Recovering Water

Can Decanter Centrifuges Solve One Of Your Most Urgent Processing Challenges?

By Eric Gentis

Before the mining industry can find the most effective and efficient solutions, it is important to first understand the most pressing challenges it faces. Removing valuable resources from the ground involves more than just digging it up, making the material smaller and then loading it on a big truck and hauling it away. It must go through the process of taking all the desired materials out of the product mined, and then finding a way to dispose of all the other things that are pulled from the ground in the process. 

Sand and gravel quarries are small-scale mines, although sometimes miners don’t see them that way. Mines are different from manufacturing in that you are not erecting a building and then pulling something through it while converting it. Instead mining companies must buy a piece of property and then go through the process of winning out the material. Then, they must return the natural resources, like water and overburden, back into the earth before moving on to the next piece of real estate. 

Processing ores and product rarely produces purely dry materials. Water is used to control dust, in washing product, and in the winning process of taking raw material from the ground and turned into something valuable that people want. The difficulty with using water in mining (including sand & gravel production) involves four key challenges. 

Challenge 1 – Shortage of Water
Many regions face a significant lack of water. The shortage of water is not going to improve thanks to global warming, which is exacerbating the consistent shortage over time. Part of the problem is that there are more people in the world, therefore, the demand for water is greater – particularly for domestic, agricultural, and industrial uses. There is also less water available as the worlds regions dry out. 

Because of the global water shortage, water can no longer be left in the material that gets dumped in tailings in mining operations. The water must be put back where it was extracted or at the very least re-used. In some applications, 95% of the recovered water can be recycled back into its original space. 

Fortunately, there are new methods and technologies that can reduce the water consumption that is required in the mining process. For example, the dewatering of slimes and tailings may be required. The bottom line is that the processed water needs to be reused and cannot be wasted.

Challenge 2 – Shortage of Space
Mining currently faces a huge public relations challenge. The perception is that the industry is similar to the mining company in the movie, Avatar, in which the mining companies were the bad guys trying to destroy the earth. 

While that is the general public’s perception, the reality is that while mining must account for some things, the simple fact is that the process of mining will continue. Mankind has been mining for thousands of years – ever since rocks were replaced with metals. Whether it’s a base metal (like copper, nickel or iron), or a precious metal (like gold or silver) there is less of it available to be mined, but the demand for it is higher. 

As the ore deposits around the world are mined, the better quality ores are extracted first, so now the bad news is that to win the same amount of material that was previously extracted requires more mining, which produces more tailings. The mining industry must figure out where to dispose of them. Many mining companies are forced to enlarge their tailings storage facilities. In addition, there must be an environmentally sound plan for replacing the water that is extracted.

It’s difficult to acquire permits for mining, and even when the permits are secured, that doesn’t mean that land is available. The term NIMBY has become common in the mining industry. It means “not in my back yard” and reflects the resistance from the public in general to mining operations near residential areas. With the shortage of space, miners are competing with communities, agriculture and industry for space. And the governments are less likely to make more land available for mining. 

One solution to the shortage of space is to remove more of the water using separation processes. That way, more of the desired materials can be produced, which saves a large percentage of the space required. 

Challenge 3 – Stricter Regulations
Government regulations for mining have become stricter through the years. In many European countries, the sand and gravel pits are restricted to pulling up only a certain amount of water and none of it can be put back into the groundwater for fear of contamination. Mining companies around the world have built large dams that are becoming overloaded. 

With the increase in stricter regulations, some governments are also sensitive to the “social license” to operate – compliance is required without specific regulations, based on the safety and environmental concerns of the community. A big part of the social license to operate involves water conservation. 

Challenge 4 – Improvement of Operational Efficiency 
There are limited options available for mining operators to improve profits. Miners are only paid for the minerals produced, but now more effort is required to get them. Efficiency and productivity become directly connected to profits, creating a continually rising focus on improving operational efficiency to help reduce costs.

Advantages of Using DecanterCentrifuge Technology
Decanter centrifuge technology has become a clear choice for recovering water in the processing of ore and minerals. Some mining companies use filter presses, but these large, clunky machines require permanent structures, take up too much space, and require continuous maintenance. 

There are several advantages to using decanter centrifuge technology for mining operations:

1. Typically, when materials are spent through a decanter, 70-75% of the water will be removed without too much effort. That process significantly reduces fresh water costs as the recovered water can be reused.

2. There is also a high percentage of high-water recovery. Turbidity of reclaimed water may be between 30-50 NTU (Nephelometric Turbity Units).

