Jack Caldwell - Mining Engineer - Robertson GeoConsultants

This past weekend I was thinking of mine tailings water balance calculation and quantification of make-up water requirements. These thoughts were prompted by work I have done this past week and will meet about in the coming weeks. Here is a summary of my thoughts:

The basis of a Make-Up Water Model is described in these papers:

The model enables the quantity of make-up water that is required at a tailings management facility to be estimated. Previous use and calibration of the model for tailings management facilities have established that the make-up water is required primarily to account for water lost by entrainment in the newly deposited tailings. Secondary water loss is the result of evaporation from the tailings beach. Some additional water loss results from downward seepage from the newly deposited tailings into the dry tailings of previous tailings lifts over which the new beach develops.

Key parameters used in the model include:

  • Climate and in particular evaporation (annual, monthly, weekly, or daily.)
  • The quantity of tailings deposited (generally per day.)
  • The initial void ratio of newly deposited tailings (This may be obtained by field measurement on an operating facility, estimated from laboratory settling tests, or calculated from laboratory or field density measurement or estimates.)
  • The method of operation of the tailings facility and in particular the spigot rotation period, beach size, and return water pool area.

The model does not account for the loss of water that may occur on initial deposition of tailings on to a dry, permeable foundation. This may be considered to be initial start-up water loss or as the beach grows, loss on the perimeter of the growing beach. Similarly the model does not account for water losses that may occur as tailings flow against permeable perimeter dike, berms, or embankments. To account for such losses, a detailed tailings deposition plan would have to be formulated and evaluated.

The following factors that may significantly affect water consumption from time to time, and for shorter periods may also affect overall make-up water requirements:

Start-Up Period Tailings. The start-up period may be as long as one to three years and involve the deposition of tailings that differ from those for full operation in the long term.

Start-Up Period Deposition Rates and Beach Geometry. The volume of tailings deposition for the start-up period may be less than for longer-term operation of the tailings facility. In addition, during the start-up period, deposition will be closer to the downgradient perimeter embankment. It is to be anticipated that initial deposition onto native foundation soils and what has been termed edge-effects may significantly impact actual water losses.

Tailings Variability. In practice, at many mines, there is considerable variation from time to time in the tailings, particularly its clay type and content. This variability arises from recovery of ore from different parts of the mine: sometime mining is focused near the surface where the rock may be weathered; sometimes mining is focused deeper in the pit or underground mine, where there is less, or unweathered material. In addition, as copper, gold, uranium etc. content varies from place to place in the pit, the mill may implement different processing approaches, from changes in grind to alternative flocculents.

Seasonal Variability. The model accounts for seasonal variability by using monthly average precipitation and evaporation numbers. The results indicate a corresponding seasonal variation in make-up water requirements. In practice, operation of a seasonally-affected tailings facility usually involves the use of water pools where water may be retain (stored) in low make-up water requiring periods, and used when make-up water requirements increase. The size and variability of the pool may affect short-term (monthly and/or seasonal) water losses.

Ongoing Deposition Edge Effects. If deposition involves discharge from perimeter spigots around the entire circumference of the tailings facility, the edge effects may be less significant than may be the case if deposition is from one or a limited number of spigots at the short end of a long, relatively narrow tailings facility.

And more that I do not record here. The point is that an accurate reckoning of make-up water requirements is complex and much more thought is merited.