When reading a recent article, I was struck by the adage “When the diagnosis is made, the thinking stops.” The article further states, “Such decision-making bias can have devastating consequences.”
Cause and effect are often singular in wash plant diagnosis, but the root cause may be farther upstream than thought. Jumping to a conclusion can lead to lost time and incorrect/delayed diagnosis.
Since many issues can affect production quality and quantity in sand plants that are still being commissioned, I’m going to focus on issues surrounding plants that have been up and running for a while.
A McLanahan Frac Sand Plant.
With modular, standardized-type plants, the system may be rated at 250 TPH, but it is limited by a maximum content of certain parts of the gradation, typically fines at max 15% passing 200 mesh. As deposits get lower in quality, it is not uncommon to see a 30% washout. However, the standard plant may only be able to handle 150 TPH at that level of contamination.
Custom plants would typically be designed to handle the level of contaminants expected, but deposits are inherently variable, so the highs may require a reduction in feed rate or higher levels of blending of coarser sand.
Understanding what's wrong with the product
Much depends on the feed but also the type of process being used to create the product(s). It is also important to understand what is wrong with the product.
- Are there too many fines or too much coarse material?
- Is there not enough mid-sized material or too much mid-sized material?
- Is the product failing at the plant or at the customer’s yard?
Materials handling can also affect the result. Softer materials that were in spec after being washed could suffer increased breakage due to transfer points and result in a failed gradation.
The first and most important thing to ask is, “What has changed?”
- What has changed since yesterday?
- What has changed since an hour ago?
Here are five quick things to check:
In our experience, the No. 1 thing that knocks a product out of spec is the feed material. This is the first thing to check if you are out of spec. Consider the following factors:
- Are you in a new mining area?
- Has strata in the deposit changed?
- Has the feed rate increased?
- Is the feed blending incorrect?
A quick belt-cut sample and sieve analysis can answer these and a lot of other questions.
Some sand plants are designed to simply wash out excess 200 mesh, so they will not address excess or lack of other fractions.
In more sophisticated plants with the ability to re-blend a coarse and fine stream, getting the material back in spec could be as simple as moving the blend gate.
In the most sophisticated plants, such as those using the recipe or fractionating principle, the automatic blend program will typically address these issues, adjusting on-line.
But nothing will make up for a feed lacking those fractions.
A McLanahan Recipe Sand Plant.
Besides the feed material, the following process and mechanical reasons can also knock the product out of spec.
2. Feed preparation
No amount of washing will work if you do not first liberate the feed materials into their discrete components.
As would be the case with any mineral process, liberation is key. If you do not liberate the gold, you won’t recover it. If you do not liberate the clays up front in an aggregate plant, they will dog you downstream, contaminating every part of the process.
There are multiple types of feed preparation equipment, but not all of them are designed for plastic clays should you encounter them. This part of the diagnostic process is heavily related to point No. 1 above — this is often a feed issue.
Worn or failed components, such as paddles, are the most obvious culprits here. Check the power draw on the Attrition Cell motors. If power draw is down, then investigate internally. Also, ensure the water volume pressure is correct.
Confirm the water Pumps are delivering the right pressure and volume of process water to the plant.
If your sand is out of spec, check that the Pumps are operating correctly.
Confirm the water source level in the ponds. Lower levels can lead to lower output with floating pier- or pontoon-type Pump mounting.
Confirm the inlet pressure at the Cyclones. If Pumps are worn and not delivering the required volume, dilution water will be compromised. Cyclones and Separators™ will change in performance, allowing valuable coarser particles to report to the overflow stream or fines misreporting with the coarse
In addition, the Sump may be overflowing at the weir and coarse fractions lost to waste.
Was any pipework changed? For example, changing from 10” Steel to 10” DR 11 HDPE (high-density polyethylene) can make significant differences to friction loss and lower delivered volume.
In the case of a Classifying Tank, which is typically used to address a mid-range belly often in the 30 mesh x 50 mesh fraction, an out-of-spec product could be due to:
- A stuck or sticky discharge valve(s)/solenoid
- A lack of rising current water
- A lower or higher volume of slurry feed — dredge plants often suffer this condition
- Programming that has been compromised perhaps by a lightning strike or short circuit
Occasionally with changing feed, the overflow weirs need to be adjusted to increase/decrease velocity.
Cyclones and Separators™ may have damaged rubber internally, which can severely compromise their performance. Check the apex diameter against original supply. The larger the diameter, the more dilute the underflow typically is — dragging unwanted fines with the water — and you may have problems controlling the Separator™ with the siphon valve.
Check the condition of the underflow regulator and the siphon boot, as both affect the control and performance. Replace as necessary.
Check product moisture. Increased moisture in the final product can bring undesirable fines.
Is the Screw being starved of wash-back or rising current water due to a malfunctioning/worn water Pump?
A Dewatering Screen may be malfunctioning if a vibratory motor goes out or if it has end-of-life springs/buffers. A circular or irregular motion on the screen will be evident in this case.
Check the Dewatering Screen media for holes, which create a higher recirculating load and lower efficiency.
Dewatering Screen media should be free of holes that can lower the screen's overall efficiency.
Bonus - water quality
Check your tailings management — muddy water doesn’t do a great job of washing sand and aggregates.
Are you actually getting good quality water back from your ponds or your Thickener? We know that excess mud can overwhelm effluent processing, including settling ponds, and it can come and go when a Pump returns mud from a malfunctioning filtration process.
Preparation is key
Louis Pasteur’s quote “Chance favors the prepared mind” has excellent synergy here. Good maintenance practices, OEM training and a thorough knowledge of the operational manual can help prepare for some, but not all, of the eventualities. Never be shy of contacting the OEM.