Know Your Chemistry Suspension and Compaction Behaviour of Paste

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1 International Seminar on Paste and Thickened Tailings Paste and Thickened Tailings Paste March 2 April 2004, Cape Town, South Africa Know Your Chemistry Suspension and Compaction Behaviour of Paste Andrew Vietti De Beers Consolidated Mines Limited Abstract An attempted is made to provide an explanation for the observed suspension and compaction behaviours of kimberlitic clay slurries based on three mechanisms affecting clay colloidal properties. In addition, three models are proposed which allow for the prediction of both the suspension and compaction behaviours based on an understanding of the slurry system parameters. 1. Introduction Current diamond winning metallurgical processing relies heavily on water as a process medium. Throughout the kimberlitic ore treatment phases, process slurries are generated which contain a variety of suspended minerals. Since water recovery and re-use is paramount to the operation of the treatment plant, considerable attention has been paid to the slurry thickening and tailings disposal processes, principally through the adoption of new technologies such as Paste and Thickened Tailings Disposal (P&TTD). Enhanced water recoveries are achieved by P&TTD systems, by concentrating the suspended solids within so-called low-density slurries through novel high compression thickening processes. These high-density tailings are then transported hydraulically to a surface disposal site where they are deposited. The operation of the P& TTD thickening and pumping processes are themselves critically dependant on the settling and rheological behaviours of the clay slurries. For instance, the thickening process requires that the suspended clays be in a colloidally unstable state for solid/liquid separation to take place and the subsequent compaction to a high-density state. It is equally important that the rheological behaviour of the high-density slurry be such that hydraulic transport is possible. The suspension and subsequent rheological behaviours of both low and high-density clay slurries are dependant on a number of ore and water related parameters which affect the colloidal properties of the suspended clays. This paper attempts to explain the mechanisms affecting kimberlitic clay slurry suspension and compaction behaviour.

2 2. Saline Agricultural Soils It would appear that processed kimberlitic ores (slimes), in general, bear a striking similarity to the behavioural characteristics of a class of agriculturally problematic soils known as the saline and alkali soils. These soils can be further grouped into: Saline soils Saline-alkali soils Non saline-alkali soils Three criteria are used to classify the soils (Richards 199): 1) The conductivity of an extract taken from a saturated soil sample provides a measure of the water-soluble cations within the soil (i.e. the salinity of the soil). 2) The Exchangeable Sodium Percentage (ESP) provides a measure of the amount of sodium ions bound to the clay fraction in the soil (i.e. the sodicity of the soil). 3) The ph of the saturated soil. 2.1 Saline Soils These soils have high conductivity; the ESP and ph values are low. Because of the high soluble salt content, the soils are in a flocculated state and will form settling slurries if suspended in water. 2.2 Saline-Alkali Soils The ESP of these soils are high, however, these soils behave either as saline soils or non-saline-alkali soils depending on the amount of soluble salts present (i.e. their conductivity). If the conductivity of the soils is high, and the ph of the soil is low the soils remains in a flocculated state. If however, the soluble salts are leached out of the soil, the properties of the soil change and they begin to behave as non-saline-alkali soils. 2.3 Non-saline-Alkali Soils The conductivity of these soils are low, the ESP and the ph values are usually high. The ESP has a profound effect on the chemical characteristics of the soil. The higher the ESP, the higher the ph and the more the soils tend to disperse. Typically, problematic processed kimberlite ores show behavioural characteristics similar to this category of soil. 3. Alkalisation of Soils Most importantly to the process of alkalisation, is the fact that it is the clay fraction in the soil, which is able to adsorb and exchange cations (notably sodium, calcium and magnesium) from the surrounding aqueous medium by ion exchange mechanisms. Normally, calcium and magnesium are the principle ions found in a saturated soil extract, however, under certain conditions, the sodium ion can become the dominant ion if the salts become concentrated through evaporation. In this case, the saturation limit of various salts (e.g. calcium/magnesium

