18 IE(I) Journal-MN

(Ms) K Panchal is with Larsen and T(Ms) K Panchal is with Larsen and T(Ms) K Panchal is with Larsen and T(Ms) K Panchal is with Larsen and T(Ms) K Panchal is with Larsen and Toubro Ltd, Voubro Ltd, Voubro Ltd, Voubro Ltd, Voubro Ltd, Vadodara,adodara,adodara,adodara,adodara,Gujarat and Prof M Bulsara is with the MechnicalGujarat and Prof M Bulsara is with the MechnicalGujarat and Prof M Bulsara is with the MechnicalGujarat and Prof M Bulsara is with the MechnicalGujarat and Prof M Bulsara is with the MechnicalEngineering Department, G H Patel College ofEngineering Department, G H Patel College ofEngineering Department, G H Patel College ofEngineering Department, G H Patel College ofEngineering Department, G H Patel College ofEngineering and TEngineering and TEngineering and TEngineering and TEngineering and Technologyechnologyechnologyechnologyechnology, Gujarat 388 120., Gujarat 388 120., Gujarat 388 120., Gujarat 388 120., Gujarat 388 120.

This paper (modified) was received on May 30, 2006. Writtendiscussion on the paper will be received until April 30, 2007.

Effect of Circulating Load and Test Sieve Size on Bond WorkIndex for Natural Sand, Limestone and Bituminous Coal in aLaboratory Ball Mill

(Ms) K Panchal, Non-member

Prof M Bulsara, Member

The Bond Work Index can be used not only for determining the grinding power but also to estimate thedifference in power consumption while handling material of different hardness. Every material has acharacteristic bonding strength at molecular and grain level. The power required to break the bonds ishigher for harder material. This study has been carried out with an aim of comparing power consumptionfor three materials having different hardness. The effect of circulating load (CL) and test sieve size (Pi) onthe bond grindablity (Gbg) and Bond Work Index (Wi) based on samples of limestone, natural sand andcoal has been investigated. In the experimental studies, the relationships between the Bond Work Indexwith the circulating load and test sieve size are also examined.

Keywords :Keywords :Keywords :Keywords :Keywords : Circulating load; Bond grindability; Bond work index; Ball mill

NOTNOTNOTNOTNOTAAAAATIONTIONTIONTIONTION

Fc : powder filling volume, %

F80 : sieve opening through which 80 % of the feedpasses through the specified sieve, mesh

G : net weight of undersized material per mill rotation

Gbg : bond grindability, g/rev

J : ball filling volume fraction, %

P : power required, kW

Pi : test sieve size, mesh

P80 : sieve opening through which 80 % of the productpasses through the specified sieve, mesh

T : throughput required, t/h

U : interstitial filling, %

W : work input, kWh/t

Wi : bond grindability index, kWh/t

INTRODUCTIONINTRODUCTIONINTRODUCTIONINTRODUCTIONINTRODUCTION

Three materials, namely, limestone, natural sand and coalwere identified for grinding based on their requirement inindustry and availability in market.

Limestone (or minerals) is one of the widely usedsupplementary binding materials to enhance the strengthand durability of plastic moulded parts. Also, it is used asa main constituent in paint and pigment industry. Natural

sand is widely used as a major constituent in glassmanufacturing industry. Major application of natural sandis found in ceramic industry, glass industry, etc. The majorapplication of pulverized coal is found for fluidized bedcombustion in power generating station. Pulverized coalfiring system is used for steam generation. Large quantitiesof very fine particles of coal, which passes a 200-meshsieve, must exist to ensure ready ignition because of largesurface-to-volume ratio. Greater surface area per unitmass of coal allows faster combustion reactions becausemore carbon becomes exposed to heat and oxygen. Thisreduces the excess air needed to complete combustion.

In the design of grinding circuits in a mineral processingplant, the bond method is widely used for power needs.Despite having many advantages, this method has somedisadvantages too, which are given below. It is tiring andrequires long test time. It needs a special mill.

EXPERIMENTAEXPERIMENTAEXPERIMENTAEXPERIMENTAEXPERIMENTATIONTIONTIONTIONTION

A Bond Work Index test is a standard test for determiningthe Ball Mill Work Index of a sample of ore. It was developedby Fred Bond in 1952 and modified in 1961. This test maybe required for the design of a new mineral processingplant.

For the purpose of the test, 1.4 kg of sample was takenfrom each material, namely, natural sand, limestone andcoal. The bulk density of each sample was measured, thedetails of which are given in Table 1. The test methodelaborated by Deniz1 was followed to extent possible,however, variation in the ball mill specification is givenin Table 2. Motor specifications of ball mill are given inTable 3.

