backfilling in the underground mining industry

BACKFILLING IN THE UNDERGROUND MINING INDUSTRY.

By Ray Brown.

I have often been asked by people outside the mining industry what backfilling is and why it is

necessary to the industry.

Because of this I have put together the following information which has been documented through

observation, research, experience and utilising industry best standards. I have had personnel

experience with all of the following methods of backfilling in the mining industry.

Throughout this information package you will notice the mention of cement/binder. The cement

referred to is General Purpose or G.P. cement which is used in conjunction with acid neutral tailings.

Binder is a blend of ground steel furnace slag, GP Cement and lime. The extra lime is used in

conjunction with more acidic tailings to bring the acid level down to prevent the fill from eating away at

the cement causing it to crumble and the ground steel furnace slag is used to help strengthen the

overall mixture.

The type and percentage of cement/binder to be used within the different types of backfill is

determined during the initial testing of the tailings or product to be used within the process. The

percent of cement/binder required is also calculated out depending on the number of sides that will be

exposed as well as the height and width of the stope to be filled. I.e. A small stope requiring 1 side of

exposure will require a lot less cement/binder than a large stope with all 4 sides requiring exposure.

Another factor is whether or not the backfill will have a requirement of being minded back through it

again to reach another stope.

Another chemical that may be required, especially in pastefill, is what’s known as an additive. High

clay content in the tailings will require a chemical that will break down the clay. Some tailings will

require a chemical to slow down the initial time it takes for the cement/binder to start curing as well as

adding some extra pliability to the end product. Using the correct chemical in the process can assist in

the overall solids content of the fill and this in turn can assist in lowering the amount of cement/binder

that is required to achieve the same final strength in the fill which can be a large financial saving.

The first part that I tell people is to always remember that safety is the top priority in all mines. If the

following information is adhered to there should never be a problem, not only the mining industry but

in all sections of industry and at home.

If the task you are about to perform is not detailed enough, if you are unsure or you do not fully

comprehend the correct procedure, contact your Supervisor before continuing.

Always remember safety first, no job is so important that your safety or the safety of your fellow

workers should be put in jeopardy.

If at any time you come across a procedure or a section within a procedure that doesn’t work, or

that you believe can be improved on, don’t keep it to yourself. Let your supervisor know of the

problem and, if possible, suggested improvement and it will be investigated and where required

the procedure will be updated with the improvements.

Always remember to be on the lookout for better ways to do a job.

You must never work on equipment unless you have a work permit and it is fully isolated and

locked out by using your own personal locks and tags.

Each person working on the equipment must place their own locks and tags on each piece of

equipment being worked on.

If the piece of equipment that is being worked on is in line with other pieces of equipment always

ensure that the equipment before and after it are also locked out.

Do not use the interlock system as part of your lockout procedure

It is your duty of care to:

• Work safely.

• Look after your work mate.

• Maintain a safe work place.

• Report all accidents, incidents and hazards.

Why is backfill used?

Backfill is used in the mining industry for several reasons.

• Ground stabilisation.

• To extract ore from adjacent or under another block of ore/stope.

• To extend the life of the mine.

• To lower the cost of mine closure. (Environmental).

There are several different types of filling techniques used in the mining industry. Following are the

types that I have had personnel experience with and the good and bad points that I have found for

using them.

1. Rock Fill:

Good points:

• This type of fill can be used effectively if there is no mining next to or under the stope being

filled.

• There is no waiting time once the stope is filled if firing a stope from above.

• This type of fill uses the waste rock from the underground workings of the mine to fill the void

which cuts down on the size of the waste stockpile on the surface.

• Low cost and can actually save money as the waste rock would otherwise need to be

transported to the surface and a separate storage space would need to be set up.

Bad points:

• Contaminates the ore if used next to or over another orebody/stope.

• Difficult to fill the stope completely due to the natural rill angle of the rock once tipped.

2. Hydraulic Fill, (HF):

Good points:

• Uses the wet tailings directly from the milling process which cuts down on the size of the

tailings dams required on the mining lease.

• Pumps this product through banks of cyclones to remove most of the water and very fine

material.

• Uses a waste product.

• Can be used effectively if there is no mining next to or under the stope being filled.

• Short waiting time if firing a stope above. This will depend on the type of drainage system

used in the stope being filled and the amount of very fine sand material still used in the fill.

Bad points:

• The plant needs to be relatively close to the mill.

• The amount of H.F. produced depends on the mill’s throughput & the type of raw ore being

treated.

