Building a small dredge
Nov 15, 2008 8:42:34 GMT 12
Post by RKC on Nov 15, 2008 8:42:34 GMT 12
From: RKCNZ Sent: 13/08/2008 4:29 p.m.
G'day Matt ( flapperthermal ),
The reason for placing a classifier screen over riffles in a single sluice box surface dredge is to prevent rocks collecting in the riffles and then boiling out the riffle area.
If you were intending to dredge in an area where you were sure there was only fine gold, then you could confidently cover all riffles with a classifier screen over the entire length of the sluice box. However, in nearly all situations some riffles have to be exposed so any nuggets can be caught. If I was making a 4" surface dredge, that I intended to be used in different areas, I would probably leave the first two or three riffles exposed and cover the riffles about another third or half the way down the box. On one dredge sluice box I made myself I had the classifier screen covering the lower third of the sluice box, which is something I copied from the Precision dredges ... but I'm now not so sure this is a particularly good way to go. On another dredge ( img.photobucket.com/albums/v172/Coochee/Eductor%20dredges%20and%20equipment/nzgoldenA.jpg?t=1218590086 ) I had my classifier only covering the top quarter of the sluice box ... which seemed to work OK most of the time. However when using this dredge in Otago, the Maori stones (black and heavy stones which are extremely common in rivers such as the Arrow river) would collect between the riffles and boil out the riffle area. I made it a habit that each time I would surface I would pick up and remove the larger Maori stones from the sluice box. Because they were heavier than most river stones they would tend to sit between the riffles and not budge an inch unless picked up by hand and thrown back into the river. That dredge had significant gold loss with one of the main reasons for the gold loss being that I did not have enough of the riffles covered with the classifier screen.
This Alaskan production dredge ( img.photobucket.com/albums/v172/Coochee/10-inch%20dredge/10inchproductiondredge5.jpg?t=1218591969 has a classifier screen covering the entire length of the sluice box. It was built to dredge in Alaskas 40 mile river where they must have been expecting to only dredge fine gold.
If you wish, you can use riffles of the same height the length of the sluice box (In the sluice box I made pictured above, I had the riffles, under the classifier, smaller in height compared to the riffles in the bottom two thirds of the sluice box).
You would need at least a couple of inches between the top of riffles and the classifier screen.
Unbacked Nomad carpet is preferable to backed carpet as its very much easier to clean. You don't need anything under the carpet as the aluminium floor of the sluice box seals the carpet. You do need to have the riffles held down tight to hold the carpet securely in place and there are a number of ways to do this. Common sense will probably be enough to tell you how best to hold the riffles down. All the clips I've seen on the Keene dredges have been very poor quality and will fall apart with use. Its possible to get stainless steel clips that are used on boat hatches that would do a good job ... and the ones I'm thinking of are adjustable tinyurl.com/5cyf7u . On this sluice box img.photobucket.com/albums/v172/Coochee/Eductor%20dredges%20and%20equipment/nzgoldenAwingnutby2.jpg?t=1218591349 I had the riffles held secure by two wing nuts, which is a very crude way of doing it. They needed to be replaced quiet often as the rocks that came down the sluice box would hit the wing nuts and damage them beyond repair.
The Alaskan production dredge pictured above has an interesting way to hold the riffles securely in place that could be worth copying for your 4" dredge. The Precision dredges used something similar img.photobucket.com/albums/v172/Coochee/Precision%20dredges/bbbbnzgoldZ.jpg?t=1218592337 ... so thats probably where the Alaskans copied it from.
Regards,
Rob (RKC)
G'day Matt ( flapperthermal ),
The reason for placing a classifier screen over riffles in a single sluice box surface dredge is to prevent rocks collecting in the riffles and then boiling out the riffle area.
If you were intending to dredge in an area where you were sure there was only fine gold, then you could confidently cover all riffles with a classifier screen over the entire length of the sluice box. However, in nearly all situations some riffles have to be exposed so any nuggets can be caught. If I was making a 4" surface dredge, that I intended to be used in different areas, I would probably leave the first two or three riffles exposed and cover the riffles about another third or half the way down the box. On one dredge sluice box I made myself I had the classifier screen covering the lower third of the sluice box, which is something I copied from the Precision dredges ... but I'm now not so sure this is a particularly good way to go. On another dredge ( img.photobucket.com/albums/v172/Coochee/Eductor%20dredges%20and%20equipment/nzgoldenA.jpg?t=1218590086 ) I had my classifier only covering the top quarter of the sluice box ... which seemed to work OK most of the time. However when using this dredge in Otago, the Maori stones (black and heavy stones which are extremely common in rivers such as the Arrow river) would collect between the riffles and boil out the riffle area. I made it a habit that each time I would surface I would pick up and remove the larger Maori stones from the sluice box. Because they were heavier than most river stones they would tend to sit between the riffles and not budge an inch unless picked up by hand and thrown back into the river. That dredge had significant gold loss with one of the main reasons for the gold loss being that I did not have enough of the riffles covered with the classifier screen.
This Alaskan production dredge ( img.photobucket.com/albums/v172/Coochee/10-inch%20dredge/10inchproductiondredge5.jpg?t=1218591969 has a classifier screen covering the entire length of the sluice box. It was built to dredge in Alaskas 40 mile river where they must have been expecting to only dredge fine gold.
If you wish, you can use riffles of the same height the length of the sluice box (In the sluice box I made pictured above, I had the riffles, under the classifier, smaller in height compared to the riffles in the bottom two thirds of the sluice box).
You would need at least a couple of inches between the top of riffles and the classifier screen.
Unbacked Nomad carpet is preferable to backed carpet as its very much easier to clean. You don't need anything under the carpet as the aluminium floor of the sluice box seals the carpet. You do need to have the riffles held down tight to hold the carpet securely in place and there are a number of ways to do this. Common sense will probably be enough to tell you how best to hold the riffles down. All the clips I've seen on the Keene dredges have been very poor quality and will fall apart with use. Its possible to get stainless steel clips that are used on boat hatches that would do a good job ... and the ones I'm thinking of are adjustable tinyurl.com/5cyf7u . On this sluice box img.photobucket.com/albums/v172/Coochee/Eductor%20dredges%20and%20equipment/nzgoldenAwingnutby2.jpg?t=1218591349 I had the riffles held secure by two wing nuts, which is a very crude way of doing it. They needed to be replaced quiet often as the rocks that came down the sluice box would hit the wing nuts and damage them beyond repair.
The Alaskan production dredge pictured above has an interesting way to hold the riffles securely in place that could be worth copying for your 4" dredge. The Precision dredges used something similar img.photobucket.com/albums/v172/Coochee/Precision%20dredges/bbbbnzgoldZ.jpg?t=1218592337 ... so thats probably where the Alaskans copied it from.
Regards,
Rob (RKC)