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.

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.

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.

Advantages of Paste Backfill Systems

In underground mines the introduction of hydraulic sandfill systems over 40 years ago improved ground support and reduced labor and material costs. Paste backfill systems offer advantages over hydraulic sandfill 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 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 sandfill 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 backfill system because strength is achieved earlier than with hydraulic sandfill.
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-fill 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-fill can be used in irregular or narrow parts.

Applications – Case Histories

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 (60m) to a vertical shaft.

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 hydrocyclone 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.