3 sulphates or carbonates) is exceeded and they are precipitated out of solution, increasing the relative proportion of sodium ion. Under such conditions, sodium replaces the original exchangeable calcium and magnesium cations on the clay surfaces, the ESP of the soil increases and the soils become alkali. 4. The Effect of Water Another factor, which plays a vital role in determining the ion exchanged nature of the clays, is the chemical quality of the water, which contacts the soil. The alkali hazard potential of a water used for irrigation is determined by the absolute and relative concentrations of the cations in the water. If the proportion of sodium in the water is high, the alkali hazard is high and conversely if calcium and magnesium predominate, the hazard is low. An easy to remember rule of thumb is hard water makes soft land and soft water makes hard land (Richards 199). A unit which is used to determine whether a water quality is likely to create clays, which are highly sodium ion exchanged is know as the Sodium Adsorption Ratio (SAR) value of the water. The SAR is the ratio of sodium ions to calcium and magnesium ions in solution and the figures are derived from a normal chemical analysis of the water (in meq/l): + Na SAR = ( Ca + Mg ) / 2 Since there is a fairly good correlation between the SAR value for a water and the ESP value of a suspended clay, the ESP value of the clays in the irrigated soil can be estimated if the SAR of the irrigating water is known (Richards 199). 5. Kimberlitic Clay Characteristics Kimberlite ore is an ultrabasic igneous rock consisting of a matrix of cementing material in which mineral inclusions of various crystal elements such as diamond are found. In most cases, the cementing matrix is composed of a range of clay minerals, of which those from the smectite group are dominant, comprising anywhere from 50% to 90% of the clay mineral fraction (-2 micron). The Montmorillonite clays are thought to be the major clay species. In order to understand the behaviour and interaction of clays in slurries (or soils), it is essential to understand their structure. Smectites are classified as 2:1 type clays as a consequence of the particular arrangement of the clay particle crystal lattice (Van Olphen 1977). The crystal lattice is composed of a single octahedral gibbsite layer (if the central atom is Al 3+ ) which is sandwiched between two silicon tetrahedral layers (Hurlbut & Klein 1977) (Figure 1).

4 Figure 1: Crystal Lattice Structure of a Typical 2:1 Clay Mineral (Hurlbut & Klein 1977) These crystal lattice layers are however, not uniform with respect to the chemical nature of their central atoms. Often, isomorphus substitution of the existing atom by an atom with a lower valance can take place (for example Mg 2+ for Al 3+ in the octahedral layer) that results in an excess negative charge, which is distributed at the tetrahedral surfaces. The excess negative charge is compensated for by the adsorption of cations onto the outer surfaces of the clay crystal lattice structure. These cations are present even in the dehydrated forms of the clay, however, in the presence of water, the compensating cations may be exchanged by other cations in solution depending on how strongly they are bound to the clay surface. For this reason, they are known as exchange cations and their concentrations can be used as a measure of the amount of lattice charge or cation exchange capacity of the clay (Van Olphen 1977). Smectite clay surface charge accounts for about 0% of the total cation exchange capacity while the remainder is accounted for by the clay particle edges. It is generally concluded that under aqueous conditions, hydroxyl (OH - ) groups will attach to the exposed silicon tetrahedral and metal ion octahedral atoms at the clay edges (Svarovsky 191). As a consequence, the ph of the surrounding environment will have a profound effect on the crystal lattice edge charge. Under acidic conditions, the OH - bearing groups will become protonated to carry an overall positive charge. As the ph is increased, the OH - groups will become deprotonated until a point of overall edge neutrality is achieved. This ph is known as the Point of Zero Charge (PZC) of the clay crystal edge. Further increases in ph will result in total deprotonation of the OH - groups until an overall negative edge charge dominates (Figure 2).

