1. DEFINITION AND DESCRIPTION OF PASTE BACKFILL
Paste is a high density mixture of water and fine solid particles with a relatively low water content (10-25%) such that the mixture has a consistency as measured by the ASTM slump cone test from slightly greater than zero up to nearly 12 inches (305mm). Particles of different size classes will not segregate or settle when the paste is not being agitated or when it is stationary in a pipeline. Cement may be a component of paste. Larger particles of aggregate can generally be added to a paste without greatly changing the pipeline transport characteristics.
A paste may bleed water slightly when it is allowed to be motionless for periods of up to several hours, but the solids do not become so dense that the paste cannot be easily re-fluidized. The dividing line between the definition of a high density slurry and a paste is not sharp. It is a practical matter that must be considered when designing a pipeline transportation system. If the pipeline is very short, there is very little risk of plugging the pipeline with a high density slurry when flow is stopped for short time periods. With longer pipelines the mixture must remain stable without motion for periods as long as several hours to avoid pipeline plugging.
The fine particles in a paste mixture can be composed of mine tailings or naturally-occurring clays, silts and fine sands. The mineralogy of the particles thus can have a wide variety from quartz and feldspar to clays, micas and even salts.
Tailings from the milling or processing operation of a mine are the most common source of solid material for paste back-fill in the mining industry. The use of tailings will be emphasized in this paper. Paste backfill would generally be used in underground mines but could be used for waste disposal in open cast and strip mines.
The moisture content or density of a paste for a given slump consistency depends on the size distribution of the particles. The finer the particles, the more surface area that must be wetted thus yielding higher moistures and lower densities for a given consistency. With larger particles, the surface area is smaller resulting in lower moistures and higher densities for a mixtures of the same given consistency. The specific gravity of the particles also affects the density of the mixture when expressed by weight. In practice, paste mixtures range from 73% solids by weight to 90% solids by weight.
Paste mixtures are non-newtonian fluids. Viscosity is a measure of the resistance to movement between adjacent layers of fluid, or the resistance of the fluid to flow. Newtonian fluids exhibit constant viscosity regardless of the flow rate and the yield stress, which
must be overcome before flow commences, is zero for newtonian fluids. Paste mixes are generally classed as Bingham plastic fluids which have a significant yield stress but have a relatively constant viscosity as flow rate increases. However, experiences with pipeline transportation of pastes have shown that the theories of paste flow are not well understood. Viscosity can either increase or decrease with time or flow rate depending on the characteristics of the paste. Attempts to develop predictive models for paste rheology have so far been relatively unsuccessful.
2. PREPARATION OF PASTE MIXTURES
Tailings from a milling operation are usually discharged as a dilute slurry. Excess water may be recovered for recycling in the milling operation by use of a tailings thickener, but the tailings slurry is always too wet to be considered a paste. Therefore, dewatering of the tailings slurry is usually the first step in preparing a paste backfill mixture. Fine particles, often referred to as "slimes", must not be lost during the dewatering operation so a conventional gravity thickener becomes the equipment of choice for the first stage of dewatering. If more than a sufficient amount of fine particles are present in the tailing stream, part of the stream can be processed with a hydrocyclone and the overflow discarded thus removing some of the water and increasing the thickening and filtration rates. This is called partial classification. Cyclones cannot generally be used solely as the first stage of dewatering because slimes are lost in the overflow. However, cyclone overflow can be dewatered in a thickener and re-mixed with the cyclone underflow to form a paste. For a common milling operation with quartz, carbonates, and feldspars as the predominant mineralogy, the underflow from the thickener should be 65-70% solids by weight.
The thickener underflow should be a stable slurry. A stable slurry does not exhibit segregation of particle sizes or rapid settling of larger particles. A stable slurry can be easily pumped with centrifugal pumps, and pipeline velocity is not so critical as with dilute slurries. Filtration is generally the final dewatering step in preparation of a paste backfill. Many different types of dewatering filters can be used including disk and drum vacuum filters, horizontal belt vacuum filters, belt filter presses, and hyperbaric disk filters. Capital and operating costs are the criteria used for filter selection. The product of the final dewatering step is a moist filter cake which can be handled with belt conveyors.
