Dewatering: Thickening, Filtering, CCD, Water Treatment & Tailings Disposal

Dewatering: Thickening, Filtering, CCD, Water Treatment & Tailings Disposal 2017-04-04T06:57:46+00:00
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Horizontal Vacuum belt filters (18 replies)

Ace Levy
2 years ago
Ace Levy 2 years ago

Fines handling can be an ultimate nightmare, especially when your ultra-fines refuse to settle in a thickener or your cake won’t dry. Typically what are the matters of interest to a process engineer in the day to day running of a Horizontal Vacuum belt filter? Ideally, filter presses provide better moistures but their inherent batch nature makes them less attractive throughput wise.

1. What measured variables contribute positively to the good running of these units?

2. How does one effectively control the moisture below the usual 30%.

3. What are the typical remedies for clay rich particles on a belt filter

4. Is there any recent 'fish-tail' (flocculant/slurry mixing units) designs that maximise contact

5. Is it useful to send samples for PSD and if yes at what frequency? 

Dizzy Flores
2 years ago
Dizzy Flores 2 years ago

Percent solids being fed to the press and filtrate clarity are good starting points. It's also important how you lay the flocked slurry onto the press.

Reagent selection and belt selection.

Reagent addition strategy and conditioning.

Due to the abrasive nature of slurry direct stream addition will have to occur on the outer limits of the boundary layer of flow through the pipe and another method of mixing to be utilized. 

Bill Rico
2 years ago
Bill Rico 2 years ago

Fine can control by selecting proper grinding media and slurry feed to bed filter contain suitable solid and liquid proportion and flocculent addition up to proper quantity, Horizontal Vacuum belt filter cloth must be checked time to time and it should be proper quality.

Marshal Meru
2 years ago
Marshal Meru 2 years ago

To improve the filtration of very fine clays sediment, one factor that you must consider is the cake resistance. Polymeric flocculent like polyacrylamide (non or cationic) can form a loosely packed sediment with large and continuous pores. This facilitates the flow of water from the sediment or the cake on the filter (less cake resistance) so that cake with less moisture can be produced. More advantages we may get are those the filtration rate increases and power requirement decreases.

Sugar Watkins
2 years ago
Sugar Watkins 2 years ago

Yes it is correct. Flocculation will enhance filtration. But many industries fail to use them with poor knowledge of storing, preparation and dosing.

Carmen Ibanz
2 years ago
Carmen Ibanz 2 years ago

In general, a poly-DADMAC would be first utilized well upstream to increase the energy and the amount of shear it encounters. Things like velocity of the slurry or multiple pipe bends imparting energy into the mixing. Then a PAM could be introduced into the head box where a minimal amount of shear and little energy will be imparted on the flocks. Depending on the process upstream and the efficiency of thickening (and the reagents used there), will dictate charge densities and MW. Previous experience says 98% of the time the DADMAC will come first and then a anionic PAM. To further control moisture content a surfactant can be added. The cleaner the filtrate, the closer you are getting to destabilize the colloids. Efficient filtering is more about charge alignments and neutralization strategies. Water temperature, pH, water hardness start playing a larger effect as your PSD trends down.

Zander Barcalow
2 years ago
Zander Barcalow 2 years ago

In designing of Thickener feed well for proper mixing and distribution of solids and mixing with flocculants keep in mind that such an application proved good for 1500 mr/hr slurry, thickening and recovering 80% clear water.

Carl Jenkins
2 years ago
Carl Jenkins 2 years ago

The residual polymer can affect product selection in unit operations occurring after thickening occurs. This usually occurs when the thickener/ clarifier (s) are too small and polymer additions are high.

In this scenario when the slurry is pumped and exposed to high energy new surfaces are exposed and the colloids should start to stabilize again, however the overdose in polymer will attempt to bind with these sites, effect rheology and hinder performance in later occurring unit operations. If feeding some sort of filter lower percent solids are usually safer.

Jean Rasczak
2 years ago
Jean Rasczak 2 years ago

If feeding some sort of filter lower percent solids are usually safer.

Sucking more water to the underflow of the thickener will also let you get a clear overflow as the rise rate will be lower.

In my experience polymer additions are either high due to poor control (pump left in manual at 100%) or poor feed-well performance (extra floc being added to try to help with mixing).

Marshal Dienes
2 years ago
Marshal Dienes 2 years ago

In that case increase the time of drying due to ultrafine cake will hinder the passing of air. And you can also reduce the cake thickness.