Tests were carried out on a ball mill. Electronic weighingbalance mechanical shaker were used as supportingequipment.

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The material was packed to 700 cc volume using a vibratingtable. This is the volumetric weight of the material to beused for grinding tests. For the first grinding cycle, themill was started with an arbitrarily chosen number ofmill revolutions (30 rpm, as variable speeds were notpossible in the set-up). At the end of each grinding cycle,the entire product was discharged from the mill and wasscreened on a test sieve (Pi). The oversize fraction wasreturned to the mill for the second run together with freshfeed to make up the original weight corresponding to 700 cc.The weight of the product per unit of mill revolution, calledBond Grindability (Gbg) was calculated. It was used toestimate the number of revolutions required for the secondrun to be equivalent to a circulating load of 150%. Theprocess was continued until a constant value ofgrindability was achieved1.

Bond WBond WBond WBond WBond Work Indexork Indexork Indexork Indexork Index

The samples were taken and standard Bond grindability testwas performed. The observations are given in Table 4. BondWork Index values (Wi) were calculated from equation (1)as2

(1)

To calculate the work input W for grinding from somefeed size to a specific product size1, one may write

(2)

and

(3)

(4)

(5)

(6)

The Gbg and W i values for each sample are given inTable 4.

RESULRESULRESULRESULRESULTS TS TS TS TS AND DISCUSSIONAND DISCUSSIONAND DISCUSSIONAND DISCUSSIONAND DISCUSSION

Comparison of Bond WComparison of Bond WComparison of Bond WComparison of Bond WComparison of Bond Work Index for Differentork Index for Differentork Index for Differentork Index for Differentork Index for DifferentMaterialsMaterialsMaterialsMaterialsMaterials

From Figure 1, it can be seen that for a given mesh size,the bond work index for natural sand is highest followedby limestone and coal is the least as expected. At 100 %circulating load and Pi =100 mesh, the correspondingvalues being 65.205 kWh/t, 60.715 kWh/t, 50.562 kWh/t.It is also noticeable that the variation in Bond Work Indexreduces at higher circulating load. At 400% CL, thevariation being 30.5 kWh/t, 33.259 kWh/t and27.698 kWh/t. Similar trend was also observed at Pi =150mesh.

Effect of Circulating Load on Bond WEffect of Circulating Load on Bond WEffect of Circulating Load on Bond WEffect of Circulating Load on Bond WEffect of Circulating Load on Bond Work Indexork Indexork Indexork Indexork Index

Figures 1 shows the variation in the Bond Work Indexwith the circulating load. It has been found that thecalculated work index is increased with the lowercirculating loads for each test sieve size.

Effect of TEffect of TEffect of TEffect of TEffect of Test Sieve Size on Bond West Sieve Size on Bond West Sieve Size on Bond West Sieve Size on Bond West Sieve Size on Bond Work Indexork Indexork Indexork Indexork Index

Table 4 gives the variation in the Bond Work Index withthe test sieve size.The test results indicate that, the BondWork Index increased as the finer sieve size was used.

The earlier results are in line with the results availablefrom the literature.

TTTTTable 1 able 1 able 1 able 1 able 1 Actual mill conditionActual mill conditionActual mill conditionActual mill conditionActual mill condition

Qual i tyQual i tyQual i tyQual i tyQual i ty C e r a m i cC e r a m i cC e r a m i cC e r a m i cC e r a m i cBalls Assumed porosity, % 40

Ball filling volume fraction (J), % 43.5Material Material used Limestone, Natural

sand, CoalPowder gravity, g/cm3 2.2, 3.1, 3.5Interstitial filling (U), % 12.07Powder filling volume (fc), % 2.1

TTTTTable 2 Comparison of grinding conditionsable 2 Comparison of grinding conditionsable 2 Comparison of grinding conditionsable 2 Comparison of grinding conditionsable 2 Comparison of grinding conditions

StandardStandardStandardStandardStandard A c t u a lA c t u a lA c t u a lA c t u a lA c t u a lcond i t i oncond i t i oncond i t i oncond i t i oncond i t i on cond i t i oncond i t i oncond i t i oncond i t i oncond i t i on

Mill Dia, mm 300 275Length, mm 300 580Volume, cm3 21 205.75 34 450

Mill speed Operational (f), % 75 30Balls Dia, mm 15 - 38 25.4

Number of balls 285 1 100Mass of one ball, kg 0.0706 0.0179Total mass of ball, kg 20.125 19.69

