design considerations for a tailings disposal system

7/28/2019 Design Considerations for a Tailings Disposal System

1/4

More mines are considering tailings disposal systems such as this system at ENAMI in Chile.

The disposal area can be seen in the left background behind this deep cone thickener.

(Photo courtesy of FLSmidth)

Design Considerations for a Tailings Disposal SystemAn effective plan not only considers the thickener, but also evaluates transport distances and the deposition

siteBy Steve Fiscor, Editor-in-Chief

The drivers behind paste or high densitythickeners are water recovery and cost savingswith impoundment construction. Tailingsimpoundments have become a limiting factor formines. In some parts of the world, permitting anexpansion, let alone a new impoundment, isdifficult. There are serious safety andenvironmental repercussions brought about byhigh runoff. By using paste or thickened tailings,many mines find they can stack more material insmaller foot print and safeguard the operationfrom issues associated with runoff.

In other parts of the world, rainfall is not anissue and water is a scarce resource. Someoperations in Chile are pumping sea water to

high altitude plants making that water anexpensive commodity. By properly engineeringthe tailings disposal systems, water can berecycled effectively and in some cases capitaland operating costs for the facility can bereduced as well.

The days of assembling a conventional (or highrate) thickener next to the plant and pumping aslurry to a settling pond will soon be history.Most mines today are considering an array ofthickeners that also include high densitythickeners and deep cone paste thickeners. The

object is to deposit thickened tailings or a pasteusing the least amount of energy, while recyclingthe most water. The costs for these systems vary,but the exorbitant cost of engineering andconstructing a tailings impoundment usuallymakes the decision easy.

The trick, according to the experts, is to look atthe deposition site and determine the facilitysneeds, then work backward through the process. By assembling a team of engineers and geo-technicians involved in designing allthree aspects of the system the thickeners, the transport and the impoundmentmines can effectively evaluate the criteria andengineer an appropriate site-specific system.

Cost Considerations for Paste Thickening Systems

Prior to the advent of paste thickeners, a mine would specify and buy a high rate thickener. The vendor would design and install it.Because the underflow from a conventional thickener, a tailings slurry, flowed according to Newtonian principles, each of the otheraspects of the system could all be designed independently and they would work well.

The principles behind high rate thickeners are based on unit area and the slurry it produces will eventually segregate into solid andliquid phases. A high density thickener produces thickened tailings. The difference between a high rate and a high density thickener isthe underflow will not segregatethat water remains bound into the mixture. A deep cone, or deep bed, thickener produces paste.Both high density and deep cone thickeners depend on retention time rather than area. Yield stress is the dividing line between pasteand thickened tailings, and deep cone thickeners produce a yield stress of 120 to 150 Pascals (Pa) and greater. Unlike slurries, pastedoes not behave according to Newtonian principles. Engineers have to consider rheology and how it affects the entire tailingsdeposition system.

All of us in the paste business agree the best approach to designing a thickened tailings system is to decide on the end product and

gn Considerations for a Tailings Disposal System http://www.womp-int.com/story/2010vol08/stor

03/04/2013


2/4

A Siberian gold plant opted f or a 45-m high density thick ener. (Photo courtesy of WesTech)

site requirements and work upstream, said Mark Niederhauser, global paste manager, FLSmidth. Whether its surface deposition orpaste backfill , which is a little different, the system needs to maintain a yield stress or more likely a certain moisture content. Theparameters vary based on water recovery, which has an environmental aspect, but maximizing recovery also allows the mines toincrease production.

With a thickened tailings system, transport costs can be a big variable. Moving paste 500 m vs. 30 km can be a huge difference.However, a higher yield stress translates into less of a requirement for impoundment where the cost of the equipment would beinsignificant compared to the costs on the deposition zone. The engineers at FLSmidth have a general rule of thumb, Niederhausersaid. If a high rate thickener represents $1 in cost, then a high density thickener would be $1.30, and paste would be $2.

As far as bang for the buck, high density is probably the way to go if it comes down to a dollar decision, Niederhauser explained. Ahigh density thickener recovers a significant amount of water and does not increase the transportation cost proportionately. As anexample, sometimes the difference between a high density and a paste unit is 3% to 5% moisture, but the cost to recover that extramoisture might be 50% to 70% higher. Without taking into consideration the effect on the impoundment, there is a point of diminishedreturns, where going to the next level, paste, is significantly more expensive.

According to Niederhauser, more mines are looking to stack dry tailings using filtration (or a filtered cake). There is a role for ahigh density thickener in this process as well , Niederhauser said. He cited a U.S. coal application that uses a plate-andframe press.The mine filters thickened tailings from a high density thickener.

By increasing the underflow density from 27%-30% up to 50%-52%, Niederhauser explained, it reduced the number of filters byone-half. That high density thickener cost about $1 million to install, but it eliminated three filters at $2.5 million each by thickeningit up to an optimum solids concentration, Niederhauser said. This is the first time the combination of a high density thickenerfollowed by pressure filters has been used for coal refuse.

