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. In some cases we have seen holes where no replaceable string is present, resulting in water ingress and diluting the paste.
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.
Click here to see camera footage of worn casing due to paste fill operations.
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:
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:
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. We typically recommend 4 1/2″ casing for the main string below the surface feeder joint.
Alternative surface feeder casing:
We typically recommend 4 1/2″ casing for the main string below the surface feeder joint. 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:
Alternative sizes available (less common):
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.
To find out more or to discuss any specific needs you have, contact AGE Developments.