sucking gravel at last,black sand problem

[bluethermal]

Hey fellas,

The new forum looks good,nice work Rob. Its being a harder road then i had thought but i have my new toy sucking gravel and the odd bit of Au. I have kept things nice and simple and for the moment have the motor/pump on the river bank and diffuser and sluice in the river, my only concern is the testing i have been doing in the Arrow the riffles seem to get clogged with black sands and iron stones. Is increasing the angle of the sluice going to help this problem or is the fine gold going to get washed out all so? Or is this a common problem dredging in iron sand rich spots? Is a half inch to the foot a good starting point for fall? Thanks in advance
may gold be with ya
matt

[RKC]

G'day Matt,

You have come across the most significant impediment that presents itself to Kiwi dredgers which makes fine gold recovery extremely difficult from South Island rivers.

Its great that you have recognized the problem first time out. Most Kiwi dredgers of the past have not even acknowledged the problem as significant because their experience has been limited to dredging in New Zealand and they have had nothing to compare the black sands they have seen, with. But those of us who have dredged in other countries knew of the problem from the very first time we dredged a New Zealand river. My educated guess would be that the heavy black sands that are found in such massive quantities in the South Island rivers are in much greater quantity than in any other country ... anywhere! Because of this problem a very different approach needs to be taken to dredging in New Zealand's South Island (I'm referring to production dredging, recreational dredging is a different proposition as some gold loss is generally more acceptable to a hobbyist).

The solution is to use Hydraulic riffles in a sluice box. But ... hydraulic riffles are not all that practical for installing on a small portable dredge. A dredge can easily be built with hydraulic riffles installed, but it would then be a large dredge (essentially a production dredge).
tinyurl.com/4tenwu

If you increase the angle of your sluice box you will start to lose more of the fines. The problem is the packing of the black sand between the riffles that do not allow the fines to go to bottom and much of the fine gold will sail down the sluice box over the black sand and back into the river.

Its worth mentioning here that because of the abundant black sands in New Zealand South Island rivers, subsurface dredges are even more useless in New Zealand than they are in other countries. Otago rivers that have been dredged by Suitcase dredges (probably even the river you are to dredge) have a great deal of fine gold remaining. The remaining gold has gone straight through the Suitcase dredges and has sat in the rivers waiting for someone to come along with a surface dredge. One spot I dredged on the Arrow river, I got over 50 ounces from, and all but about 10 or 11 ounces of chunky gold that came from one small area, was fine gold that had gone straight through Suitcase dredges.

There have been some previous references to the black sand problem on the GDF and following are some references I have found from previous posts.

From:Re: Constructing a dredge hot water system
« Result #3 on Nov 14, 2008, 9:05am »

When designing a sluice box for use in Otago you need to be aware that the massive quantities of black sand in all Otago rivers (and to only a slightly lesser extent in other auriferous rivers of the South Island) necessitates an innovative approach to sluice box design if the intention is to aim to capture fine gold in 'payable' quantities from a 'production' dredge.

A conventional sluice box with standard Hungarian riffles will clog with black sand in minutes when dredging in any Otago river (these examples
img.photobucket.com/albums/v172/Coochee/Hot%20linked/728b398a.jpg

img.photobucket.com/albums/v172/Coochee/Eductor%20dredges%20and%20equipment/nzgoldB_013.jpg?t=1229728756


are from the Arrow river) and pack solid. Much of the fine gold then travels straight down the sluice box and back into the river. (This example
img.photobucket.com/albums/v172/Coochee/Hot%20linked/3672fa87.jpg

is from a Nth Queensland river and dramatically shows the difference between a river with just some black sand -- which is most auriferous rivers throughout the world -- and a river with massive amounts of the stuff.)

Re: Dredge under construction
« Result #5 on Nov 12, 2008, 1:03pm »

It’s a significant advantage to have the sluice box vibrate, as vibration aids gold recovery.

Having a sluice box vibrate is probably not as important to a dredger who intends to dredge rivers with little black sand as it would be to any New Zealand dredgers who dredge the South Island rivers that carry massive amounts of the heavy black sands which clog a sluice box.


The heavy black sands found in New Zealand ’s South Island rivers will fill the space between each riffle and prevent the riffles working as intended. However, the vibration from the motor agitates the black sand to allow gold to sink down into the carpet under the riffles (what occurs is similar, but not quiet as efficient, as using hydraulic riffles in a dredge sluice box). Without the vibration, the black sand would tend to pack solid and much of the gold would jump riffles until it reaches the end of the sluice box and is returned to the river.

Its important that any dredges with conventional riffles, that are to be used in New Zealand ’s South Island rivers, have the motor directly bolted to the dredge frame without any damping added. If a motor and pump is separated from the dredge (as it is on the NZ Suitcase dredge) the problem of the dredge riffles packing solid will result, and gold recovery would be significantly impaired.

Please note: I'm not saying that there is no vibration if the dredge motor is not attached directly to the frame, I'm just saying that vibration is reduced.