Zander Barcalow
2 years ago
Zander Barcalow 2 years ago

What filter cloths are you using? A woven polyester cloth will give higher throughputs and lower moistures. Pre-flocculation of coal fines prior to the main addition point in the thickener feed box will also assist. What strength are you mixing your floc at, additional dilution at the addition points will aid dispersion. As for the fish tail feeder are yours smooth or do they have V mixers? I am familiar with the large units at Grooteguluk, you are welcome to contact me if you wish to discuss in more details.

Carmen Ibanz
2 years ago
Carmen Ibanz 2 years ago

Cloths is the initial starting point of your investigation. You then need a filter vendor (such as Roymec Technologies!) to do a lab filtration study. This study will be able to show the effect of cloth type on filtration rate and moistures.

However, I am afraid that it is most likely the problem lies in the PSD of the feed, especially ultrafines (clay). Vacuum filters do not like ultrafines! And the way to deal with this typically is flocculant, which is what you are doing. But you must be aware that in 99% of cases flocculant will increase filtration rates, but also increase moisture.

The lab filter audit will give you a moisture vs throughput graph for your particular size filter, and will also give you the absolute minimum moisture that can be achieved. Based on this, you can see if your filters are not running properly (and can takes steps to remedy), or if you have no chance of getting to where you want to be.

For the latter, I suggest there are two alternatives: more filtration area (new VBF's), or deal with the feed. To deal with the feed, you need to consider cycloning to take out some of the ultrafines. This will immediately give you much higher throughput and lower moistures. However, you will then need to thickener the cyclone overflow and filter this in a filter press, or dispose to tailings if a tailings facility exists.

Without any doubt, taking out ultrafines will make a quantum change to your VBF operation.

Carl Jenkins
2 years ago
Carl Jenkins 2 years ago

I had the experience of working on a belt filter that was designed to treat a cyclone overflow after leaching. A float plant was added in between the mill and leach circuit meaning the belt was treating a floated product. Much finer than the mill OF consequently the belt was wetter than it was drier, Increasing flock only made the problem worse as it held the moisture as mentioned, and it was because of the ultra-fine material that it wouldn't filter. What fixed the problem but we were not able to use was heavy doses of coagulant into the feed/disperser box of the belt filter. This enabled those fine particles of flocculated material to clump together into bigger particles and this then enabled the filter belt to suck that liquid out better. But Coagulant is expensive and thus was not continued. The belt filter was the tails end of the line so was not justified using it.

We did have some minor success breaking the surface area of the belt cake by speeding the belt up (thinner cake) and having a series of brushes scrape across the top of the cake making a larger surface area for the vacuumed to affect.

Jean Rasczak
2 years ago
Jean Rasczak 2 years ago

Your problem is simply that your process is out of balance due to inadequate performance - fix the cause not the symptom. You can put an expensive sticking plaster on the gaping wound by changing filter cloth or flocculant but your first approach must be varying thickness to reduce the differential settling effect.

Then go back and look at what has changed and then as suggested invest in expert advice - just make sure you force a nope minded analysis. 

Marshal Meru
2 years ago
Marshal Meru 2 years ago

It seems that you have a problem in settling the fines in the thickener and in addition you would recover more water. The filter could be an expensive way to achieve these goals. You should find a remedy for the lack of settling in the thickener which could be the result of the clay present. The challenge to settle clay in the thickener can be effectively achieved in various ways and the de-watering to higher densities in the thickener can be achieved, but is not always desirable as the pumping and transporting become very problematic quickly as the density increases. Electrokinetic de-watering of the thickener underflow could be an alternative to using a filter press and it has been reported in literature that a Larox filter was successfully replaced by this technology.

Dizzy Flores
2 years ago
Dizzy Flores 2 years ago

Another alternative would be to look at Centrifuge technology to recover the fines. A simple spin test can usually determine if the centrifuge is a good alternative. We are working with many fine recovery applications (without flocculant) lately, including thickener overflows and belt press overs. The centrifuge is an enclosed and continuous operation, a huge advantage in today’s facilities.

Sugar Watkins
2 years ago
Sugar Watkins 2 years ago

You can use Larox vertical filter press. They are automatic with good throughput and allows pretty good control over the cake moisture.

Bill Rico
2 years ago
Bill Rico 2 years ago

Please let us know what you are trying to filter in the first place.

What is the mineral /Is it concentrate or tailings /what is the PSD up to the finest size normally determined in a process plant/ which fractions are high in clay/is it originating from a thickener u/f / what is the pulp density/ and what is the objective of filtration, is it feed to a next process stage requiring low moisture or mainly water recovery.

Marshal Dienes
2 years ago
Marshal Dienes 2 years ago

Usually, ultrafines will make a thin film on the top of the layer due to vacuum from bottom and does not allow air from atmosphere thus reduced effect of vacuum. Instead try to break the top layer without damaging the cloth which allows you to utilise vacuum much efficiently and lower in moisture.

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