Commonly, some storage of filter cake is necessary to level out surges in the paste preparation process. Storage of large quantities of filter cake should be avoided because the material is often sticky and causes problems with bulk material handling
It is possible to avoid the filtration step in the preparation of paste backfill. The thickener underflow slurry can be mixed directly with a suitable dry alluvial material to produce a paste mixture. Moisture content and particle size distributions of the components are the determining factors to be considered. Also, as mentioned above, in some cases it is possible to cyclone the tailings, thicken the overflow, dewater the underflow on a gravity screen, and re-mix the two components to produce a paste, thus avoiding the filtration step.
The components of a paste mixture including filter cake, cement, aggregate, and water must be mixed thoroughly to produce a homogenous paste for pipeline transportation. A paste mixing plant is very similar to a concrete batch plant. Components must be weighed accurately and supplied rapidly to the mixing process. A batch mixing process is easier to control and thus usually preferable to a continuous mixing process. High intensity mixers developed by the concrete industry are suitable for mixing paste backfill.
Precise control is necessary to operate a paste backfill plant. Small variations is moisture content will result in large variations in pipeline friction. Modem instrumentation and PLC control have made paste backfill a practical backfill material.
After mixing, the paste backfill can be discharged into a vertical pipeline or into a conventional concrete pump hopper. Practical pumping distances range up to 3,280 feet (1,000m) and vertical dropping distances are unlimited. A horizontal leg at the bottom of a vertical pipeline can also transport paste up to 3,280 feet (1,000m) with energy supplied by the vertical column of paste.
3. PASTE MIX SPECIFICATIONS AND DESIGN
The most important requirement to produce a paste is the presence of a sufficient amount of fine particles. In most cases, pastes must contain at least 15% by weight of particles less than 20 microns in diameter. Mineralogy and particle shape will affect the amount of fine particles necessary.
A program of laboratory and pilot scale testwork is necessary to determine the suitability of a material to be used for paste backfill. Table 1 contains an outline showing most of the testwork that could be performed to determine if a material is suitable fur a paste mixture. Experienced engineers can perform simple mixing tests and, with the knowledge of the particle size distribution, can make judgments about the suitability of a material.
Testwork to determine suitability for paste backfill
Laboratory phase Grain size distribution by laser Mineralogy and density Grain shapes Compaction curves and optimum density Liquid and plastic limits Porosity Permeability Abrasiveness Bin Flowability Slump vs. water content Thickening tests Filtration tests Cycloning tests Paste mixing - visual observation Pipe column flow tests Strength tests
Pilot plant phase Thickening at plant site Filtration at plant site Slump vs. mixing power requirement Full scale pumping loop test
Experience to-date has shown that it is difficult to scale paste flow characteristics from small scale tests to full scale pipeline conditions. This generally results in the necessity of performing full scale pumping tests.
A paste backfill can be engineered to yield the necessary strength for the intended application. Strength is increased by increasing cement content, reducing moisture content (stiffer mixes) and by the addition of aggregate. In many cases, mine backfill with unconfined compressive strength of 100-300 psi (0.7-2.1 mPa) is adequate. This can be achieved with 3-5% by dry weight of Portland cement.
4. ADVANTAGES OF PASTE BACKFILL SYSTEMS
In underground mines the introduction of hydraulic sandfil1 systems over 40 years ago improved ground support and reduced labor and material costs. Paste backfill systems offer advantages over hydraulic sandfil1 systems:
1. Greater strengths can be achieved with less cement.
2. It is not necessary to decant water from stopes being filled with paste as it is with hydraulic sandfill.
3. Generally, all of the tailings can be used for paste, but with hydraulic systems only the coarse