Mining Engineers Toolbox - Types of Mine Backfill

Paste backfill is defined as an engineered mixture of fine solid particles (with binder) and water, containing between 72% and 85% solids by weight. Particles in a paste mixture will not settle out of the mixture if allowed to remain stationary in a tank or in a pipeline. It can be placed in stopes with or without binder addition depending on the strength requirements for backfill. Improved pumping technology, environmental concerns, and the need for a low cost/high strength fill in mines, are driving mine operators to consider paste backfill as a tailings management and mine backfill alternative.

The main advantages and disadvantages of a paste backfill system over a conventional hydraulic backfill plant are generally accepted as:

Advantages of Paste Backfill

Disadvantages of Paste Backfill

higher strengths can be achieved with an equivalent cement content;

drainage of water and slimes from the fill are minimized, reducing the need for bulkhead construction and extensive drainage works. This feature also reduces maintenance on sumps and mine de-watering pumps;

in some cases, unclassified tailings can be used to make paste rather than just the coarse fraction as is the case for hydraulic backfill;

shorter stope cycle times can be achieved because an equivalent strength can be achieved in a shorter time with paste backfill;
paste backfill systems achieve lower porosities than conventional fill thereby increasing the tonnage of material that can be disposed of underground; and,

since paste backfill is deposited as a non-segregated mass of backfill (because cement particles are not displaced by the internal movements of the draining water), more predictable strength properties for the fill can be achieved.

paste backfill systems typically have higher capital costs compared to conventional hydraulic backfill plants;

the pumpability of a paste is very sensitive to small changes in water content and grain size distribution; and,

the distribution network in the mine requires a greater level of engineering design to control pipeline pressures.

The key characteristics of tailings or other materials being assessed for suitability as paste backfill are:

If unclassified mill tailings are to be used as the primary constituent of the paste backfill, it must be dewatered to an acceptable pulp density. Dewatering can be achieved by a variety of means including cycloning, thickening, or filtering. The rate of dewatering is a function of the specifications of the de-watering unit, as well as the characteristics of the material (including grain size distribution and specific gravity).

The pumpability of the paste is dependent on the viscosity of the tailings as well as the type of pump employed and the geometry of the distribution system. In turn, the viscosity of the paste itself is influenced by pulp density, grain size distribution (principally the fines content), and binder content.

Increasingly, backfill is being used to achieve high ore extraction ratios in mining operations. For such operations, a structural fill having predictable strength properties is required. The strength of the final backfill product is a complex characteristic which is influenced by many parameters including pulp density, binder content, grain size distribution, specific gravity, curing time, and curing temperature.

The bulk density of the placed fill is required in order to predict the proportion of the tailings tonnage that can be placed underground versus the proportion that is required to be stored on surface. The in-situ density of paste backfill is higher than conventional hydraulic fill and this can have positive benefits in terms of the size and cost of a surface impoundment.

The grain size distribution of the paste backfill has been shown to be important by studies which indicate that pipeline pressures are sensitive to the percentage of minus 20m m size material (slimes) in the mixture. Landriault et al. (1987) conducted a series of pump loop tests that showed the pressure losses in a system at various slime contents (Figure 2). As a general rule, a paste fill should contain a minimum of 15% (by weight) in the minus 20m m size fraction. The finer material forms an annulus of slower moving material around the pipeline walls. It then acts as a lubricant that surrounds the central plug of coarser particles, allowing them to flow through the pipeline at substantially reduced frictional resistance

Figure 4 is a plot of grain size distributions from a number of operations employing paste backfill. It indicates that the minus 20m m fraction ranges from a low of 13% at the Lucky Friday Mine to a high of 68% at the Greens Creek Mine. The flow characteristics of paste fill are not greatly influenced by the material greater than 20 m m in size. For example, gravel size material from a heavy media separation circuit has been used successfully in a paste fill plant at the Grund Mine in West Germany (Broicher).

The pulp density is defined as the ratio of total weight of solids (including binder) to the weight of water plus solids. Small changes in the water content can result in a dramatic increase in line pressure. This phenomena is illustrated in Figure 5 which shows the influence of pulp density on line pressure losses in a typical system. In laboratory scale testing, the pulp density is carefully controlled by adding metered proportions of water to dry solids. In a paste backfill plant the pulp density must be carefully monitored and controlled. To achieve this, paste backfill plants commonly use a PLC control and batch processing to weigh and mix the paste constituents prior to transport.

Viscosity is a measure of the resistance to movement between different layers in a fluid or mixture. In concrete terminology this is also known as the workability. The viscosity of a paste mixture is difficult to predict and is influenced by many factors including: pulp density, grain size, mineralogy, and grain shape. The concrete slump test has generally been used as a measure of the viscosity of paste mixtures. Paste mixtures commonly exhibit slumps of 15-20 cm (6-8") on a standard (12") cone.

Paste mixtures behave as non-Newtonian fluids, that is, they do not exhibit constant viscosity with variation in flow rate. The yield stress of a paste is greater than zero before flow commences. Research and experience to date, indicates that paste backfill can be considered to be a Bingham plastic fluid, exhibiting constant viscosity with increased velocity, once the yield stress has been overcome. It can also be a pseudo-plastic fluid, exhibiting decreasing velocity as velocity increases.

Binders are used in paste backfill where structural strength is required of the backfill and where resistance to liquefaction is necessary. Portland (Type 10) cement is the most common binder used for these purposes. Cement addition rates of 2 to 6% are often used to achieve typical strength requirements for mine backfill. In addition to cement, blast furnace slag, fly ash, and natural pozzolans can be added to the paste fill to partially replace cement and thereby reduce costs.

The final strength of the cured backfill is influenced by the curing period temperature. Figure 7 shows the typical increase in strength that is developed over a period of 90 days. A mining operation that exposes a backfilled stope wall shortly after it is poured must use more binder in the fill than if it exposes the wall several months after pouring. This is a mine planning issue which can have a serious impact on backfilling costs since the cost of cement represents the single largest cost for most backfill plants.

When carrying out laboratory scale testing of paste backfill samples, the curing temperature and humidity should simulate conditions expected in the mining operation. Figure 8 illustrates the significant influence curing temperature will have on the 28-day strength of backfill samples.

The specific gravity of the material used to make the paste will influence the pulp density that can be achieved. For a given slump, a higher specific gravity will yield a higher pulp density.

The porosity of the cured backfill product is important in determining the quantity of backfill that can be placed in a given volume. Porosity is defined as the volume of voids in a material to the total volume of the material. Porosity is generally lowest in well-graded mixtures. The porosity measured in uncompacted laboratory scale samples will be higher than what is achieved in a stope, since some degree of self-consolidation of the fill occurs as the stope is filled.