Paste Fill

Paste filling is a method used to dispose of mine tailings underground, to backfill underground mine stopes, or to achieve both simultaneously. One of the key difficulties in sustaining an effective paste fill operation is to maintain a serviceable conduit in which the paste can travel from above ground to below. This is most often achieved by cementing a large diameter conductor casing from surface to underground, then suspending a replaceable wear casing inside through which the paste can travel.

A common occurrence in these replaceable strings is the premature failure of the walls and subsequent cementing of the string into the borehole. This can make retrieval for replacement extremely difficult or impossible, forcing the need for a smaller diameter casing to be installed within the old string, or a new hole to be drilled entirely.

The premature failure is caused by the highly abrasive paste freefalling at high velocity which produces slurry erosion in localised areas of the string. Once this erosion pattern is established, a channel will form and promote further erosion in the same region. This channel will scallop extremely rapidly until the casing breaches - typically this will result in the paste fill casing being cemented into the outer casing. If removal is possible, it will usually reveal that the majority of the casing material shows little wear.

It has been found that the key factor in avoiding this situation is reducing the velocity of the falling paste. By ensuring that the paste material remains backed up inside the string, the corrective action is twofold:

  1. The velocity is controlled, reducing wear rates; and
  2. The paste material is controlled inside the casing string, which prevents 'slopping' and promotes a laminar flow regime.

We have had tremendous success for over 15 years in making use of premium threaded oilfield casing to fulfil the requirements of such wear casing. Our unique approach to this problem has seen us involved in many paste plant casing installations for mine sites throughout the country. We have selected premium threaded oilfield casing as the material of choice for several reasons:

  1. When installed, the internal diameter of the casing is flush at the threaded joint meaning turbulent flow of the paste material is avoided
  2. The material grades have far greater tensile strengths than standard line pipe material
  3. As the casing is threaded, there is no change in mechanical properties as a result of welding
  4. There are many diameters and wall thicknesses available
  5. It is typically available as used casing ex-oilfield at competitive prices (Note: ex-oilfield product not used for wear applications in original use)
  6. The threaded nature allows for very fast installation when compared with alternative welded line pipe materials

Typically we recommend installing a surface feeder pipe consisting of one joint of 7” casing that acts as a funnel for a smaller diameter casing. It is this smaller casing that controls paste velocity and maintains an even, laminar paste flow to avoid premature wear channelling. There are many wall thicknesses and grades available for the 7" feeder casing joint, though the common size that will typically be available is as follows:

  • 7" 29# premium threaded casing
  • Pipe body OD 7" (177.8mm)
  • Nominal ID 6.184" (157.07mm)
  • Coupling OD 7.625" (194.46mm)
  • Most common grade L80
  • Pipe body yield strength in tension 676,000lbs (306ton) based on L80 grade
  • Pipe body internal yield strength 8160psi (56MPa)
  • Physical weight 29lbs/ft (43.1kg/m)

Alternative surface feeder casing:

  • 7 5/8" 33.7# premium threaded casing
  • Pipe body OD 7 5/8" (193.7mm)
  • Nominal ID 6.765" (171.83mm)
  • Coupling OD 8.5" (215.9mm)
  • Most common grade L80
  • Pipe body yield strength in tension 778,000lbs (352ton) based on L80 grade
  • Pipe body internal yield strength 7900psi (54MPa)
  • Physical weight 33.7lbs/ft (50.1kg/m)

We typically recommend 4 1/2" casing for the main string below the surface feeder joint. Depending on paste density, this will typically facilitate flow rates of 150 - 180m3/hr. Again there are many wall thicknesses and grades available for the 4 1/2" casing however the common size that will typically be available is as follows:

  • 4 1/2" 11.6# premium threaded casing
  • Pipe body OD 4 1/2" (114.3mm)
  • Nominal ID 4.00" (101.6mm)
  • Coupling OD 5.00" (127mm)
  • Most common grade L80
  • Pipe body yield strength in tension 267,000lbs (133ton) based on L80 grade
  • Pipe body internal yield strength 7780psi (53MPa)
  • Physical weight 11.6lbs/ft (17.3kg/m)

Alternative sizes available (less common):

  • 5" 18# premium threaded casing
  • Pipe body OD 5.00" (127mm)
  • Nominal ID 4.276" (108.6mm)
  • Coupling OD 5.563" (141.3mm)
  • Most common grade L80
  • Pipe body yield strength in tension 350,000lbs (175ton) based on L80 grade
  • Pipe body internal yield strength 8290psi (57MPa)
  • Physical weight 18lbs/ft (26.8kg/m)

  • 5 1/2" 15.5# premium threaded casing
  • Pipe body OD 5 1/2" (139.7mm)
  • Nominal ID 4.95" (125.73mm)
  • Coupling OD 6.05" (153.67mm)
  • Most common grade L80
  • Pipe body yield strength in tension 361,000lbs (163ton) based on L80 grade
  • Pipe body internal yield strength 7000psi (48MPa)
  • Physical weight 15.5lbs/ft (23.1kg/m)

As is the case with any surplus material, used premium threaded casing is subject to availability. This means the wall thickness, grade and thread options will vary frequently and will need to be confirmed at the time of order placement.

If the quantity of casing required is high enough, specified mill runs can be ordered with wall thicknesses on the casing as great as 15mm in even higher tensile grades. Mill orders can take in the vicinity of 6 months to land into ports in Australia from time of order.

Paste fill casings are most efficient when installed in a vertical orientation rather than set at an inclination. Inclined casings will preferentially wear on the lower side of the casing material and reduce the operating life of the string. Additionally, the installation and removal of the string is significantly more efficient when servicing a vertical hole.

Threaded oilfield casing is installed using hydraulic tong equipment to torque the threads to the figures recommended by the manufacturer. Handling equipment is available off-the-shelf and is typically certified to 150ton lifting capacities. This allows for a fast, efficient installation and negates the requirement for any custom equipment to be manufactured. Paste plant casing strings are lowered into the borehole through the use of a main slew crane, and an auxiliary franna crane is used to suspend hydraulic tong equipment. Depending on crane operator proficiency, typical installation (or removal) times for a 400m string of 4 1/2" casing will be in the vicinity of 4-6 hours.

For more information, please contact AGE Developments