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

1

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

equipment.

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.

2

Table 1

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

. i

particles can be used. Often there is a shortage of coarse particles, so a paste system solves a material balance problem.

4. Because of reduced porosity, paste backfill is more dense than hydraulic sandfill and has a higher confined strength. In some mines more than 30% of the weight of the ore is sold as a product, and the entire tailings stream can be disposed of underground.

5. Slimes draining from hydraulic sandfiII operations often pose housekeeping problems, cause wear on mine dewatering pumps, and pose safety problems if dumped in ore passes. These problems are solved with paste backfill.

6. The mining cycle time is less with paste backfiII system because strength is achieved earlier than with hydraulic sandfilI.

7. With paste backfill a stope can be filled continuously without worrying about liquefaction and washout of the lower barricade. Thus the mass flow rate can be less with paste fill because of fewer stops and starts than is typical with hydraulic systems. With stiff mixes, barricades can actually be eliminated or simple barricades such as piles of waste rock can be used.

8. A paste backfill system facilitates the use of a mechanized undercut-and-filI mining system which increases safety, reduces dilution, and can be used with nearly any orebody shape. A paste backfill system allows flexibility in mining methods. For example, vertical retreat mining can be used for the more massive parts of an orebody and mechanized undercut-and-filI can be used in irregular or narrow parts.

RockfiII systems are used in some mines requiring backfill. Rockfill systems are generally used because of the unavailability of hydraulic fill because the mill is distant from the mine or the milling operation grinds too fine to produce a permeable hydraulic sandfiII. Paste backfill systems have the following advantages over rockfill systems:

1. Pipeline transportation is less expensive than transporting rockfill which requires fill raises and truck or rail transportation.

2. Rockfill is also more expensive than paste because most of the rock must be mined due to the fact that development waste is usually insufficient. .

3. Tailings impoundment costs are higher with rockfiII because all tailings must be impounded, and reclamation costs arise because of an open pit rockfiII mine.

3

Paste systems require higher capital investments than hydraulic sandfilI systems and roughly similar investments to rockfill systems. In most cases considered so far, the investment in a paste fill system can be easily justified by reduced operating costs and increased productivity. The only reason a paste system cannot be justified in the cases studied was insufficient reserves to recover the pay back of capital.

5. APPUCATIONS - CASE HISTORIES

5.1 Lucky Friday mine - Idaho, USA

Paste backfill is prepared by dewatering flotation tailings from a lead-zinc concentrator. The tailings are partially classified with a hydrocyclone to increase thickening and filtration rates. After thickening to 65% solids, a standard vacuum drum filter produces filter cake with 13% moisture. The filter cake is stored in a bunker and reclaimed, when necessary, with a bucket-chain excavator. A PLC-controlled batch process is used to mix paste with a consistency of 8-10 inch (203mm - 254mm) slump. Components are weighed and a high intensity mixer delivers product to a concrete pump which pumps the paste about 200 feet (6Om) to a vertical shaft. The vertical pipeline in the shaft is vented so a gravity flow situation exists. Paste is delivered to stopes 5,100 feet (1,500m) beneath the surface and as far as 2,000 feet (600m) horizontally from the shaft. Pressures in the pipeline are less than 1,000 psi (7 mPa). A two phase flow system exists in most of the shaft column with air being alternately taken in and expulsed at the top. The paste backfilling rate is 130 st/hr (120 t/hr) into horizontal undercut-and-fill stopes. Pipelines are emptied after using because cement is added by the surface backfill plant.

5.2 Bad Grund Mine - Germany

Paste fill is prepared from tailings and float-sink reject aggregate from a lead-zinc concentrator. The first stage of tailings dewatering includes a hydrocycIone and a thickener, and the second stage is a vacuum belt filter. The paste backfilling process is continuous as opposed to batch, and there is minimal storage of solids. Approximately equal parts of float-sink aggregate and tailings filter cake are mixed in a continuous mixer to form a paste of about 12% moisture and pumped about 260 feet (80m) to a vertical shaft about 1,640 feet (500m) deep. The stopes are located from 1,312 feet (400m) to 7,545 feet (2,300m) from the shaft, and an intermediate pumping station is necessary for the more distant stopes. Portland cement is conveyed pneumatically in a separate pipeline to the backfilling sites and is injected into the paste pipeline about 164 feet (5Om) from the end. The cemented paste backfill is relatively stiff as placed in

the horizontal undercut-and-fill stopes, forming a steep angle of repose. The backfilling rate is about 1,060 ff /hr (30m3/hr). It should be noted, that the Bad Grund Mine closed in early 1992.

5.3 Other applications of paste or high density backfill

At the Greens Creek Mine in Alaska, filtered tailings, mixed with cement, is prepared in a batch plant, hauled underground and mechanically placed by spreading or ramming the fill tightly into drifts. A paste systems similar to the one at the Grund mine is used at the Bad Bleiberg mine in Austria. Several mines in Africa are using high density fill and research work continues on paste fill. Inco Ltd, Ontario Division, uses high density fill prepared from alluvial materials and metallurgical wastes. Inco is currently developing paste systems using tailings and alluvial materials.

6. DISPOSAL OF INDUSTRIAL WASTES

This paper does not address the regulatory aspects of disposing waste in underground sites. The fact that orebodies containing heavy metals have existed for as long as billions of years without significant migration into ground water systems is proof that wastes could be stored permanently in appropriate underground sites.

Industrial wastes meeting the requirements of paste mixtures could be prepared and transported economically with pipelines to underground sites for permanent disposal.

7. CONCLUSIONS

Paste backfill is a relatively new technology, but it is being used at producing mines. Many mines are considering its use because of the economic and environmental advantages.

To produce a paste, tailings slurry can be dewatered with conventional equipment. Testwork is necessary to optimize the dewatering system and to obtain data for engineering design of the paste pipeline. Paste backfill systems require state-of-the-art PLC controls because only slight changes in moisture content cause wide variations in viscosity. Paste backfill can be delivered to underground voids in pipelines using pumps or gravity flow.

REFERENCES

Lerche, R. & H. Renetzeder 1984. The development of "pumped fill" at Grund mine. Hydrotransport 9: rome. Sultan, A A 1988. Sizing pipe for non-Newtonian flow. Chern. Engr 12/19/88: 140-146. Vickery, 1. D. & c. M. K. Boldt 1989. Total tailings

4

backfill properties and pumping. Innov. in Mining Backfill Tech. Rotterdam: Balkema. Whipple, R. & R. Patterson 1991. High density fill at Garson mine. CIM 10th Underground Operators Conf.