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Gold Prospecting Forums - General => Scientific Journal => Topic started by: RotGrub on June 15, 2015, 09:15:18 PM

Title: Possible theories for my problem
Post by: RotGrub on June 15, 2015, 09:15:18 PM
My tailing screen is plugging up and I can’t explain why. I’m at the point that I think there might be a physics issue occurring which is beyond my knowledge.

Background: I have designed and built a recirculating highbanker which consists of a plastic 70 gallon supply tub which feeds a powder coated steel vortex cone that is isolated with rubber and plastic washers to a 24 gallon aluminum sediment tank. The vortex is connected to a 2.5hp Honda water pump with 1 1/2” flex PVC pipe and brass couplers.   The pump feeds the highbanker with 1 1/2” flex PVC pipe and brass couplers connected to the PVC spray bar located in the highbanker’s plastic   hopper. The highbanker is constructed from aluminum with a short rubber v-mat, miner’s carpet under zinc expanded metal, under a zinc riffle system.

The tailing screen is made from ash & poplar wood with a stainless steel 60 mesh screen mounted to the wood frame. The bottom of the tailing screen is fabricated with galvanized sheet metal which slopes from both the front and rear to the center where a 4” PVC discharge outlet is located. The cavity between the SS mesh and bottom GSM is 1 1/2” on the front & rear and 4” in the middle at the drain outlet. 

Operation: The pump pulls water from the 70 gal supply tub up into the vortex where the heavier -60 particles are spun down the vortex into the 24 gal sediment tank. The sediment tank has a vacuum gage which operates between 60-30 psi. By design the vortex pulls cleaner water up into the pump and pressurizes the highbanker spray bar system.

The highbanker washes the material and discharges the tailings onto the tailing screen which are brushed off with a nylon street broom. During the process the tailing screen is brushed constantly to remove the tailings and prevent the tailing water from filling up the screen. The tailing screen sits on top of the 70 gal supply tub with the 4” outlet extending below the water line of the supply tub  into a funnel pipe to collect the -60 material into the vortex.

I have used a similar system without any problems in other areas with different material with the exception of the vortex tank. The 60 mesh never plugged up as I’m now experiencing. My first thought was the new material had a higher concentration of fine particles which was the cause of the tailing screen plugging up.

But after running 150 buckets of -1/2” material I can say the condition seems to not be created by the concentration of fine material. The more material you run the concentration of suspended clays increases which should result with more plug ups as the suspended clays build up.

This is not the case. I have experienced these plug up within a 10 bucket run with relatively clean water and have no plug ups in 40-50 bucket runs where the suspended clay concentration is at its highest. It has no pattern related to the water viscosity as it occurs randomly through the runs; at different times and different days.

On closer inspection it seems like there is a vacuum on the bottom side of the 60 mesh. It almost has a sheen created by a film of water. If I beat on the screen with the brush and brush both forwards and backwards rapidly it seems to release the water and quickly drain and I can the proceed to run several more buckets without incident.

So here are my questions:
1. Could a vacuum be created under the screen?
2. Could the minerals in the material and the brushing action with a nylon brush over the SS screen  create a magnetic field or some other issue and be attributed to the problem?
3. Could there be magnetic particles in the material which are sticking to the SS screen which their concentrations are equally as random as the plug ups?

It doesn’t make any sense to me. I was going to just replace the 60 mesh with 50 mesh and see what happens but I really believe something else is going on.

There are videos of this system in operation if you are interested in helping me solve this problem.


https://www.youtube.com/user/R0TGRUB

Title: Re: Possible theories for my problem
Post by: overtheedge on June 16, 2015, 08:18:35 AM
I doubt that magnetism is the problem. Most stainless steels aren't magnetic.
Sounds more like you have a bunch of material that is near screen size and once it is wetted, it pins the screen.
The surface tension of water is a fairly powerful force.
As the system is a recirc, have you tried using a surfactant such as Jet-Dry?

