; the inner barrel is designed to stand still and thusminimise disturbance to the core. The barrel is progressed down the hole by addingsections of drill pipe and is rotated via a clutch mechanism driven by a diesel engine.Downward pressure is applied by hydraulic rams. A means of both cooling and flushingany rock debris is necessary; usually water is pumped down the drill string and thisreturns up the borehole to the surface. Additives can be used’ to improve perf orrnanceor reduce water usage if necessary; compressed air is an alternative to water.

The advantages of chip sampling are that it. is cheaper and faster than core drilling, andto reduce costs could be done with a {lam-y drill rig used for blasthole ~illing.Disadvantages are that the chips can become contaminated when passing back up thehole and some of the dust is lost to the atmosphere. Use of cyclones to collect thesamples can improve representativeness, and there are rigs purpose designed with drillpipes which incorporate an extra tube to carry the chippings to the surface. Coring,although more expensive, provides not only more representative samples but also anopportunity to describe the strata in considerable detail.

However, much depends on the integrity of the driller who must record the depth drilledto obtain the core samples and also the occurrence of cavities. The softer, clayey bandsthat are often present in limestone strata may be flushed away during drilling so thatalthough the core barrel is drilled 3 metres into the strata the recovered core lend I

will be less. It is extremely important that the geologist interprets this loss whenlogging the core, as described in the next section.

4.3 Core Logg@

When core is removed from the core-barrel, its depth is noted and it is placed in a boxand appropriately labelled. This core will then be studied by the geologist who will notonly make a careful description of the strata and note such iterns as the angle of dip~but will also calculate core losses. This is necessary since although the depths fromwhich the top and bottom of a section of core were recovered are known, it is’only bycalculating losses within the core that the accurate depth of any point within thesequence can be determined. In his description of the core, the geologist willdifferentiate carefully between all the rock types present and describe in detail eachof these rock types and the exact depth down the borehole of each junction betweenthem. It is essential that as much information as possible is obtained from theboreholes since it is by correlation of individual strata, or groups of strata, in differentboreholes that the structure of the unexposed rocks is worked out.

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If a core is to be analysed chemically, the geologist will divide the core into suitable lengths foranalysis. Such sample lengths are normally based on differences in the rock which indicate achange in the chemistry regarding its use as a cement raw material, although if a large thicknessof identical material occurs it will be divided at regular intervals.

Two important considerations when choosing sample lengths are (1) the analyses will be used topredict the quality of the limestone for stages in quarry development and therefore the chosenlengths should not span boundaries between strata which may be quarried separately (2) wherecore losses have occurred the quality of the limestone as quarried may differ from that indicatedby the recovered core (e.g. loss of softer, clayeybands means the recovered core analysis is highergrade than would have been the case if all the strata drilled had been recovered) and this must berecognised when assessing the deposit.

4.4 SamRle Preparation

The aim of samplingand sample preparation is to obtain small portions of bulk material, such assections of borehole core, for chemical a.dor physical tests. Such samples must represent thebulk material as closely as possible.

Where it is necessary to not only analyse the core but also either retain a fill core record of aborehole or use the core for physical testing, the first stage in sample preparation is to split thecore longitudinally, this being carried out either by splitting with a chisel or preferably using adiamond saw. The split core which is to be analysed is then crushed, using a laboratory jawcrusher, to a maximum size of 6mm. The crushed material is then thoroughly mixed and a samplerepresenting about 2°/0 of the total taken. Accurate sampling of the crushed material can only bepetiormed mechanically, and the use of a riffle or rotary splitting device is highly recommended.The small sample of crushed material can now be ground, ready for analysis, using either a pestleand mortar or a ring roller mill (e.g. a Tema Mill).

Efficient sampling is a task which must be performed conscientiously; accuracy and cleanlinessare essential as is clear, unambiguous labelling. The sample for analysis will only be of the orderof 30 grams, whereas the initialcore from which it was obtained may have been over 6 kilogramsin weight. Much time and money spent on recovering borehole cores or other samples can beentirely wasted if insufficient attention is paid to the preparation of representative samples foranalysis.

4.5 Planning of Drilling ProEramme

Dnillingprogrammedii.dfiltwo needs; one is to determine the structure of the strata and the otheris to provide samples. The number of boreholes required is, therefore, a consequence of thenature of the deposit. A simple structure with very little variation in grade requires a fewboreholes to confirmthe predictions made from surface information. Boreholes maybe required

to locate faults or major folds, although they may not be necessary for quality reasons. Conversely, adeposit which could have the simplest structuremight vary laterallyto such an extent that a series ofboreholes on a grid systemis requiredto be drilled.