TTTTTable 3 Motor specification of the ball millable 3 Motor specification of the ball millable 3 Motor specification of the ball millable 3 Motor specification of the ball millable 3 Motor specification of the ball mill

Motor : Model/typeMotor : Model/typeMotor : Model/typeMotor : Model/typeMotor : Model/type Crompton greaves/1542 J/GDG 01Crompton greaves/1542 J/GDG 01Crompton greaves/1542 J/GDG 01Crompton greaves/1542 J/GDG 01Crompton greaves/1542 J/GDG 011010101010rpm 1405Power, kW (hp) 1.5 (2)

TTTTTable 4 Vable 4 Vable 4 Vable 4 Vable 4 Values of alues of alues of alues of alues of GbgGbgGbgGbgGbg and and and and and WWWWW iiiii of the samples of the samples of the samples of the samples of the samples

PPPPP iiiii C L ,C L ,C L ,C L ,C L , Coa l ,Coa l ,Coa l ,Coa l ,Coa l , Natural sandNatural sandNatural sandNatural sandNatural sand L i m e s t o n eL i m e s t o n eL i m e s t o n eL i m e s t o n eL i m e s t o n eM e s h ,M e s h ,M e s h ,M e s h ,M e s h , % G b gG b gG b gG b gG b g ,,,,, WWWWW i i i i i ,,,,, G b gG b gG b gG b gG b g ,,,,, WWWWW iiiii ,,,,, G b gG b gG b gG b gG b g ,,,,, WWWWW iiiii ,,,,,

BSS g/rev kWh/t g/rev kWh/t g/rev kWh/t150 100 0.050 46.060 0.037 59.399 0.047 48.741150 150 0.058 40.911 0.040 55.309 0.053 43.686150 250 0.071 34.684 0.061 38.956 0.080 31.329150 400 0.085 29.810 0.083 30.298 0.092 28.020100 100 0.050 50.562 0.037 65.205 0.040 60.715100 150 0.062 42.262 0.047 53.505 0.051 49.659100 250 0.085 32.619 0.071 38.074 0.075 36.393100 400 0.104 27.698 0.093 30.531 0.083 33.259

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CONCLUSIONCONCLUSIONCONCLUSIONCONCLUSIONCONCLUSION

l The Bond Work Index is minimum for coal, followedby limestone and maximum for natural sand for thesame test sieve size, as shown in Figure 1. This meansthat more amounts of fine particles can be achieved incase of coal, followed by limestone and least by naturalsand.

l The Bond Work Index value increases as test sieve

number decreases. This is because larger diameter ofparticles and more amount of material will go out forthe same time period.

l The Bond Work Index values increases as circulatingload decreases, that is, the Bond's Work Index value ismore for 100 % circulating load and less for 400 %circulating load, as given in Table 4. This is because for400% circulating load, power is already consumed inprevious grinding and feed for it is finer as compared tofeed for 100 % circulating load. This is because of methodof the test selected.

REFERENCESREFERENCESREFERENCESREFERENCESREFERENCES

1. V Deniz, N Sutcu and Y Umucu. 'The Effect of Circulating Loadand Test Sieve Size on the Bond Work Index Based on NaturalAmorphous Silica'. The Eighteenth International Mining Congressand Exhibition of Turkey-IMCET, 2003.

2. V Deniz, A Gelir and A Demir. 'The Effect of Fraction of MillCritical Speed on Kinetic Breakage Parameters of Clinker andLimestone in a Laboratory Ball Mill'. The Eighteenth InternationalMining Congress and Exhibition of Turkey-IMCET, 2003.

3. V Deniz. 'Relationships between Bond's Grindability (Gbg) andBreakage Parameters of Grinding Kinetic on Lime Stone'. TheEighteenth International Mining Congress and Exhibition of Turkey- IMCET, 2003.

4. W L McCabe, J C Smith and P Harriott. 'Unit Operations ofChemical Engineering'. McGraw Hill Inc, New York.

5. Perry's Chemical Engineer's Hand Book. McGraw Hill Inc,New York.

0 100 200 300 400 500Circulating load, %

Figure 1 Comparison of Bond WFigure 1 Comparison of Bond WFigure 1 Comparison of Bond WFigure 1 Comparison of Bond WFigure 1 Comparison of Bond Work Index at differentork Index at differentork Index at differentork Index at differentork Index at differentcirculating loadcirculating loadcirculating loadcirculating loadcirculating load

Bon

d W

ork

In

dex

, k

Wh

/t

70

60

50

40

30

20

10

0

Limestone – 150 mesh Natural sand – 150 mesh Coal – 150 mesh Limestone – 100 mesh Natural sand – 100 mesh Coal – 100 mesh