5 Zeta Potential (mv) ph PZC Figure 2: Surface Electrical Charge Characteristics of a 2:1 Clay With Respect to ph Finally, all members of the smectite group of clays share one common feature, in that they have the ability in the presence of moisture to absorb water and other polar molecules between the particle unit layers and cause swelling of the matrix hence the term swelling clays. Two forms of swelling mechanism are known which depend on the moisture content to which the clays are exposed. Firstly under low moisture content conditions a limited step-wise expansion of the unit layers known as Interlayer (or Type I) swelling occurs. As more water molecules are drawn between adjacent clay platelets up to three layers of water molecules are covalently bonded to the tetrahedral surfaces and in a semi-crystalline structure that resembles that of ice. This mode of swelling leads to at most a doubling in the volume of the dry clay (Grimm 19). The second form of swelling occurs at high moisture contents and can lead to an unlimited or complete separation of individual layers and is known as Osmotic (or Type II) swelling. Under this condition, the exchange cations dissociate from the clay surface and move to the hydrated region between clay particles and as such they are regarded as being in solution and hence lower the activity of the water between the particles. This allows more water from the surrounding to move into the interlayer region by osmotic forces thereby increasing the interlayer swelling. This form of swelling may continue indefinitely until normal electrical double layers separate the individual clay particles (Figure 3).

6 Interplanar Spacing (Å) C -1/2 Figure 3: Change in Layer Spacing of a 2:1 Clay with Increasing NaCl Concentration (Sequet et al. 1975). (Type I swelling is step-wise and limited while Type II swelling is linear and unlimited) The degree of osmotic swelling of a particular clay depends largely on the nature and concentration of the cations in the contacting water and the degree of octahedral substitution of the clay type. Monovalent cations (such as Na + ) in solution tend to cause unlimited swelling since they are small and are able to dissolve more easily in the semi-crystalline water layers which surround the clay particles thereby drawing more water between the adjacent particles by osmotic action. Divalent cations (such as Ca 2+ and Mg 2+ ) on the other hand tend not to cause unlimited swelling since they have a disruptive effect on the water layer structure and as they are able to provide links between charged sites on adjacent silicate sheets (Sequet et al. 1975; Mering 194).. Kimberlite Ore Processing and Clays Conventional kimberlitic ore processing practices (particularly in the water recovery and tailings disposal circuits) are undergoing rapid change. The concept of Paste and Thickened Tailings Disposal (P&TTD) is gaining acceptance within the minerals industry, primarily as a means of reducing water consumption, as well as improving disposal site stability and safety (Robinsky 1999; Paterson et al. 1999). However, the technique is more sensitive to the variability in the behavioural characteristics of clay suspensions than are the conventional water recovery and tailings disposal circuits. Within a typical kimberlite processing circuit, ore is crushed and scrubbed before being screened and processed further. Scrubbing represents the first contact between the ore and the plant process water and may be critical in determining the behavioural characteristics of the subsequent low-density tailings stream. Should a P&TTD circuit be installed, this stream would typically be routed to a high compression thickener where the solid/liquid separation would take place and the solids would be compacted to a high-density underflow product,

7 which would have certain unique non-newtonian rheological characteristics. These characteristics would determine the pump and pipeline design requirements and subsequently the disposal site sizing requirements. 7. Paste Behavioural Models As stated, low-density kimberlitic tailings slurries ( 1.5 mm) would typically constitute the feed to a high compression thickening unit. The slurry is generally classified into two fractions according to particle size, namely the +75 micron 1.5 mm grits fraction (which is easily settlable) and the 75 micron slimes fraction containing the clay minerals. The most important parameter affecting the thickener performance would be the suspension or settling behaviour of the slimes fraction within the slurry. Three mechanisms affect the colloidal properties of clay slurries. 1. Ion exchanged nature of the suspended clays:- increasing the sodium ion exchanged nature of the clays will lead to increased dispersion behaviour of a clay slurry suspension. 2. ph of the suspension:- as alluded to earlier, the ph of the suspension greatly affects the charge associated with the clay particle edges. Below the clay edge PZC, edge-to-face particle interactions take place, resulting in particle aggregation and settling under gravity. At suspension ph values above the PZC, clay slurries will tend to remain dispersed as a consequence of negative particle repulsive forces (Figure 4) Figure 4: Clarity Profiles of a Sodium Exchanged Clay Suspension (left) and a Calcium Exchanged Clay Suspension (right) with ph. 3. Ionic concentration of the suspension:- A third and overriding mechanism affecting the colloidal properties of clay suspensions is the absolute ionic concentration of the suspension. The electrical double layer surrounding individual clay particles becomes progressively compressed at high ionic concentrations, reducing interparticle distances and allowing particles to interact. An ionic concentration is reached, known as the Critical Coagulation Concentration (CCC) at which, the forces of attraction between adjacent particles become greater than the repulsion forces and particle aggregation and settling can occur.

