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Backfill basics

A Q&A with Paul Rantala Paul Rantala operates a consulting practice, Par Innovation Inc., specializing in backfill.

A Q&A with Paul Rantala

Paul Rantala operates a consulting practice, Par Innovation Inc., specializing in backfill. A mining-mechanical engineering graduate from Queen’s University in Kingston, Ontario, he began his career at Inco’s Sudbury operations and has been operating his own consulting practice for seven years. Rantala consults globally, spending much of his time in Europe, Africa and Australia. In an interview with freelance writer Marek Krasuski, Rantala offers an overview of the types of backfill, their applications and benefits to the mining industry.

Q. How has backfill methodology evolved over the years? A. Backfill has been around for a long time. Fifty years ago only sand and water was used to fill in areas left open from ore extraction. But since then, other products and new technologies have been developed. Novel rock mixing systems and paste fills are now used. What is important is that backfill should be cost effective and perform as required. Universities and research institutes especially have focused on studying various aspects of backfill technology.

Q. What are some of the challenges associated with backfill? A. The number one challenge is cement. Probably 10 per cent of cement produced in Canada is used for backfill in mines. It is very expensive and could comprise as much as five per cent of a mine’s total cost of operations. Cement also raises environmental considerations as it produces a considerable amount of C02 emissions through its manufacture. Q. What is hydraulic backfill? A. It is essentially any kind of backfill carried by water through pipelines. Solid particles are sluiced through the water quickly without having the chance to settle until they reach the dumping point.

Q. What are the advantages and disadvantages of hydraulic fill? A. It is a simple and effective method of transporting fill. Hydraulic fill can be placed without cement as it drains water and does not create a hazard due to impounded water. Q. What is paste fill and what are its advantages? A. Paste fill is a muddy material that is as thick as toothpaste. Compared to sand and water, it performs better and requires smaller amounts of cement to produce a very strong product. It is also much more uniform in texture after placement. Q. Any disadvantages? A. Paste fill is bound with cement, which can make it more expensive to produce. Q. What is gel fill and what are its advantages? A. Gel fill is a hybrid of sand fill and paste fill, though it is practically indistinguishable in appearance from sand fill. Gel fill has a chemical additive that thickens the water into a glue-like substance, making it a consistently uniform product. Gel fill’s advantage is that it is a better utilization of cement. It flows like sand through the pipelines. It is also more robust because it allows for a change in water content, unlike paste fill, for example, which is more expensive to produce and requires more exacting water proportions. But the best type of back fill is different for each location. Test programs need to be established in order to determine the best match. Q. What are some other kinds of mine fill? A. Practically any material that is cheap, readily available, and performs well can be used as back fill. Rocks, slag, sand, and tailings are examples. Q. How much is back fill used in the underground mines of Canada as compared to the rest of the world? A. I wouldn’t want to comment about the rest of the world. But I would suggest that two-thirds to three quarters of the mines in Canada use backfill. Countries like South Africa, where mining methods may be different, may not use backfill at all. Instead, they would put in wooden posts, about one meter in length, or use some sort of cribbing technique. In some cases they may use sacks that are filled with sand to help support the openings.  Other countries, like Australia, are doing good things with backfill. But much of the mining technology spreads globally throughout the industry. South Africa, Canada, Australia and the United States, for example, are home to mining companies that work all over the world. So the technology is shared and transferred quickly. And every one of those countries does something well and something horrible. So we as Canadian mining producers should not be too high and mighty about ourselves because there are a lot of mining advancements emerging from other countries as well.  It should be noted, however, that Sudbury is doing a lot of good stuff, especially in the mining supply and service sector. Q. Are there environmental issues associated with backfill? A. One good thing is that a lot of waste product can be utilized by putting it back underground. Q. Can backfill have a future for carbon sequestering? A. The mining industry needs performance from its backfill. But if it serves as a secondary purpose such as carbon capsuling, then that too is useful. Q. Are there new developments on the horizon that will advance Canada’s expertise in backfill technologies? A. There are companies that are refining gel fill with more additives. In general terms, I would say that advancements are always a possibility. I too am working on that. Q. What will be the role of backfill in very deep mining? A. Backfill will play a vital role in the extraction process of ore in deep mining. It just takes more effort to install pipelines that reach down to the 7,000 to 10,000- foot level. Basically, fourth-fifths of mining is at the deep level. Other countries such as South Africa are beyond the 12,000-foot level. Q. Does backfill have a future for storage of products such as waste materials? A. The mining industry’s primary directive is to make ore, and backfill is a part of that process. So it makes environmental sense to do what is already being done – that is to put the waste derived from mining back into the ground. It’s probably the cheapest thing that can be done since this material has already been processed into tailings and crushed rock. But is there a future for other waste materials? There may be, if future plans make economic sense..
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