Jigging Is Generally Described As A Multi-Phase Process That Optimizes Stratification Of Particles. Why On The Mohr Project Did Jigging Technology Prove Economically Successful?
By Jared Mohr, Andrew Snoby, Mark Bothwell
Apologies to our third-grade grammar teachers. This story began a long, long time ago, more than a decade ago. Mohr Sand Gravel & Construction LLC was founded by Jared and Tami Mohr in 2007 when they opened the Mohr Pit outside of Lake City, Iowa, selling a wide variety of aggregates from fill sand to native rock.
In 2013, they expanded and opened Mohr Concrete in Auburn, Iowa. Mohr Concrete services all commercial and residential needs for concrete. It is no secret the shale content of the aggregate in this area of Iowa is high, too high to routinely meet concrete specifications.
As early as 2010 Mohr was trying to reduce the shale content. Other operations with shale and chert contaminations were successfully using teeter bed separators. So, rising current technology was tried first. Unfortunately, it failed, and it was a costly failure.
Jigging technology was known to Mohr, and tests looked promising, but given the failure of wet, teeter-bed equipment, the risk wasn’t worth taking. Then in 2022 a new company was established called The Jig Rental Company. Suddenly, the risk wasn’t too great.
In 2023, Mohr rented a jig plant for two months, then four months, then the whole season. The task of changing sand with 3% shale into sand with 0.6% to 0.8% shale was possible, and it eliminated the need for a 40-mile truck haul.
Nice story. Right? Is this the right decision for everyone? Not likely.
Why on the Mohr project did jigging technology prove economically successful? The short answer is efficiency.
The next logical question is how is efficiency measured? Efficiency can be evaluated in a variety of ways including, actual yield at an achieved quality level verses theoretical yield at the selected quality level. For instance, if a grade vs. yield curve exists that shows a product with 1% lightweights can be achieved at 96% yield and the plants performance generates a 1% lightweights’ product at 90% yield, the efficiency could be calculated as 90/96 = 93.8%.
Expected results can also be calculated. Experienced process engineers often reference an Ecart probable error number or an imperfection number to distinguish efficiency differences among varying pieces of equipment.
Figure 1 shows a typical graph of an imperfection curve. The “Y” axis is the distribution factor, and the “X” axis is specific gravity. The curve shown is making a separation at 1.8 specific gravity. Basically, that means any particle of 1.8 specific gravity in the feed to the machine represented by this graph will have a 50:50 chance of reporting to the sink product or the float product. A particle that is 1.6 specific gravity has about a 15% chance of becoming part of the sink product and an 85% chance of finding its way to the float or lightweight product.
The steeper the slope of the curve (the more vertical) the less probability there is that a low-density particle will end up in a high-density product. So, does this mean that if the cost parameters are the same one should buy the more efficient machine? Well, yes!
Is there ever any justification not to buy the most efficient machine? Once again, the answer is yes.
If the feed to a gravity separation machine only has coal with a 1.2 specific gravity and gravel with a 2.6 specific gravity, the imperfection curve could be steep or shallow, and the resulting products would be identical. In the Figure 1 graph, 100% of the 2.6 specific gravity material will report to the high-density product without any of the 1.2 specific gravity material. Thus, even if there was a competing machine with a perfect separation, the quality of the aggregate product would be the same. In this instance the lowest cost equipment wins.
What is the secret? How can aggregate producers faced with low-density contaminants know what process is right for them? The cost and efficiency of aggregate separation machines vary widely.
One of the most important, if not the most important, parameter to consider is the characteristics of the feed. When the contaminants are comprised of particles with densities near 2.0 it forces the separation density to be higher. Referencing Figure 1 again, there would be relatively little removal of 2.0 particles if the separating gravity of the machine is 1.8. About 85% of the 2.0 specific gravity particles would remain in the high-density product.
A separating gravity above 2.0 is necessary. This condition illustrates the influence of near gravity material. Near gravity material is defined as that material found within plus or minus 0.1 specific gravity units from the actual separating gravity which is found at the 0.5 distribution factor point. If the feed contains a high amount of near gravity material, it is usually important to use the most efficient unit operation possible. In Figure 2 below, the separation is now at 2.10 specific gravity. Now, only about 25% of the 2.0 specific gravity particles report to the high-density product. In this application efficiency can be extremely valuable.
In Figure 3, machines with differing efficiencies are compared. Once again note, the more efficient, steeper curve will send approximately 25% of the 2.0 dense particles to the sink (high-density) product. Stated another way, 75% of the 2.0 specific gravity particles are rejected to a float (low-density) product. By comparison, roughly 38% of the 2.0 specific gravity particles will report to the high-density product with the less efficient machine. Additionally, the less efficient machine is only able to retain 92% of the high-quality 2.6 aggregate sink product. For the less efficient machine to retain more of the high-quality aggregate it must operate at a lower separating gravity, but to do so will result in higher amounts of contaminants to remain in the aggregate product. Efficiency is critical.
While not all jigs or teeter bed separators are equal, for the purposes of this article the jig used by Mohr is the more efficient machine compared to the less efficient teeter bed separator. So, what are the features that influence the efficiency of jigs and teeter bed separation equipment? For teeter bed separators, classical free settling and hindered settling are most important. Figure 4 presents a set of curves constructed by Hazen Research. Please note the range of sizes that are separated according to density is roughly 3 – 5 to 1. The greater the difference in densities, the greater the effective size ratio for density separations.
It is well established that the presence of particles in the flow path reduces the settling rate of a particle. In hindered settling, the range of sizes that can be separated by density is expanded slightly, but the concept remains the same, and the profile of the curves remains the same, only at lower velocities.
Jigging is generally described as a multi-phase process that optimizes stratification of particles by differential acceleration during the pulsion phase of the jigging stroke, followed by free falling stratification and hindered settling and finally consolidated trickling where the finest highest density particles work their way down through the now stratified bed of material.
Once the jigged bed of material is stratified, a bed level sensor detects the depth of a selected density strata and material with greater density is withdrawn from the stratified bed. Lower density material overflows a fixed weir. Jigging is typically effective with a particle size ratio of 10:1 or greater.
As might be suspected, there isn’t a clear winner between jigs and teeter bed separators. Both technologies have advantages and limitations. Selection of the machine OR circuit that will give the most cost-effective yield must consider not only the capital and operating cost of the machines and their respective separating efficiencies, but also the characteristics of the material to be separated, especially the amount of near gravity material.
Sometimes the least expensive machine is the best option. Sometimes it isn’t. Sometimes the most efficient machine is the best option. Sometimes it isn’t. Good luck.
Jared Mohr is with Mohr Sand Gravel & Construction; Andrew Snoby is vice president of operations for Snoby Separation Systems LLC.; and Mark Bothwell is a mining consultant at Central Service and Supply.