Analysis of the slump test for on-site yield stress
measurement of mineral suspensions
S.Clayton a ,T.G.Grice b ,D.V .Boger a,*
a Particulate Fluids Processing Centre,Department of Chemical Engineering,University of Melbourne,
Parkville,Melbourne,Victoria 3010,Australia
b Australian Mining Consultants,Melbourne,Victoria 3000,Australia
Received 14May 2002;received in revid form 16July 2002;accepted 9October 2002
Abstract
The slump test,originally ud to determine the ‘workability’of fresh concrete,has since been adapted for u in the minerals industry.The slump test finds extensive industrial application for monitoring material consistency in tailings disposal operations.The parameter ud as the indicator of consisten
cy is the slump height,an empirical value,which is only relevant for the specific material being tested.We propo that the yield stress,a unique material property,is a better measure of consistency.Models relating the slump height to yield stress have been developed for the cone
[Mate
´riaux et Constructions (Paris)17(1984)117;Christenn,G.,1991.Modelling the flow of fresh concrete:the slump test.PhD thesis,Princeton University;Canadian Geotechnical Journal 28(1991)457;Journal of Rheology 42(1998)865]and cylinder [Journal of Rheology 40(1996)1179]slump tests.In this investigation,a direct comparison of the cone and cylinder models for yield stress measurement of mineral suspensions is undertaken.The analysis clearly shows that the cylinder model more accurately predicts the material yield stress.A strong ca is made for the replacement of the widely ud cone test with the simpler,cheaper and more accurate cylinder test.
D 2003Elvier Science B.V .All rights rerved.
Keywords:slump test;mineral suspensions;paste fill;yield stress;rheology
1.Introduction
The ASTM standard cone slump test (ASTM,1998)originated for testing the ‘work-ability’or consistency of concrete.A schematic of the test is prented in Fig.1.The test
0301-7516/03/$-e front matter D 2003Elvier Science B.V .All rights rerved.doi:10.1016/S0301-7516(02)00148-5
*Corresponding author.Tel.:+61-3-8344-7440.
E-mail address:d.boger@unimelb.edu.au (D.V .Boger).
/locate/ijminpro
Int.J.Miner.Process.70(2003)3–
21
involves filling a frustum of a cone in a specified way,removing the cone vertically and measuring the distance that the concrete ‘slumps’.The distance is defined as the slump height.The slump height must fall within a given range for the concrete to be acceptable.If the slump height is too great,then the flowability is incread but the final strength of the concrete is reduced.Converly,if the slump height is below the desired value,the concrete will be too stiff and will not flow into the tight corners of moulding.
The slump test has since been developed to measure the workability of a variety of time-independent inelastic fluids,including mineral tailings suspensions.The slump test prently finds extensive industrial application in surface and underground tailings disposal operations in which the dilute tailings produced in processing are concentrated to a high solids concentration for disposal.A result of the highly concentrated nature of the tailings is the prence of an appreciable yield stress,which is the minimum shear stress for irreversible deformation and flow to occur.The yield stress increas exponentially with solids concentration (Boger,1998)so a small change in concentration can result in a large change in the yield stress.Conquently,control of material consistency in waste disposal operations is critical.
The slump height measured via the slump test is generally ud as the control parameter.The slump height,an empirical measure of consistency,is dependent on both the material yield stress and density,which in turn are dependent on chemical composition,particle specific gravity and particle size.In a mining context,the factors may vary with changes in ore origin or changes in ore processing.As a result,utilisation of the slump height as the sole parameter of consistency for waste disposal systems could potentially lead to problems.Therefore,the yield stress,a unique material property,is the preferred indicator of consistency.If the slump height could be related to the yield stress,then the slump test would be a simple and ideal technique for on-site yield stress measurement.
Several analytical models have been developed to relate the slump value to a corresponding yield stress,and to predict the slumping behaviour of the material.The slump models are derived from first principles with model variables expresd in dimensionless form.Thus,the slump models are not empirical and provide a
material-
Fig.1.Schematic of the cone slump test.
S.Clayton et al./Int.J.Miner.Process.70(2003)3–21
4
S.Clayton et al./Int.J.Miner.Process.70(2003)3–215 independent,unique relationship between yield stress and slump height.The first analysis was made by Murata(1984),followed by Christenn(1991)who corrected a simple integration error made by Murata.Rajani and Morgenstern(1991)and Schowalter and Christenn(1998)have further investigated the conical test.There is some uncertainty in the yield stress measurement techniques ud in the papers so it is difficult to make any firm conclusions as to the validity of the model.
The slump test was first adapted to a cylindrical geometry by Chandler(1986)for the alumina industry.Chandler realid there was a relationship between the slump height and flow behaviour of the bauxite residue he was testing,but did not analytically relate the two.Pashias et al.(1996)developed a model for the cylindrical geometry,comparing the results from the model with thos
e from the static vane test.The results compared favourably.The authors also investigated the nsitivity of the slump height to sample structure,material,aspect ratio,lift rate and measurement time and found that slump measurement is esntially independent of the factors.