Most plants today are looking at high density thickening and potentially paste thickening, said Fred Schoenbrunn, globalsedimentation manager, FLSmidth. As little as 10 to 15 years ago, people just looked at high rate thickening. The typical copperconcentrator tailing was 55% up to 60%. Now most requests are looking at 62% to 70% solidsthats a major increase in densitybeing considered right off the bat.

High density thickeners are designed to get the highest solids concentrations that can be moved with a centrifugal pump. Its similarto the 80:20 rule, getting most of the benefits for an extra 20% to 30% expenditure without going to the expense of positivedisplacement pumps, Schoenbrunn said.

Water is an expense and it drives up the cost of a plant, Schoenbrunn said. The industry is rapidly adapting by improving waterrecovery within the plant and the consistency of the deposition zone. To expand capacity, the plant will need more water and the bestway to get there is better recovery from tailings.

Engineering a Successful Paste System

Designing a successful paste thickeningrequires a cooperative effort. The rheology,and the distance and elevation between thethickener and the deposition site determinesthe pumping and pipeline requirements,explained Tyson Gollaher, paste groupmanager, WesTech Engineering. Once thepumping and transport requirements aredetermined, the thickener can be designed tomeet the paste production and rheologicalneeds, Gollaher said.

A good approach is to assemble a team thatincludes the engineers from the miningcompany, the geotech designing theimpoundment, the pipeline designer and thethickener designer, Gollaher said. Themine explains its strategy for the tailingsdisposal site and the team evaluatesalternatives for an effective system, workingupstream from the deposition site to the pastethickener. Adjustments can be made based onwhat is reporting to the paste thickener.

Its also very important to confirm the plan


03/04/2013


3/4

The paste dries after 30 days at a bauxite deposition site in a tropical setting. (Photo courtesy of WesTech)

In as little as 90 days, men can walk on these dry bauxite tailings. (Photo courtesy of WesTech)

with bench scale testing, looking at the yieldstress and the rheology, Gollaher explained.Bench scale testing would be able todetermine what would happen with shearthinning for example, Gollaher said. Thepaste leaves the thickener with a certainyield stress, but once it passes through thepumping system and a certain length of pipe,the rheology, yield stress and the flowcharacteristics of the paste can change even

though the solids concentration remains thesame.

When a mine first starts to consider thedifference between the ultra high densitypaste and thickened tailings, the customer isusually leaning toward paste, Gollaher said.They want to maximize water recovery andthey want to produce a really thick paste.After a needs assessment and designconsiderations, especially capital andoperating costs, they compare paste withthickened tailings, which has the

non-segregating qualities at a lower cost.They will often reconsider.

As an example, Gollaher cited a recentproject at a Siberian gold mine. The mineneeded to process 800 mt/hr of flotation goldtailings and wanted to recover the mostwater possible. The pastes non-settlingcharacteristics reduced the expensesassociated with barrier construction at thedeposition site. The flow rate was veryhigh, Gollaher said. To make an ultra highdensity paste would have required severaldeep bed thickeners and the capital costs andpumping costs overwhelming favored a highdensity thickener.

Mines working in relatively flat, arid areasalso face a similar dilemma. Damconstruction costs for a high rate thickenerapplication would be extremely high and water would be lost due to evaporation. WesTech recently installed a deep cone thickenerfor an iron ore facility in South Africa that faced these concerns.

The processing plant was designed to handle 10 million mt/y. The tailings stream was first processed by a high rate thickener torecover a large portion of the water near the plant. The underflow then traveled 4.5 km to a deep bed paste thickener, located adjacentto small valley where the tailings would be surface stacked.

From the plant, the volumetric flow rate was 15,000 m3/hr, Gollaher said. The primary purpose of the conventional thickener wasto reduce the volumetric flow to the paste thickener by 90%. They looked at a paste system design without the conventional thickener,but the number of paste thickeners was cost prohibitive compared with the high rate/paste thickener combination. More recently,Gollaher said WesTech provided a similar system for an iron ore facility in China with this same layout, high rate thickener near theplant pumping to a paste thickener near the deposition site.

Rainfall can also be a problem for conventional tailings disposal sites. In Brazil, WesTech recently designed thickeners for a bauxitebeneficiation facili ty in a very wet tropical location. The tailings system consisted of two high density paste thickeners that produce alow yield stress paste. This type of paste, which could be pumped with centrifugal pumps, was expected to produce a low anglebeach slope. The site consisted of six different ponds. Four of the ponds were used for tailings deposition and the other two wereused to collect rainfall and runoff.

They rotate the deposition point between the four ponds at 30-day intervals for an optimum drying cycle, Gollaher said. Even with


03/04/2013


4/4

the amount of rain they receive, after about 30 days the paste dries and begins to crack. After 90 days, they are walking on it. Thisfacility has been a showcase for people who have concerns for runoff. The ponds are tied together so the runoff just passes by to thefinal buffer pond. Its a system thats working quite well.

With paste, the mining business is just starting to see all of the possibilities. If a mining operation looks at particle size distributionand it has fines that are 20% passing 20 microns, it has a paste application. Paste technology provides distinct advantages andGollaher believes it should be considered for applications of all types.

As featured in Womp 2010 Vol 08 - www.womp-int.com


03/04/2013

design considerations for a tailings disposal system

Documents