• Hydraulic fill needs to be pumped into the stope via pipelines.

• A good drainage system is required within the stope.

• Creates a lot of excess water that needs to be pumped out of the mine.

• Contaminates the ore if used next to or over another orebody/stope.

• Relatively high risk of a bulkhead failure due to pressure caused by the water in the fill if the

correct procedures aren’t strictly adhered to.

• Medium cost due to the continual maintenance associated with pumps, cyclones and the

removal of the waste water and fines. The drilling of holes between the surface and the drives

underground, installation of the underground pipelines to the top of the stope being filled and

the building of bulkheads at the draw points.

3. Cemented Hydraulic Fill, (CHF):

Good points:



• Uses hydraulic fill & then adds cement/binder to the mix.

• Can be used effectively if there is mining next to or under the stope being filled.

• Uses a waste product.

Bad points:



treated.

• Cemented Hydraulic fill needs to be pumped into the stope via pipelines.





• High cost due to the cement/binder content required, continual maintenance associated with

pumps, cyclones and the removal of the waste water and fines. The drilling of holes between

the surface and the drives underground, installation of the underground pipelines to the top of

the stope being filled and the building of bulkheads at the draw points.

• Depending on the cement/binder content in the mix it could take up to 3 months for the fill to

cure sufficiently before mining can be allowed on the stope next to or under it.

Cemented Aggregate Fill – There are 2 methods of using CAF:

1. Uses the cemented hydraulic fill & adds aggregate at some point in the system. This

could be in the line at the surface or from a conveyor system at the end of the line as it

goes into the stope.

Good points:



• Uses waste rock from the underground workings of the mine which cuts down on the size of

the waste stockpile on the surface.

• Fills the stope quicker as 2 waste products are being used.

Bad points:



treated.

• Cemented Hydraulic fill needs to be pumped into the stope via pipelines.



• High cost due to the cement/binder content required, the cost of crushing the rocks down to

the required size & having it delivered to the plant & then into the system, the drilling of holes,

installation of pipelines & building of bulkheads at the draw points.



• Wears out the pipeline very quickly if the aggregate is added into the pipeline.

• Must be added at the correct ratio to maintain the required strength.



2. Uses a batching plant to mix dry tailings from a tailings dam, aggregate, cement/binder

and water.

Good points:

• Uses 2 waste products.

• Short curing time, around 1 to 2 weeks.

• Very little waste water produced.

• Needs a hole directly from the surface into the top of a stope.

Bad points:

• Plant needs to be mobile to move from hole to hole or uses an agitator truck to deliver the

CAF to the hole.

• Doesn’t like being pumped or running around bends in a pipeline.

• High cost due to the cement/binder content required, the cost of crushing the rocks down to

the required size, delivery of the tailings & drilling a hole into the top of a stope and if required

the installation of a conveyor system or the cost of an agitator truck.

• The drilled hole into the top of the stope usually only services 1 stope.

• Limited in the depth of the stope.

Cemented Rock Fill:

Good points:



• Uses waste rock from the underground workings of the mine which cuts down on the size of

the waste rock stockpile on the surface.

• Fills the stope quicker as 2 waste products are being used.

Bad points:


• The CHF needs to be pumped into the stope via pipelines & the waste rocks need to be

trucked or conveyed to the stope.



• High cost due to the cement/binder content required, the drilling of holes, installation of

pipelines, building of bulkheads at the draw points and if required the installation and

maintenance of a conveyor system.


• The rock fill needs to be placed into the centre of the stope with a good beach of CHF

between the stope walls and the rocks.




Pastefill – There are 2 methods of making Pastefill.

1. Dry Plant.

A dry plant is so named because it uses dry tailings from tailings dams that are no longer in use

because they are full and have had time to dry out. This reduces the need for the mine to find room

for another tailings dam once the dam is emptied.

This type of plant uses a continuous run mixing system to mix dry tailings from a tailings dam,

cement/binder and water.

Good points:

• Uses a waste product that could otherwise be an environmental issue.

• Has a short curing time, 2 to 4 weeks depending on the size of the stope, the solids content

and the binder content used.

• Very little waste water produced as all of the water in the paste is used in the curing process.

• Generally Pastefill doesn’t require pumping as it is gravity fed via pipelines for deep mines.

Shallow mines may require a positive displacement pump to obtain the required continuous

flow rate.

• Relatively low risk of a bulkhead failure due to the lack of pressure caused by the water in the

fill if the correct procedures are strictly adhered to.