3. Compared with a filter press, a decanter requires a third of the footprint. Sometimes it’s even smaller. The space required is not much more than two pick-up trucks. (e.g. 7x Z92 Decanters -> Area required = 30 m x 50 m (1,500 m²).

4. Filter presses require a permanent, concrete base and enclosed structure. Decanters require no cover or permanent structure. Electronics are protected within a shipping container next to the unit that is dust proof, wind proof, and temperature controlled. The system sits on a highly-portable metal skid and can be moved easily. It’s possible to install centrifuges without covers because they are durable and resistant to extreme temperatures. 

5. Decanters are cost efficient. Because they are fully automated and can run continuously 24/7, decanters require one-tenth the number of operators that a filter press needs. 

6. Decanters have sophisticated plug & play solutions in a standardized system. There are also optional features such as placing a centrifuge on a platform or adding a conveyor to remove solids. 

7. Decanters are easy to transport. The low profile, relatively small size is designed to fit standard road trucking, making them easily transported to another mines. 

8. Decanters have reliable and proven technology with high availability.

9. Decanters are easy to maintain. They are resistant against wear and corrosion, making them more efficient to operate and reducing maintenance costs. Some companies, like Flottweg, offer full aftermarket services that make maintenance and operation simple and worry-free for the operators. 

10. Filter presses use +20% less electricity, but because of the robust mining design, decanter centrifuges are still more cost efficient to operate and have higher productivity. 

11. Another specific advantage a decanter centrifuge has over a filter press is that there is no filter cloth which can become blinded with fine particles. This means that the flow rate through a centrifuge remains constant.

How Decanter Technology Works
A decanter is a sedimentation tank that is wound around an axis. In the sedimentation tank, the solid particles, which are heavier than the liquid, move to the bottom by gravity and form a sediment (solid phase) at the bottom of the tank. 

In a centrifuge, the solid and liquid phases are separated by means of centrifugal acceleration. In the rotating bowl of the centrifuge, the solid particles, which have a higher density and are therefore heavier than the liquid, move outward because of centrifugal force. The sediment is formed on the inner wall of the centrifuge bowl. 

Since centrifugal forces of approximately +1,000 g are exerted in a mining centrifuge as opposed to 1 g in a gravitational field, the separation of the solid particles from the liquid is much faster and more efficient.

The product is fed into the inlet chamber of the decanter scroll through the centrally arranged feed pipe. From there, after gentle pre-acceleration, it passes through the distributor openings into the decanter bowl.

The decanter bowl has a cylindrical/conical shape and rotates at a pre-set speed optimally adjusted to the application. The slurry rotates within the bowl at the operating speed and forms a cylindrical layer at the bowl wall. Due to the higher density, the solids contained in the product settle on the inner wall of the bowl under the action of centrifugal force. The length of the cylindrical bowl section and the cone angle are selected to meet the specific requirements of an application.

The decanter scroll rotates at a slightly different speed than the bowl and conveys the separated solids toward the conical end of the bowl. This differential speed determines the residence time of the solids in the decanter bowl. Residence time is a critical factor for cake dryness. It can be adjusted by changing the differential speed of the scroll providing optimal separation. Depending on the application and task, the scrolls are designed differently.

Settled solids are ejected through ports at the conical end of the bowl into the solids housing and fall through the discharge chute.

The clarified liquids (fluid phase) flow to the cylindrical end of the bowl where they exit over weir plates. In these openings there are very precisely adjustable weir plates (overflow weirs) with which the pond depth (liquid layer) in the bowl is adjusted. The liquid is collected in the drain housing and discharged without pressure.

As an alternative to the overflow weir, the clarified liquid can also be discharged via an impeller and discharged from the bowl under pressure in a closed system. This eliminates the need for a separate chamber pump. A further developed variant, the adjustable impeller, allows infinitely variable adjustment of the pond depth during the run. This allows a fast and fine adjustment to changed conditions without having to take the centrifuge out of operation.

The use of decanter centrifuges for recovering process water and dewatering tailings is an economically viable and attractive alternative to other existing technologies for the mining industry.

Based on environmental impact, space requirement and costs, the mechanical separation of solids and liquids by centrifugal force is being accepted as the preferred solution in mining and minerals processing operations. The dewatering and tailings operation is attractive, economically viable, and it’s easy. Just plug it in, and it runs continuously with little maintenance and fewer operators. The portability makes it possible to move the operation to other sites and the smaller footprint allows for accessibility even in the most unusual locations.

Eric Gentis is with Flottweg Separation Technology.

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