5. INTERPRETATION OF BOREHOLE RESULTS

A geologist, having decided that enough boreholes have been drilled,must interpret the itiormation to reach,’

a conclusion as to the disposition of the strata at depth. The simplest explanation is applied first, andincreasing complexity is added only if justified by both the borehole and sutiace information. Figure 1serves to illustrate how two borehole records could be interpreted; it will be appreciated that whicheverinterpretation the geologist chooses it will be put to the acid test once quarrying begins.

In cases where the boreholes have only encountered one major stratigraphical unit, e.g. a major limestoneor chalk body, then correlation of the boreholes would have to be by studying detailed features such as shellbands, minor clay lenses and textures.

The second importantuse of borehole informationis thatof raw materialquality. Average analysescanbe :calculatedfor chosen stratain eachborehole (hence the need for carefid selection of samplelengthsduringcore logging) andthese, in relationto theirlocations, can be used to calculate an overall average analysisfor therawmaterialdeposits. Furthermore,predictionscanbe madeas to the chemistryof the raw materialsastheywillbe deliveredto theworks, particularlywithregardto variationsover the life of the works. Suchqualityinformation%togetherwith the physicalnatureof the materials(e.g. moisture content and hardness)will be factors in choosing the type of process for the new works.

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Bomhole Results::::::;::;::;::,,.;:::::

A. A horizontal bed B. A faulted horizontal bed C. Faulted dipping beds

Fault

F. Deeply eroded G. Repetition ofsimilar successions

...----#

H. Latemi facies changes

FIGURE 1: POSSIBLE INTERPRETATIONS OF TWO BOREHOLES

6. RESERVES ASSESSMENT

Having built up a picture of howreserves can be made as follows:-

the limestone occurs at depth, an estimate of the

(i) Area of limestone deposit multiplied by its thickness will produce a volume.

(ii) Multiplying this volume by the density of the limestone (typically 2.6 tonnes percubic metre) will give a tonnage

i.e. Area x thickness = volume

Volume x density = tonnage

The above calculations will provide a gross reserve for an area of interest, but it is alsonecessary both to design a quarry and to estimate how much of that gross reserve willbe won by operating the designed quarry.

Quarry design is illustrated by a simplified example shown in Figures 2 to 6.

The rectangular property shown in Figure 2 is known, from surface investigations in theregion, to contain limestone beneath a cover of soil and weathered material which willhave to be removed and tipped/dumped. Contours (i.e. lines of equal height or elevationin metres above a fixed point e.g. sea level) on the ground surface are also shown inFigure 2.

A series of boreholes were drilled to determine the thickness of overburden and theirlocations are shown on Figure 3. For each borehole a thickness of overburden has beenrecorded and this can be subtracted from the ground surface elevation to deduce theelevation of the top of the limestone. This point information can be interpreted as anunseen surface representing the top of the limestone and contours drawn as shown inFigure 4; these contours will be useful in both designing the quarry and calculatingvolumes

A slice or section drawn through the middle of the property gives a profile showing theoverburden overlying the limestone - see Figure 5.

When designing a quarry, allowances have to be made for:-

A margin to the company boundary.

A slope angle in the overburden for reasons of stability.

A bench, for access, between the base of the overburden and the top of the firstface in the limestone.

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Figure 5

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Deciding at what elevation the first bench should be to avoid excessive faceheights (not greater than 15 metres).

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Benches between the vertical (or near vertical) faces in the limestone for reasonsof stability.

Figure 6 compares two views of the proposed quarry - one is in section, the other inplan view. Note that the sloping face in the overburden is shown with squiggles and thelimestone faces with cliff symbols.

What can be seen is that, because of the need to maintain stability, the area over whichoverburden is to be moved is greater than that which will be quarried for limestone.Volumes can be calculated for the overburden required to be removed and also thelimestone to be won. Note that when calculating the overburden volume, the averagethickness required could be averaged from the borehole results or by the better methodof estimating the thickness of overburden at points on a grid by comparing the groundsurface and the surface representing the top of the limestone. This latter surface wouldhave to be used to average the limestone thickness to be won from the top bench.These volumes of overburden and limestone can be expressed as a ratio which is a usefulguide to the viability of working a limestone deposit. For example, having to remove1 part of overburden for 1 part of limestone to be won might be an acceptable cosu ifthis ratio rises to 2 parts overburden to 1 part limestone then it may be preferable toquarry elsewhere.

It is these basic principles of maximum bench heights and the need for each bench tobe inside the area of the one above which are applied to the examples of quarry designdiscussed in the next paper.

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