Bad points:

• This type of plant can’t operate when it’s raining.

• High cost due to the cement/binder content required, delivery of the tailings, the drilling of

holes, installation of pipelines & building of bulkheads at the draw points.

2. Wet Plant.

A wet plant is so named because it uses the wet tailings directly from the milling process which cuts

down on the size of the tailings dams required on the mining lease.

Good points:

• Uses a waste product that could otherwise be an environmental issue.



• Very little waste water produced underground as all of the water in the paste is used in the

curing process.



• Generally Pastefill doesn’t require pumping as it is gravity fed via pipelines for deep mines.

Shallow mines may require a positive displacement pump to obtain the required continuous

flow rate.

• Environmental issues like rain don’t have much of an effect on the plants operation.

Bad points:

• Plant needs to be relatively close to the mill.

• Pumps the tailings through banks of cyclones to remove most of the water and very fine

material and then dries the tailings over a vacuum drying conveyor, then has cement/binder

and water added to the mix.

• Higher costs due to the extra equipment required and the associated maintenance, the

cement/binder content required, drilling of holes, installation of pipelines and building of

bulkheads at the draw points.

Paste & Rock Fill Co-Disposal.

Uses Pastefill and adds waste rocks from the underground headings at the discharge point into the

stope.

Good points:

• Uses 2 waste products. Uses waste rock from the underground workings of the mine which

cuts down on the size of the waste rock stockpile on the surface.

• Fills the stope quicker.





• Very little waste water produced underground as all of the water in the paste is used in the

curing process.

Bad points:

• The pastefill needs to be delivered into the stope via pipelines & the waste rocks need to be

trucked or conveyed to the stope.


• The rock fill needs to be placed into the centre of the stope with a good beach of paste fill

between the stope walls and the rocks.

A simple drawing of an orebody that has been partly mined out and the types of fill that could be used.

The raw ore stope in this instance could be filled with either HF or Rock Fill.

In this illustration the RAW ORE STOPE would be filled with either HF or ROCK FILL if it is below the

next orebody/stope to be mined or CHF, CAF, CRF, PASTE or PASTE AND ROCK FILL if the next stope

to be mined is either beside or below it.

My personnel favourites are Rock fill for stopes that don’t have other ore bearing stopes around or

under them and Paste fill or Paste and Rock co-disposal for all others.

What Is Pastefill?

Pastefill is a Non-Newtonian Bingham plastic.

In simple terms this means that it doesn’t follow Newtons Laws concerning fluids.

The simplest way of explaining this is using the analogy of Toothpaste. You can hold a toothpaste

tube upside down with the lid off for as long as you like and nothing will come out. However once you

apply a little bit of force to the outside of the tube by squeezing it the toothpaste will start to flow.

Pastefill, when mixed correctly, has very similar properties.

It requires a force to make it move, the force used is gravity. (Stopes close to the surface may require

a positive displacement pump in the system).

Gravity starts the paste moving however it’s the weight of the paste above it that keeps the paste

moving in the pipeline.

The general rule of thumb for pastefill is that for every metre rise in a vertical line you will have 1.5

metres flow in the horizontal line. I.e. 300m vertical = 450m horizontal.

A good pastefill mix is when you are running the system on the verge of blocking the line because of

the thickness. I have seen a line block when the solids content has been changed by as little as

0.05% per cubic metre.

Plant Procedures.

There are a lot of procedures involved in running a plant.

They cover all of the equipment, pre-starts, start-ups, running, shut downs, line blockages, Q&A

testing and even how to fill in the shift reports.

If at any time you come across a procedure or a section within a procedure that doesn’t work, or that

you believe can be improved on, don’t keep it to yourself. Let your supervisor know of the problem

and if possible, your suggested improvement and it will be investigated and where required the

procedure should be updated with the improvements.

Always remember to be on the lookout for better ways to do a job.

WORK SMARTER NOT HARDER!

Plant Check Lists.

When you first start at a plant you may feel a little overwhelmed with the amount of plant check lists

that are in place.

These check lists are a very important part of a plants operation and they are in place for the following

reasons:-

1. They will help to keep you and your workmates safe.

2. They ensure that the plant is in a safe condition to start.

3. They will give all operators consistency in the checks.

4. They will pick up minor issues before they become larger issues.

You will find that the more often you do them the easier they become.

Well these are my thoughts. I hope that they are informative for you.

backfilling in the underground mining industry

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