8 Integration of the three mechanisms can provide visualisation models to explain the suspension and compaction behavioural observations of kimberlitic clay slurries. Typically, particle colloidal potential would increase with increasing clay ESP due to the increasing dispersive nature of the clay, however, this trend is only truly expressed in a narrow band between approximately ph and 11. At lower ph values, clay particle edge-toface interactions take place and hence settling can occur even at high ESP values. Also at ph values greater than 11, settling is once again observed at high ESP values simply due to the CCC of the suspension being exceeded (Figure 5). 12 Solids in Suspension After 90 hours (Plan view) Solids in Suspension After 90 hours (Isometric view) % 0.2 % 0.4 % 0. % 0. % 1.0 % 1.2 % Solids Concentration (%) Slurry ph ESP (%) ESP (%) Slurry ph Figure 5: Model for Describing the Suspension Behaviour of Naturally Settling Low-Density Kimberlitic Clay Slurries Observing and describing the suspended portion of a slurry provides one view of the behaviour of clay particles within such a slurry. Another view, which would be more useful to the hydraulic transport and deposition characteristics of the slurry, would be to describe the behaviour of the settled or compacted portion of the slurry under the same conditions as described in Figure 5. In this case, it would appear that two compaction zones (or clay particle interaction zones) exist a zone of relatively poor compaction surrounding a zone of high compaction which corresponds to the region of maximum clay dispersion (Figure ). Indeed, the degree of solids consolidation in the compaction zone is significant, up to 0% solids by mass (Figure 7).

9 Mud Bed Compaction After 90 hrs Mud Bed Compaction After 90 hrs (Plan View) (Isometric View) m 0.04 m 0.0 m 0.0 m 0. m ES P 40 (% ) ESP (%) H ry p Slur Figure : Model for Describing the Compaction Behaviour of Naturally Settled Low-Density Kimberlitic Clay Slurries Solids Content in Settled Bed After 90 hours Solids Content in Settled Bed After 90 hours (Plan view) 12 % 20 % 30 % 40 % 50 % 0 % 11 (Isometric view) 70 n (%) Solids Concentratio 0 Slurry ph Slurry ph 0.12 Mud bed Height (m) ESP (%) 0 0 ES P 7 0 (%) ry ur Sl Figure 7: Solids Content of the Settled Bed of Naturally Settled Low-Density Kimberlitic Clay Slurries Initially, these observations may appear to be contradictory, i.e. a colloidal condition exists in which maximum clay particle dispersion is allowed and in which maximum slurry consolidation and compaction of any settled solids to take place. ph

10 In order to explain this seemingly contradictory behaviour, the orientation of the clay particles in the settled bed was investigated and visualised using a cryogenic Scanning Electron Microscope technique. Figures and 9 represent two micrographs describing similar clay mineral suspensions with differing colloidal properties. Figure describes the clay particle orientation within a slurry in which the colloidal properties allow particle interaction to take place (i.e. within the particle interaction zone as described by the compaction model). It is noted that significant edge to face interactions are allowed with the accompanied presence of a significantly high void ratio and low compaction density. Figure. Scanning Electron Micrograph Describing Clay Particle Orientation Associated with a Slurry in which the Colloidal Properties allow for Particle Interaction Figure 9 describes the clay particle orientation of a slurry in which the colloidal conditions are such that particle interaction is not allowed. In this case, a high degree of compaction of the solids is achieved. It is noted that almost exclusively face to face interactions are allowed with the accompanied presence of a significantly low void ratio and high compaction density.

