Introduction: A Recirculating Sluice Box for Gold Prospecting
Sometimes there isn't any running water available to power a sluice. A lot of my favorite gold panning streams dry up almost completely during the summer months. So I decided to build a recirculating sluice that would recycle a little water over and over again.
Here is a video of the finished sluice in operation in its final configuration. This sluice went through several revisions and lots of tweaking before getting to this finished product. You can see the entire evolution of this project on my web site. See also my gold panning instructable.
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Step 1: Planning the Sluice Box
So what is a sluice box and how does it work? Basically, a sluice box is a long, narrow box with a series of obstructions called riffles in it. If the sluice is placed in a running stream of water, and gold-bearing gravel and dirt is fed into the upstream side, the heavy minerals, including gold, get caught in the eddies created by the riffles, and the bulk of the lighter material gets washed through the box and out the end. Over time, as more and more material is fed through the sluice, more and more gold builds up in it. A sluice box can process much more material, much more quickly than a person, or even a team of people can pan material with gold pans.
I drew up a simple plan for a cradle that would sit on top of a plastic storage bin full of water. The cradle would hold the sluice and allow me to adjust the angle of tilt. Water would be pumped out of the bin to the top of the sluice. Water and debris would fall back into the bin at the bottom of the sluice. This would be great! I'd be able to use it in the field where there was limited water available. I'd designed a recirculating sluice or highbanker. I couldn't wait to build it.
The sluice itself is just a simple three-sided box. I decided to keep it simple and cheap, so I made it out if wood. I used a 1 x 6 pine board 36 inches long for the base and 1 x 3's for the sides. It is all held together with Gorilla Glue and screws. I marked out the location where the riffles would be. I also built and attached a spraybar to spray water into the top of the sluice. More on all this in later steps.
Step 2: Building the Box and Cradle
The sluice itself is just a simple three-sided box. I decided to keep it simple and cheap, so I made it out if wood. I used a 1 x 6 pine board 36 inches long for the base and 1 x 3's for the sides. It is all held together with Gorilla Glue, screws and nails. I marked out the location where the riffles would be. I also built and attached a spraybar to spray water into the top of the sluice. More on all this in later steps.
The cradle that holds the sluice over the tub is constructed similarly. It is made from 1 X 4 pieces of pine glued and screwed together. Notches cut into the cradle allow it to lock onto the rim of a plastic storage bin. I have two sets of notches in the cradle, which allows it to fit on two different size bins.
A 1 X 6 piece of wood is mounted diagonally at one end of the cradle to act as a splash board to direct water falling out of the lower end of the sluice back into the tub. The cradle is about 1/2 inch wider in inside width than the sluice (this is important). The wood of the sluice will swell when it gets wet. If you don't provide sufficient clearance, the sluice and cradle can lock together. I found that out the hard way in an earlier version of the sluice. You can see the entire evolution of this project on my web site.
The cradle has two hinge points made of heavy sheet aluminum screwed onto it. The sluice has two 1/4-20 t-nuts installed near its lower end to accept bolts passed through the hinge points. This allows the sluice to pivot up and down to adjust the angle of fall.
Step 3: Finding a Pump
I saw an old bilge pump at a yard sale. The guy was asking $5 for it. I talked him down to $3. It looks pretty beat up, but it works great. You can also find pumps like this at boating supply stores and on Ebay. I glued a PVC fitting on the outlet of the pump so I could attach a hose barb and a 1 in. ID hose.
I carry some sealed lead-acid batteries into the field with me to power the pump. They last a for a few hours on a good charge.
Step 4: The Spraybar
The photo below shows the spraybar in action. It is made from some 1in PVC pipe with lots of holes drilled in the bottom to allow for water flow. A hose connects one end of the spraybar with the bilge pump through a valve to regulate the water flow. The other end is just capped off.
I attached the spraybar to the top of the sluice with some steel strapping and screws. Below is a photo of an early test run.
Step 5: Making the Riffles
My biggest challenge on this build was making the riffles for the sluice out of steel angle stock. I am an old carpenter. I can make anything out of wood. Metalworking though is more of a challenge for me. So I hesitated for a while before taking the plunge and using steel. But I decided it was high time I learned how to weld anyway. So I bought some 1/2 x 1/2 angle stock and some 1/2 x 1/8 flat stock. My intent was to use the flat stock as rails on either side of the sluice and weld the angle stock between them to make riffles. The entire steel riffle assembly could then be lifted out of the sluice during cleanups.
