Deep dive into the workings of a Bellows Drywasher
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Thread: Deep dive into the workings of a Bellows Drywasher

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  1. #1

    Jan 2015
    California Desert
    42 times

    Deep dive into the workings of a Bellows Drywasher

    I thought it might be fun and helpful for new folks to talk in detail about the function of the bellows drywasher.

    At first glance, a bellows drywasher is a rather simple looking piece of equipment. However, behind this simple exterior is a more complex and fascinating side.

    A bellows drywasher is essentially an air sluice. Both the bellows drywasher and the sluice use a fluid to transport and separate gold bearing material. The water sluice uses water and the bellows drywasher uses air.

    The magic of the bellows drywasher is the fluid it uses air; not only transports material, but also separates the gold using differences in density, just like water. This is important, because one of the best methods to separate gold out of waste material is by using gold’s high density.

    We all know that gold is heavy, but what is important is that it is dense, in other words it is very heavy for its volume. Gold can be more than six times denser than quartz sand.

    If we dumped gold bearing material into a bucket of water, not much would happen, but if we put that same material into a moving fluid layer a lot starts to happen. The most notable thing is a type of stratification takes place, lighter material at the top and denser material at the bottom. If everything was in balance, this stratified material would go moving along happily. But, we want to recover gold so we must disturb this stratification.

    The way we do this is by placing a riffle in the moving fluid layer. The riffle manipulates and slows the fluid layer to the point that the fluid can no longer carry the dense gold, but can continue carrying the less dense waste material.

    You could theoretically have infinite numbers of riffles to capture the gold, but practicality and experimentation shows that most of the gold captured by this method requires only a small number of riffles.

    Now that we have a basic understanding of the importance of density, let’s look at how the bellows drywasher creates a fluid layer.

    The bellows of a bellows drywasher in effect is a type of air engine. The piston is the bellows bottom, the fabric sides are the cylinder walls, the flaps are the intake valves and the riffle tray fabric (along with the gold bearing material) is the exhaust port.

    The beginning of the cycle starts with the bellows moving down and drawing air in through the flapper intake valves. When the bellows reverses direction and starts moving back up, the flapper valves close and the compression cycle begins. The compressed air exhausts through the riffle tray fabric and then the cycle starts again.

    This exhausted air is the fluid in our air fluid layer.

    A closer look at the compression cycle shows an interesting and important effect of the riffle tray fabric interacting with the gold bearing material. When material falls on the fabric, the material obstructs the small holes in the fabric. The bellows must now do more work to overcome the obstruction resulting in higher pressure in the bellows.

    Initial logic says; just use bigger holes in the fabric and the drywasher will breathe like a champ. But the operation of the bellows drywasher is not always logical.

    As it turns out, we use this obstruction of the riffle tray fabric to our advantage. By tuning the size of the holes in the fabric, we get the fabric and the material to act like a seal against the compression in the bellows. When the pressure is high enough, it overcomes the seal and “Pop”; the air carries the material upward.

    Now that we have created a fluid layer, gravity can act on the material and begin the process of stratification. Light material at the top, dense material at the bottom.

    In a water sluice, the flow of water moving from a high point to a lower point moves the fluid layer across the riffles. In a bellows drywasher, we have to get a bit clever to achieve the same thing.

    By angling the riffle tray on our bellows drywasher, we create a vector to normal gravity. Each time our air engine pulses, it tosses the material at about perpendicular to the riffle tray. Gravity then pulls it straight down. The result of this is the material moves in a series of arcs down the riffle tray.

    If we were to rely only on the air to move material down the riffle tray, most likely we would have a very inefficient drywasher that could hardly get material off the end of the riffle tray.

    Why is this? Because short of a hurricane (like what happens in an air-blower drywasher), air does not have enough energy to effectively move the material.

    Here again the bellows drywasher has a trick up its sleeve. Because the bellows drywasher uses flexible fabric for the riffle tray, the interaction of the compressed air with the fabric acts like a trampoline propelling the material up in the air.

    By compressing and exhausting air along with the trampoline effect, the bellows drywasher can create a fluid layer, stratify this layer, move the material down the riffle tray and capture gold just like a water sluice.

    The design and building of an effective bellows drywasher is not always straight forward. Multiple variables interact with each other in a dynamic way. To get a good result requires sound engineering and the patience to keep trying different combinations of layout and materials.

    Bellows drywashers are certainly interesting, fun to design, build and use. There is simply nothing like hearing the sound of "Pop", "Pop", "Pop" all the while hoping that big picker is laying behind the first riffle


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  2. #2
    Feb 2014
    11 times
    Been using a bellows (puffer) type drywasher for several years here in southern AZ. Have owned a Keene and now a Thompson - both great machines. Easy to understand the basic workings of a bellows drywasher - fluid air separating the more dense from the lighter materials, you have given some very interesting insights. The explanation of the mechanics and interactions of the bellows and fabric are fascinating. Gotta love it when the dirt is super dry, the feed rate from the hopper to the riffle tray is just right, and the rhythmic sound "thump", "thump", "thump" means you have hit the sweet spot and hopefully some gold for your efforts. Thanks for the deep dive, and remember to always wear a dust mask while drywashing.
    Rdg Sluicer likes this.



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