eric
Title: Re: Possible theories for my problem
Post by: RotGrub on June 16, 2015, 09:38:51 AM
I doubt that magnetism is the problem. Most stainless steels aren't magnetic.
Sounds more like you have a bunch of material that is near screen size and once it is wetted, it pins the screen.
The surface tension of water is a fairly powerful force.
As the system is a recirc, have you tried using a surfactant such as Jet-Dry?

eric

Eric, I have not tried jet dry. I did give alum a try to see if it would flock the suspended clays together which it did, but they settled to the bottom of the supply tank and were then remixed back into the system when I ran material. I was hoping the vortex would capture the flocked material but it did not. I did not associate if this conditioned occurred or not during my use of alum.  Not sure what jet dry is (I'll do a search).  Is it the stuff you put into the dish washer? how much is used and is it safe for the environment? I hear what your saying regarding the material pinning the screen openings and that has been my theory from the start, but this conditioned occurred last weekend in the early stage of my run where the water was very clean. In addition the material drops off at the front section just below the sluice and will build up in the first foot or so if not brushed off. How is the lower section of the tailing sluice; where no material is present holding water? Your surface tension theory makes sense as the screen looks like it is encased in water and not coarse or gritty as when the screen is dry.

Thanks for your input.
Title: Re: Possible theories for my problem
Post by: Goldcrow on June 16, 2015, 09:48:19 AM
Are you creating a 'static electricity' condition?
Title: Re: Possible theories for my problem
Post by: RotGrub on June 16, 2015, 11:16:56 AM
Are you creating a 'static electricity' condition?

I have no idea Goldcrow. I'm somewhat familiar with static electricity (rubbing your shoes across a carpet and touching metal to discharge the built up electricity) but do not understand the principals behind the event or how it could contribute to my problem. Could the nylon brush be creating something when its brushed several hundred times across the screen? I tried to provide as much info in my statement above because I believe there is something else going on other than particles plugging the screen.   
Title: Re: Possible theories for my problem
Post by: azau on June 16, 2015, 01:18:41 PM
Brushing the material from above the screen may be part of your problem especially if the screen is not sloped enough.  The bristles may actually be forcing the oversize material already stacked on the screen to wedge into the openings rather than be loosened.  Overfeeding may be another factor.  Brushing from below would prevent most blinding.   A shaker screen with abrupt bump stops at each side of the stroke would/should keep the material actively moving, minimize screen blinding and encourage oversize to exit.  Maybe steepen the screen and compensate for lower retention time on a same sized screen by lengthening it.  In your old location the feed materisl may have been more rounded than what you are feeding now.  Angular material is more likely to find a way to stick in a screen opening due to it's irregular shape.

Kind of disjointed thoughts but things to consider.

Good Luck.

I think the "sheen" is simply an indication that the screen is partially/mostly blinded so the water cannot pass through as quickly and sort of stacks up until it can.  As you probably already know, when the screen is blinded some of the larger undersize material will report with the oversize stream since it has no other place to go.
        
Title: Re: Possible theories for my problem
Post by: RotGrub on June 16, 2015, 04:25:08 PM
Brushing the material from above the screen may be part of your problem especially if the screen is not sloped enough.  The bristles may actually be forcing the oversize material already stacked on the screen to wedge into the openings rather than be loosened.  Overfeeding may be another factor.  Brushing from below would prevent most blinding.   A shaker screen with abrupt bump stops at each side of the stroke would/should keep the material actively moving, minimize screen blinding and encourage oversize to exit.  Maybe steepen the screen and compensate for lower retention time on a same sized screen by lengthening it.  In your old location the feed materisl may have been more rounded than what you are feeding now.  Angular material is more likely to find a way to stick in a screen opening due to it's irregular shape.

Kind of disjointed thoughts but things to consider.

Good Luck.