11 Figure 9. Scanning Electron Micrograph Describing Clay Particle Orientation Associated with a Slurry in which the Colloidal Properties do not allow for Particle Interaction. Colloidal Properties and Paste Rheological Behaviour The rheological behaviour of a paste, are determined by a number of slurry related parameters. Very often, particle size and slurry solids content are regarded as the prime contributors to the strength and flow behaviour of the paste. On occasion, however, rheological behaviours are observed which appear puzzling and which can not be explained on the basis of particle size and slurry density alone for instance a low density paste may exhibit a higher yield strength than a similar paste at much higher density. The effects which slurry colloidal properties have on clay particle association and hence paste rheology are clearly demonstrated in Figure in which a wide range of rheological behaviours are expressed by a single thickened kimberlitic clay mineral paste. In this case, under certain colloidal conditions, the slurry (d 50 of 2 micron) was shown to have no yield strength at solids contents as high as 47% solids by mass, while in other conditions, it was shown to develop significant strength at comparatively low solids contents (30% solids by mass). These rheological observations, when simply related to the solids content within the paste provide no explanation of this apparently aberrant behaviour, however, if the same data is expressed as a function of slurry colloidal properties such as ESP and ph they can be explained clearly (Figure 11).

12 Yield Strength (Pa) Shear Yield Strength (Pa) Solids Content (% by mass) Figure : The Effect of Slurry Colloidal Properties on the Rheological Behaviour of a Single Kimberlitic Clay Mineral Suspension and Expressed as a Function of Solids Content ESP (%) ph 9 11 Figure 11: The Effect of Solids Content on the Rheological Behaviour of a Single Kimberlitic Clay Mineral Suspension (at 41% solids content by mass) Expressed as a Function of Slurry Colloidal Properties.

13 9. Conclusion As the minerals industry strives towards adopting Sustainable Development principles, possibly one of the most far-reaching process improvements has been the development of the Paste and Thickened Tailings Disposal system. As both the reliability and the understanding of the system improves, it is envisaged that these systems will become a commonly accepted alternative to conventional disposal techniques within minerals and other industries. However, as will all processes, pushing the boundaries of a technique requires greater attention to detail. With regard to P&TTD systems, the detail appears to be at the level of the surface chemistry characteristics of the suspended clay minerals. As demonstrated, the colloidal properties of a slurry can have a profound effect on the settling; compaction and rheological behaviour of many mineral tailings and particularly those containing clay minerals. In these slurries, the effects of slurry density appear to play a sub-ordinate role to the colloidal properties in determining the rheological behaviour of the paste. A complete understanding of the tailings suspension characteristics (i.e. both the mineral and aqueous components) as well as the mechanisms which affect the clay surface charge characteristics are required in order to manipulate process conditions to match the process needs and to ultimately master the P&TTD system. Based on the above and on visual observations, two models have been proposed in order to predict the suspension and compaction behaviours for kimberlitic clay mineral suspensions. Reference Richards, L.A. (ed.) (199) Diagnosis and improvement of saline and alkali soils, US Dept Agriculture Handbook No. 0. Van Olphen, H. (1977) An introduction to clay colloidal chemistry, John Wiley & Sons, New York. Klein, C. and Hurlbut, C. S. (1993) Manual of Mineralogy (21 st ed), John Wiley & Sons, New York. Svarovsky, L. (ed.) (191) Solid-liquid separation (2 nd ed), Butterworths, London. Grimm, R. E. (19) Clay mineralogy (2 nd ed), McGraw Hill, Sequet, H.; De La Calle, C. and Pezerat, H. (1975) Swelling and structural organisation of saponite, Clays and Clay Minerals, Vol 23, Pages 1-9. Mering, J. (194) The hydration of Montmorillonite, Trans. Faraday Soc., Vol 42, Pages Robinsky, E. I. (1999) Thickened tailings disposal in the mining industry, E. I. Robionsky & Associates, Toronto.

14 Paterson, A.J.C., Vietti, A.J., Derammelaere, R.R. and Hester, H. (1999) Future Trends: Waste Disposal of High Concentration Kimberlite Tailings, 1 st Annual General Meeting of Canadian Institute of Mines, Calgary, May 1999.