Re: Building a small dredge
« Result #1 on Nov 15, 2008, 9:12am »

Achieving an acceptable 90% fine gold recovery with a dredge that's designed to be portable, is difficult ... especially if the dredge is used in Otago where the abundant heavy black sands

img.photobucket.com/albums/v172/C....pg?t=1224453122

will clog a sluice box with conventional Hungarian riffles. However for the sake of portability some loss of gold would probably be acceptable to most beginner dredgers. The inclusion of hydraulic riffles on a dredge is the main way known to professional Kiwi dredgers to dramatically improve fine gold recovery when dredging in areas of abundant heavy black sands. However hydraulic riffles are not a practical option to use on a portable dredge as the design would be complicated and the whole dredge would have to be much bigger and heavier. Hydraulic riffles are therefore only for use on Production dredges.

The usual way for a beginner to get a bad start in dredging is to purchase a dredge thats a dog. And most beginners do start with a dredge that is useless or near useless ... I did, as my first dredge was the 4-inch Goldstriker subsurface dredge. The main problem that results from buying a dredge that's a dog is that it then puts most beginners off dredging altogether ... which is unfortunate as when a well designed dredge is used nothing beats dredging as a profitable and interesting means of getting gold (especially in New Zealand which has an abundance of rich auriferous waterways).

A portable 5-inch dredge such as I have described above would inevitably be priced more than what has been offered to beginner dredgers in New Zealand previously, however the difference would be that such a dredge would actually do the job. Also, there would not be a market among experienced dredgers for such a dredge, because experienced dredgers nearly always make their own dredges. And the inexperienced would probably think the dredge is overpriced and far too large as they would be comparing it with other dredges they have seen advertised. So ... it probably won't happen, and beginners will continue to throw their money away, and give up on dredging. When using a dog of a dredge beginners spend most of their time, when they should be learning how to dredge, fighting the dredge. The answer might be for someone to import quality dredge parts, such as dredge pumps, which would then allow people to put their own dredge together.

Regards
Rob (RKC)

[bluethermal]

Hi Rob,
thanks for the good info as usual, so in a nut shell i need to attach motor to sluice to get the vibrating action to help clear the black sands, and prehaps knock up some hydraulic riffles to place under classifer.Will try with motor attached first to see how this helps first. Only small problem is the creeks i want to dredge will not float a dredge, pretty small water way.
all the best
matt

[RKC]

G'day Matt,

Attaching the motor to the dredge frame will only help with the problem of the black sand packing between the riffles and will not solve the problem completely. Only you can make the decision whether it is something worth doing and forgoing the advantage of having your motor on the riverbank. Hammy used to always place his motor on the river bank when he was in the small streams of Otago. And in such small streams it can be a considerable advantage to do this.

Hydraulic riffles have only previously been used on the larger production dredges (such as 8") and to employ hydraulic riffles you would need significant upgrading of dredge components such as pumps. And, in my opinion, you are already underpowered.

I would strongly suggest that you just work with what you have ... for now. The season is upon us and its time to get in the water. The best classroom is in the river, doing it. There are bound to be some mechanical problems arise with your dredge and you need to be doing some dredging for the problems to become aparent so they can be fixed.

Put it this way ... with the dredge you have you will get more fines than you would if you were using a Suitcase dredge, but would be getting less than if you were using a big dredge with hydraulic riffles installed. And using a bigger dredge would introduce other problems such as a lack of portability ... swings and roundabouts.

The dredge I used in the Arrow when I got the 50 ounces lost a lot of the fines but I still got reasonable gold. In fact I had a reminder of the amount of fines I was losing when I went back to where I had dredged the Arrow in the previous season, and while I was dredging to try and find where I had finished up the previous season, I dredged into my old tailings. And on getting to bedrock I could see fine gold sprinkled around on bedrock glittering like a Christmas tree back at me. It was all gold that had gone through my dredge and the spring floods had then sieved the gold through the lose tailings until all those small specks stopped moving when they got to bedrock. If I had not dredged up the gold I saw, the gold would have been moved again by following floods to move further downstream where it would have collected and concentrated in traps such as bedrock crevices. (Actually, what I found when dredging the Arrow is that chunky gold collected in spots on bedrock that were clay like. The clay held the gold like glue. This was in areas that had not been dredged previously and what would happen is that I could dredge for hours and not see anything, and then come across a patch of bedrock that was clay. And in an area of only say a foot or so square there would be about a quarter ounce or more of visible chunky pieces of gold nuggets ... not necessarly in a crevice but just in a patch of bedrock that was clay. Something else worth noting is that in the Arrow a rusty red wash is always worth investigating. The rusty red wash has something to do with how the miners of the 1800s hand worked the ground.)

There always seems to be some compromises that have to be made to get gold ... with the most important point being to get in the water and put your dredge to work.

Regards,
Rob (RKC)

[dredgenz01]

I DREDGE IN A SPOT THAT HAS LOTS OF BLACK SAND IN THE TOP OF THE SOUTH NZ.
A GOOD TIP.
DREDGE OFF AN AREA IN A ARK LEAVING THE VERY BOTTOM SANDS AND ROCK. CLEAN OUT YOUR DREDGE AND THEN GO BACK OVER FINISHING OFF THE BOTTOM OF THE RIVER YOU WILL IMPROVE YOUR FINE GOLD RECOVERY THIS WAY.
THIS TECHNIQUE WILL DEPEND ON THE RIVER AND DREDGE YOU ARE USING.

[Deleted]

Happy new year all.
Heres to yellow boxes, heavy pans and a good gold price.