I settled on 6 riffles, 4 inches apart and starting 4 inches from the bottom end of the sluice. I cut up the steel pieces without too much difficulty, even though I only had a hacksaw for the job. I don't have a lot of metal working tools.
I built a short section of sluice out of scrap lumber to serve as a jig for welding the pieces together. I used a borrowed welder to weld the pieces together. My welds are ugly (I need more practice) but they seem strong enough. I also welded on two angled pieces in the middle of the riffle assembly to serve as anchor points for holding it in the sluice. Not bad for a welding newbie.
The third photo shows the nearly finished riffle assembly, looking like a mini ladder. I still needed to trim the top hold-down ear back a little.
After early tests with the sluice, I found I needed to weld on another flat piece at the top end of the riffle assembly (4th photo) to hold down the mesh and ribbed matting that would go under the riffles. More about those later.
The last photo below shows how the riffle tray is held in the sluice. There are two right-angle "ears" welded onto the center of the riffle tray. They have passage holes drilled in them to fit over hanger bolts in the side walls of the sluice. Wing-nuts hold them in place. It's a good system. The only challenge is not losing the wing-nuts when disassembling the sluice for cleanups.
Step 6: Matting and Mesh
Early sluice boxes just had riffles in them. They caught a lot of gold, but some of the finer gold tended to wash right through them. The old-time prospectors eventually learned the trick of lining the slick bottoms of their sluice boxes with materials that would capture more of the fine gold. Some materials commonly used are indoor-outdoor carpeting, expanded metal mesh, ribbed matting, and a specialty product called miner's moss, designed especially for use in sluice boxes.
I pondered what to use. Miner's moss is supposed to really catch the gold. However, it is fairly expensive, and it is also very thick, I would have had to redesign the sluice with taller sides to use it. Easier and cheaper options were carpet, ribbed matting, and expanded mesh. I found some ribbed rubber matting cheap on Ebay, and bought a roll of it. It was easy to cut down to the width of the sluice.
I went to the local homecenter store looking for expanded steel mesh. I found it in big 4 X 8 foot sheets that were kind of pricey, and looked like a nightmare to trim down to size. Just down the isle I noticed rolls of plastic mesh made to keep leaves out of rain gutters. It looked like it exactly the same shape as the expanded metal mesh, but was cheaper, and would be much easier to cut. I bought a roll of it. It worked great.
The first photo below shows the pieces of ribbed matting and plastic mesh cut to the length and width of the sluice box. The second photo shows them installed under the riffles in the sluice. Now my sluice box should catch almost all the gold that passes through it.
Step 7: Optional Refinements to the Sluice
The sluice was essentially finished at this point. However, I couldn't stop tinkering. I decided to do some refinements. These are optional, and not absolutely necessary to make the sluice work. You don't need to do them if you want to keep things simple and easy. Me though, I just can't leave well enough alone.
The first refinement was a new way to adjust the angle of fall of the sluice. I had been adjusting the angle of the sluice by just jamming whatever was handy between the sluice and the cradle, and sliding it back and forth to find the right angle. This worked OK, but I decided to get fancy. I just happened to have a bunch of old 16mm movie projectors lying around the workshop at the time (that's another story). I removed the adjustable tilt mechanism from one of them and mounted it on the back of the cradle. Now I can finely adjust the tilt of the sluice with the twist of a knob. The first two photos below show the angle adjust mechanism.
The second refinement was a PWM speed controller for the pump. The PWM (Pulse Width Modulation) speed controller for the pump motor is a fairly simple circuit. It is based on the ever popular 555 timer integrated circuit. The theory behind this controller is that it controls the pump speed by turning the pump on and off very quickly and varying the length of time the pump motor is on during each cycle. The 555 timer creates a series of fast pulses. The width of the pulses determines how long the pump is turned on during each cycle. The MOSFET is a very efficient switch. Since the motor is either full on or full off, there is no power wasted like there would be in a resistive speed controller or even a unit based on power transistors. This gets maximum life out of the batteries. Varying the potentiometer setting changes the pulse width and pump speed. There is nothing too critical about this circuit. Plus or minus 10% on the component values is close enough. The resistors should all be 1/4 Watt. Other MOSFETS will work as long as they can handle the maximum current of the pump. I just happened to have a bunch of IRF540s lying around. Size the fuse to match the pump you are using. A slow-blow fuse might be a good idea since pumps often have very high initial inrush current when started.
