Due to its name, people outside of the industry may think this is some sort of device that washes rocks, dirt and mud from all of the equipment a contractor or large-scale aggregate producer owns after a job. What are we talking about when we say “wash plant” in an aggregate-based production facility? What are some of the key features of a wash plant and what specific function does it provide? What industries can be supported by owning and operating a wash plant?
A wash plant is one of the best moneymakers in an aggregate producer’s arsenal and like all other items needs to be maintained to perfection on a daily basis in order to supply a steady stream of accurately sized and clean aggregate to the concrete batch plant.
Over the years, wash plants have evolved to provide cleaner product more efficiently in order to make concrete one of the most important products delivered to today’s construction projects. Whether it be road building, bridge building or even giant-scaled projects like dam building, all would be impossible without wash plants to wash the aggregate materials after they are extracted from raw earth. This process has so improved that today we are manufacturing much larger concrete-formed parts many miles away from the jobsite, using semi-trucks and rail cars for delivery. Many modern large scale construction builds require multiple locations and producers working together to fulfill production schedules.
Today, most wash plants start with a consistent material feeding device such as a hopper belt feeder. The feed material is completely mixed and is often dirty, with excessive fines and clay-bound crushed rock. The belt feeder sends it to the blade mill where it gets its first opportunity of dilution and breaking apart using a giant scaled mixer and the addition of water, which is often recycled many times for conservation reasons and usage costs. The aggregate is now getting roughed up a bit and is pre-scrubbed and moistened to start the breakdown effect.
Next it sees a (typically three-deck) shaker screen — horizontal or inclined depending upon factors of retention time needed based upon the cleanliness of the feed material. Horizontal screens provide longer and better shake times while adding more water to clean uglier material, while inclined screens using gravity tend to pass faster, cleaner material using less energy since the material doesn’t always require longer wash time. Either system is great and effective, but both have benefits and pricing differences that help decide which one to utilize. Sometimes portability plays into the type of screen one chooses. After screening, the larger-sized material may need to run through a coarse material washer for a final rinse and scrub to satisfy the material code/design specifications required (or just because some material is a bit dirtier and more clay-bound than others).
From the coarse material washer, the cleaned product travels up the conveyor to huge stockpiles to be loaded into the individual hoppers of the concrete batch plant where it is carefully weighed out and remixed using the pre-determined recipe of the concrete project requirements.
As with any equipment, there are multiple styles and sizes of wash plants offering a variety of productivity-specific features from basic controls to computer driven optimization and certification programming assistance.
The other critical choice is whether the wash plant should be a permanent or portable installation. Most permanent installations are near large-market urban areas where the producer has many years of virgin aggregate base material and anticipates many years of providing finished product. These permanent versions also require a permanent water reclamation system versus digging and preparing appropriately lined settling ponds, which are required for some operations (check your state and local regulations regarding water usage/recycling). Portable wash plants can be either small or large-scale based upon the equipment specified, but are generally designed as being down the center average. These portable plants are typically used in more rural locations where the demand isn’t nearly as consistent. Their productivity can still be as large as 1,000tph or as little as 50tph, depending upon screen box size and fine material washer size (AKA sand screw).
The sand screw is one of the last pieces of the puzzle. It is very similar to the coarse material washer or the blade mill, having a spiraling shaft or shafts that screw or convey material along while scrubbing. In the sand screw’s case, introducing non-turbulent water from the bottom through an aeration grate allows water to rise from below with current (of sorts) and actually floats the ultra-fines away while wet and somewhat controllable.
There are actually quite a few more pieces I have not mentioned — some of the earlier designs and some very recently designed — which can also be extremely important to wash plants choices. However, they are either not always required or require much more space and time than I have in this article. Maybe we will do a more in depth descriptive and maintenance write up of some of these left out pieces in article subjects to come throughout 2019.
Some of the other industries being supported by a typical wash plant are the frack sand producers within the oil and gas industry, gold mining exploration (when and where water is available) and some decorative rock aggregate production.
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Tim Holmberg / 2915 Idea Ave. / Aberdeen, SD 57401