Portable screening plants are a major part of the business for aggregate producers, road builders and contractors in rock mining, quarrying, demolition and recycling operations. These screens separate all kinds of valuable materials from waste, so they can be recycled and repurposed for use in a range of applications.
Any operator can tell you how important quality screens are to a business, but what’s right for one operator may not fit the bill for the next. And while it seems the only thing separating one mobile screen from another is the colour and logo, there are a multitude of factors to sift through, from small, highly customised design modifications to the overall type and size. With new technologies and development of advanced screen media, operators need to do their research and take advantage of the ever-growing options and choices.
Selecting the right screen takes time, research and a clear idea of the operation’s goals. The first step is to consider whether the company’s production has been maxed out or will continue to grow. {{image2-a:l-w:640}}
SIZE IT UP
There’s nothing more important than sizing the equipment to match the operation. Understanding the application, materials and desired production is crucial. With plenty of research and consideration of desired production, size and abrasiveness of material, and the number of end size products, an operator can find well-outfitted screens that align with the operation’s goals.
Keep these goals in mind during the selection process. Calculate projected sales for a year and break the number down to tonnages per month. For example, if a company can sell 750,000 tonnes per year, its screens need to sort 62,500 tonnes per month. If the screen is in operation three days per week (about 13 days per month), eight hours per day, the operation will require a machine capable of screening about 600 tonnes per hour (tph).
This rate is key to the success of a business. In the previous example, a screen that processes 300tph limits profits and caps the company’s growth potential. On the other hand, a machine with a potential output of 900tph will probably come with extra expenses but no added value.
Another thing to keep in mind is the end product. Most machines are two-deck screens capable of sorting two sized products and an oversize product. Others feature three-deck screens that produce an additional sized product.
Also keep in mind that screen boxes vary in size and design. For example, a few screens/scalpers will be labelled “high energy”. What exactly does it mean, and what is the benefit? A high energy screen box runs faster and produces a higher stroke than a standard screen box. Many customers notice that the more efficient, lively stroke boosts a machine’s output and produces a cleaner product in comparison with a standard screen box. Be on the lookout for these standout features, and determine what is best for your organisation before looking any further into the details. {{image3-a:l-w:640}}
SSCALP OR SCREEN
There are two main types of screens: scalpers and standard screens. Several main differences separate a scalping plant from a standard screening plant. Standard screens have a tipping grid or livehead over the feed hopper to stop large material going into the hopper. The feeder belt speed can also be adjusted to help produce a clean, sized finished product. These screens are at home in sand and gravel pits, quarries and recycled concrete and asphalt jobs, as they often are considered “finishing screens”, because they are capable of producing specific sized end products.
For applications that aren’t all about the specific sizes, there is another option. A scalping screening plant feeds material directly onto a screen as it comes out of the hopper, which eliminates blockages due to oversized, contaminated and dirty material. The machine is ideal for demolition contractors pre-processing materials such as recycled concrete or reclamation applications. Scalping screening plants are also designed to handle much larger, heavier material in larger crushing operations, or for producing a gabion stone in a quarry. They are versatile but are not an ideal choice for creating a finished product – especially when the operator needs smaller, specced materials. In fact, scalping screening plants are commonly used to process scrap metals, separate recyclables at old slag dumps and extract rock from dirt on construction sites. Afterwards, operators pull in a standard screen to perform the meticulous work.
While these units have their differences, the style of screen isn’t the only factor one must consider. Plenty of other little factors can make a big difference.
HOPPER SIZE
Let’s start where the tough gets going – the hopper. This portion of a screen fluctuates in size and durability. The industry standard hopper is 3.6m wide, with an option to upgrade to 4.2m. Very few manufacturers offer a 4.2m wide hopper off the bat. But a wider hopper is more important than one might realise.
Obviously, the wider the hopper, the easier it is to feed the machine. Just an extra 0.6m can capture more product and
prevent spillage. The size becomes most pertinent when pairing the screen with the loading machine. For example, excavators or equipment with a narrow bucket are ideal for loading a 3.6m hopper, but a 4.2m hopper is wide enough to accommodate a wider bucket. By comparison, a wheel loader bucket can hold about twice as much as an excavator bucket. The extra 0.6m loading space makes a huge difference, so using a wheel loader is a simple way for a company to pick up extra production.
LIVEHEAD TIPPING GRID
Operators can add a livehead or tipping grid to a screen above the hopper. While they perform a similar duty, they are very different. A tipping grid is the perfect solution for preventing larger materials from travelling into the hopper and through the screen.
This hinged grid catches larger materials, and operators can clear it by tipping the grid via a remote control. Although this is an affordable option, it can become a chore, particularly in wet or dirty applications where the tipping grid may become plugged frequently.
The other option, a livehead, is essentially a vibrating screen that attaches to the hopper. This piece of equipment is ideal for heavy-duty, dirty, wet and sticky applications. The unit can be used for two purposes: to scalp dirty material off and eliminate the need for manual cleaning, or to size material going into the machine, so operators can produce an additional sized product.