I I built the PWM unit into a small project box and used a long, heavy cable and large battery clips for the unit. I usually carry some sealed lead acid batteries with me to power the unit, but this arrangement also allows me to connect to my truck battery if necessary. I didn't bother with a power switch. You can add one if you like. A PWM speed controller certainly isn't absolutely necessary. However, I just love to tinker with electronics. So I had to do it. The 3rd and 4th photos below show the schematic and the finished unit.
Step 8: Testing the Recirculating Sluice
The sluice was done and ready for testing. I attached the sluice to a small blue bin for testing. I had designed this sluice to work with my big red bin, but it was full of my mining equipment and I was too lazy to dig it out of storage and empty it out. So I cut a couple of new notches in the cradle to make it fit this smaller bin for testing purposes. I also used a bungee cord to hold the cradle on since so much of the sluice's weight was cantilevered off the side.
I tested the sluice using two different kinds of paydirt. First I used some crushed gold ore from a working hard-rock gold mine, just to see how well the sluice would catch gold. I ran the crushed ore through the sluice and then did a cleanup. The 3rd photo below shows the results after panning out the material left in the sluice. It caught a lot of gold, but did it catch all of it? That was the big question. So I went to the trouble of panning out all the material that went through the sluice and into the tub, just to see how much gold the sluice missed. I found none! The sluice got all the gold from the crushed ore. Fantastic.
I wasn't done testing yet. The gold in the crushed ore was rather large and chunky, and there was virtually no black sand in it. I had expected the sluice to do well with the ore. A more real-world test would be to see how it handles fine gold mixed with black sand, like what it would be seeing if I fed it gold-bearing stream gravel.
So I ran another test. This time I fed concentrates from an Arizona dry-washing operation through the sluice. The concentrates were loaded with black sand and the gold was much finer. This was more like what I would be dealing with on my favorite gold panning streams. The 4th photo below shows what was left in the sluice when I did a cleanup. The sluice again caught a lot of gold. How much passed through this time? I once again laboriously panned out all the material in the bottom of the tub. I only found a few tiny flakes of gold. The sluice caught almost all the gold. Only a little of the very finest gold was passing through. I was very pleased with the performance of the sluice. It was ready to go into the field for a real test.
Step 9: Using the Recirculating Sluice in the Field
The first photo below shows the recirculating sluice is set up on the banks of one of my favorite gold panning streams in Arizona. It is on top of the big red tub I actually designed it to work with. The little blue tub was just for testing. Here I have dug out about 1 2/3 buckets full of paydirt from under and behind boulders in the stream bed. I classified it down to 1/4 inch and am ready to start feeding it through the sluice. The flow in the stream was down to just a tiny trickle. Later in the summer it dries up completely. Only a recirculating sluice would work here.
The second photo shows the sluice after running for a while. It is mostly full of black sand, with a few random rocks. So it is doing what it is supposed to do and is separating out the heavy minerals from the bulk of the material.
The 3rd photo shows the concentrates remaining after cleaning up the sluice. It is mostly black sand and other heavy minerals, including gold! Maybe 1/8 of a bucket is left from the 1 2/3 buckets I started with. I processed a lot of material quickly and only had to pan out a small amount of material to get to the gold. This is great!
The 4th photo shows gold in my pan as I pan out some of the concentrates. Yahoo!
The last photo shows gold starting to accumulate in my storage vial.
Step 10: Using the Sluice in a Stream
It often helps to build a wing dam to divert more of the stream flow into the sluice. Sometimes I have to divert the entire flow of a stream to get enough flow to make the sluice work. It would probably work even better if I took off the spraybar and made some kind of flare at the end to catch more water, but it works reasonably well as is if there is a good flow. My next version of the sluice may incorporate an easily removable spraybar and a detachable flare for in-stream use.
Below is a video of the sluice in use in a stream in Arizona. You can see the entire evolution of this project on my web site. See also my gold panning instructable.