I think the "sheen" is simply an indication that the screen is partially/mostly blinded so the water cannot pass through as quickly and sort of stacks up until it can.  As you probably already know, when the screen is blinded some of the larger undersize material will report with the oversize stream since it has no other place to go.
        

azau, thanks for your input. The one thing that stands out of your theories is the angular rocks or particles. This new area has angular big and small rocks they are not rounded like IBAR was. I have not looked at the small particles under my loop but will do so on my next trip. As for increasing the tailing sluice pitch, I've tried and regardless of how steep; the tailings will not move down the screen and the water just runs off. Even if the fine material is plugging the upper portion of the screen how is the lower section holding water with no material present? 
Title: Re: Possible theories for my problem
Post by: azau on June 16, 2015, 05:25:26 PM
The water may be sheeting (more water than the openings will allow free passage eg. it kind of lakes on top of the screen until it can pass) due to the increasing restriction in the upper section of the screen.  I'm not sure that that in itself is a problem but the blinding is for sure if it effects productivity.  If angular feed is the cause, you will still probably see much the same effect with slightly larger screen openings.  

Good luck.
Title: Re: Possible theories for my problem
Post by: RotGrub on June 16, 2015, 09:46:55 PM
The water may be sheeting (more water than the openings will allow free passage eg. it kind of lakes on top of the screen until it can pass) due to the increasing restriction in the upper section of the screen.  I'm not sure that that in itself is a problem but the blinding is for sure if it effects productivity.  If angular feed is the cause, you will still probably see much the same effect with slightly larger screen openings.  

Good luck.

I inspected the tailing sluice today with my loop and found approximately 10% of the screen openings were plugged with black and red particles. This was after a good cleaning with a high pressure water spray. I then verified the actual screen openings per the manufacture specifications and found this 60 mesh openings to be .009. I then looked into larger screens and found that the actual openings are based on the wire diameter and not the opening. 40 mesh screens are available in 5 sizes based on the wire diameter; net opening .012, .014, .015, .016 & .017. The 50 mesh is available in one size .011. I'm thinking of trying a 40 mesh in .015 which would be approximately 66% larger than the .009 60 mesh. I also realize that increasing the screen size  may not solve the problem as you have stated but even with 10-15% plugs in a larger screen, the additional opening area might allow the free water to pass with less restriction. The trade off will be collecting more fine and larger material into the vortex tank which might restrict the maximum amount of material I can run before cleaning the vortex tank.

Another option I found was to replace the SS mesh with a polyester mesh that states it is much more clog resistant than steel mesh. I would be concerned with its durability but for the price which is 70% less than SS I might give it a try. If it were to hold up for perhaps 100-150 buckets and eliminate the plug ups which will speed up the material processing I would be OK with replacing the screen as required.  McMaster-Carr (http://www.mcmaster.com/#9218t67/=xnoi8k)

Title: Re: Possible theories for my problem
Post by: azau on June 17, 2015, 06:25:36 AM
The polyester mesh should be easier to clean too. 

Good luck.
Title: Re: Possible theories for my problem
Post by: RotGrub on July 13, 2015, 03:15:45 PM
I removed the SS screen and found the bottom side to be plugged. The top looked and felt clean but the angular material was packed into openings and were protruding out past the screen bottom... I couldn't see this condition until I removed the screen. The next generation tailing sluice will have removable screens... I replaced the SS w/ a poly/nylon material with a similar size 58 mesh and ran some material last weekend. You can see the video here

[youtube]hFigMOef_g4[/youtube]
Title: Re: Possible theories for my problem
Post by: aumbre on July 15, 2015, 11:18:17 AM
It is posted under "Scientific Journal". Works fine here.
Title: Re: Possible theories for my problem
Post by: RotGrub on July 27, 2015, 09:39:48 AM
The nylon screen started to fail after 67 buckets. It could still run a few more buckets, but I would say under my conditions a nylon screen would get approximately 70-75 buckets of use. What I've learned from this issue is to design the next tailing screen with removable (quick change) screen trays so that field maintenance or replacement is possible. I would not rule out the use nylon but steel lasts much longer (500 buckets +) but both the top and bottom surfaces of the steel screen must be accessible and cleaned as required to prevent plugging.
Title: Re: Possible theories for my problem
Post by: azau on July 27, 2015, 10:58:05 AM
Removable screens...... and periodic cleaning from the under side first to loosen the jams then most of the material can be brushed off of the top side.  Be careful when cleaning the top side so that particles will not be driven deeper into the mesh. 

Good luck