Although these units are designed to boost production and create an additional product, they become a hindrance if used in the wrong application. They are built with thick bars that limit the open area, so operations processing finer materials might discover material on the ground that should have been in the hopper. Further, screens with 4.2m hoppers would not be used to the full potential, as the livehead measures 3.6m, leaving 0.6m of the hopper unusable.
APRON VS BELT
From the hopper, material is fed onto a standard belt feeder. The standard belt feeder is perfect for sand and gravel operations. It is cost-efficient and will hold up well in numerous applications. However, for operators working with metal, large rock or any abrasive material, a standard belt feeder is likely to tear or break. These more heavy-duty applications require an apron feeder, which is essentially a belt made of metal. Because of its durability, the apron feeder can handle nearly anything an operator throws at it, and that makes the entire machine more versatile. {{image4-a:l-w:640}}
SCREEN MEDIA
It seems there are just as many types of screen media as there are materials to screen. Most screens feature a typical wire mesh screen media for different sizing applications. However, there are a few heartier options for operators working with abrasive materials.
First, operations working with abrasive materials may want to consider stainless steel as an alternative to regular wire
mesh. Although the cost is nearly twice as much, stainless steel offers a higher wear resistance and longer wear life that is worth the extra cost.
Another replacement for the standard screen for certain scalping jobs is bofar bars. Bofar bars, formed of long bars with spaces between, are designed for materials such
as recycled concrete that commonly contain a lot of dirt and miscellaneous material. Although bofar bars do a certain amount of sizing, the process doesn’t necessarily end with thoroughly sorted product.
Punch plates are another option. These plates are essentially a piece of sheet metal with spaced holes for heavier applications. Punch plates are better for sizing in comparison with bofar bars, and are very durable, customisable and affordable.
Finally, finger decks are ideal for reclamation, landfill jobs or any other type of screening where clogging, blinding or breakage may be an issue. Finger decks can easily accommodate high impact loads mixed with recycled waste and debris.
STOCKPILING
It may look like “just” a pile of rock but stockpiling is an art. Essentially, this is what contractors work for – large piles of neatly sorted, valuable product that is ready for selling, using or building.
As you might imagine, these cone-shaped piles are formed of tonne after tonne of material. And the taller the cone, the wider the base and the more tonnage the entire pile holds. This additional tonnage with height is the reason why stockpiling height is so important. A stockpile consisting of a typical rock mixture might hold about 228 tonnes if the pile is 2.4m high with a top width of 0.6m and a base width of 4.8m.
By increasing the height of the pile by only 0.23m, the total weight of the stockpile will increase 29 per cent, to about 295 tonnes. Higher stockpiling capabilities serve as a time saver, as operators can run for longer periods without having to move material. This is why screen conveyors with the potential to stack even 0.2m to 0.25m higher can make a significant difference in an operation.
CONVEYORS
Conveyors take quite a beating. They commonly incorporate rollers, which are hot spots for damage and reduce wear life when large materials are being processed. Rollers commonly break under the pressure and belts become punctured.
As an alternative, some manufacturers offer an impact or sleigh bed conveyor. These conveyors provide more support and are more durable to handle heavy products such as large rock, metal or large pieces of wood. The impact bed is solid and runs the width
of the conveyor to best accommodate the impact of material and prevent belt damage.
Further, most discharge conveyors are about 1.2m wide. The industry offers belts up to 1.6m wide, which helps prevent clogging. Because the screen is wide and gets funnelled down to the conveyor, machines with smaller conveyors create a bottleneck effect. The material begins blinding in the corners, narrowing the opening. This creates productivity and maintenance problems, forcing operators to shut down and clean it out. Look for a machine that has less (or no) restriction as it transitions onto the discharge belt. A free-flowing machine with a low drop height is the best option to minimise wear and keep productivity elevated.
MACHINE DESIGN
Operators are guaranteed to get more bang for their buck when they choose a machine that is taller, bigger and more rugged. More robust machines often offer features that boost convenience and productivity, such as larger access areas. Double panelled engine compartments allow mechanics more space when performing service on the machine. Dual fuel nozzles provide fuelling access from both sides of the machine – a big advantage when working on a smaller site. Some manufacturers build units with access to the engine from both sides as well. This way, when the unit is parked alongside a wall, fuelling and maintenance can still be performed from the opposite side. {{image5-a:l-w:640}}
CUSTOMER SUPPORT
Customer support becomes pivotal when parts or repairs are needed. The availability of wear parts and service can make or break an operation’s numbers for any given month. Choose a manufacturer with a good reputation, not only for quality equipment and durable components, but also for exceptional customer service. Some companies offer the convenience of 24-hour online parts look-up and ordering with live service support that’s responsive and able to help customers keep their operations on track. Along those same lines, every manufacturer should be capable of providing a one-year, 1000-hour warranty. If not, let it serve as a warning.
COUNTDOWN TO PURCHASE
Purchasing the wrong screen can cause problems, slow production and result in revenue losses. Taking the time now to sift through the facts and separate the good from the bad will prevent headaches, downtime and loss of production later.
This article originally appeared in the February 2015 issue of Quarry Management (UK). It reappears in Quarry with kind permission.