A number of investigations have been completed to relate slump height and yield stress for the cone and cylinder geometries,but comparison of the two geometries for yield stress measurement of the same material has not previously been undertaken.In this inves-tigation,yield stress values as determined from the cylinder and cone slump tests and related models are compared to yield stress values determined utilising the well-established vane technique(Nguyen and Boger,1983,1985).
2.Materials,measurements,and experimental procedure
2.1.Materials
Titanium dioxide(TiO2)pigments are ud extensively in the paint industry,and have been well characterid by Liddell(1992,1996).The TiO2pigment,supplied by Tioxide Chemicals,ud in experiments had an inorganic alumina coating,an isoelectric point of 7.6and a density of4000kg/m3.The TiO2suspension was prepared by initially diluting the titanium pigment with pH-treated distilled milli-Q water(pH10).The pH of the distilled milli-Q water was altered by the addition o
f concentrated KOH.The pH of the diluted titanium dioxide sample was raid to10to completely disper the sample and then mixed well with a high shear mixer.The pH was then lowered to the isoelectric point by the addition of concentrated HNO3.
A mineral tailings paste sample supplied by BHP Minerals-Cannington is reprenta-tive of the paste from the disk filter–spiral flow mixer system on site.The paste sample has a specific gravity of3.1kg/m3and a pH of approximately7.
2.2.Measurement
Yield stress measurements were made using the vane technique(Nguyen and Boger, 1983,1985).A PHM82standard pH meter was ud to measure pH.The dimensions and material of construction of the slump cones and cylinders utilid in the investigation are listed in Tables1and2.The cone slump mould was designed as specified in Australian
Standard AS 2701.5(Standards Association of Australia,1984).In industry,the ASTM standard cone slump test is ud.The slump test procedure and the material of construction for the cone are the same for the ASTM test and the AS 2701.5test.The difference between the tests is that the dimensions (top diameter,bottom diameter and height)of the ASTM cone are double the dimensions
of the AS 2701.5cone.Conquently,the volume of the ASTM cone (5.5l)is eight times greater than the volume of the AS 2701.5cone (0.69l).For laboratory experiments,the AS 2701.5cone slump test is therefore more practical.
2.3.Experimental procedure
The conical slump test was completed as per Australian Standard AS 2701.5.There is no standard for the cylinder test so the conical test methodology was adapted for the cylinder.The cylinder was filled with sample,the top of the cylinder was smoothed over and the cylinder lifted slowly and evenly.The change in height between the cylinder and deformed material was measured.The midpoint of the slumped material was taken as the reprentative height.Heights were measured with a ruler to the nearest 0.5mm.Density and concentration were measured at the time of testing.
3.Theory
Analytical slump test models have previously been developed for the cone and cylinder slump tests.The cylinder model is generalid for any-sized cylinder,whereas the cone model is specific for a cone with a ba diameter twice that of the top diameter.This requirement led to the development of a generalid cone model to allow direct comparison with the cylinder model.The cylinder theory d
eveloped by Pashias et al.(1996)is also prented to enable easy comparison with the generalid cone theory.
Schematic diagrams for the cylinder and cone tests are prented in Figs.2and 3,respectively.The schematics display the important variables and the stress distributions involved in slumping.
3.1.Assumptions and development of cylinder and cone slump models
It is assumed that removal of the slump cylinder or slump cone does not deform the material in any way.The initial undeformed material is therefore assumed to be either a perfect cylinder or a perfect truncated cone.Practically,this will only be achieved for perfect slip at the inner surface of the slump cylinder or slump cone.
Table 1
Dimensions and material of construction of the ASTM slump cone and the AS2701.5slump cone
Cone
Height H (mm)Top radius R 0(mm)Ba radius R H (mm)Construction material ASTM
30050100sheet metal AS 2701.51502550sheet metal S.Clayton et al./Int.J.Miner.Process.70(2003)3–21
6
The only stress acting on the material is assumed to be a vertical stress associated with the material’s own weight.Therefore,the pressure (P )in the material at some height (z )below the top surface can be expresd as the weight of material above position z .
Cylinder :P j z ¼q gz
ð1ÞCone :P j z ¼q gH 3
R 0R H ÀR 0 Â1þz H R H ÀR 0R 0
À11þz H R H ÀR 0R 0 20B B B @1
C C C A ð2Þwhere H is the height of the undeformed material,q is the density of the material,g is the acceleration due to gravity and other variables are defined in Figs.2and 3.
The material is assumed to behave as an elastic solid,for which the maximum shear stress that can act on a body when a pressure (P )is applied to it in a normal direction
is Fig.3.Schematic diagram of the conical slump test,showing initial and final stress distributions (developed from Schowalter and Christenn,1998
).Fig.2.Schematic diagram of the cylinder slump test,showing initial and final stress distributions (developed from Pashias et al.,1996).
S.Clayton et al./Int.J.Miner.Process.70(2003)3–217
