Crushing rock is an age-old idea. Look at the childhood favorite cartoon “The Flintstones” and you’ll see the idea began in prehistoric times. But the reality of the task is a far cry from that of Fred and his good buddy Barney.
Though crushing rock and other materials may seem simple, the truth is that it’s really a science. Finding the right solution to maximize productivity while minimizing maintenance requires more than a large dinosaur. Selecting the right equipment can make the difference between profitability and growth or frustration and financial woes.
When selecting a crusher, several critical factors must be considered, including output requirements, materials, applications and machine design.
The first and most fundamental aspect to consider is application and material. A variety of crushers exist, and each is tailored to meet the demands of very specific applications or materials. The most critical factor with material is the level of hardness. The hardness level of the material is typically measured in compressive strength, or the maximum force that material can withstand before breaking.
Hardness levels can range from that of concrete at 7,000 pounds per square inch (psi) to 50,000 psi for ores or other hard materials. The harder the material, the longer it may take to crush. But, in addition to production times, this hardness level should also influence the specific crusher selected for the operation.
Jaw crushers, which use a moveable jaw and a stationary plate designed to form a V, crush material using compressive force. These crushers offer an ideal solution for primary crusher applications involving hard, abrasive materials, such as granite, ores or recycled concrete. Efficient and less costly to operate because of their minimal moving parts, jaw crushers are ideally suited for applications where the primary objective is reducing raw feed material into a manageable size, usually down to about 6 inches.
While the units may be appealing in light of their lower operating costs, they are not well suited for applications that require smaller material sizing and are therefore used in conjunction with other crushers. Another critical application factor to note is the effect of contaminated material. Jaw crushers do not perform well with materials contaminated with clay, dirt or even metal debris.
Cone crushers, also known for their ability to crush hard, abrasive ores and rocks, break material by squeezing or compressing it between convex and concave shaped surfaces. Best suited for secondary crushing applications, cone crushers are built to crush presized materials, usually 4, 6 or 8 inches, depending on the size of the crusher. Cone crushers can form a finished product down to one-half-inch minus.
Like the jaw crusher, cone crushers provide a relatively low-cost crushing solution, but don’t be fooled by the lower operating costs. Certain application drawbacks may make another crushing solution more appealing. First, cone crushers cannot accept all material sizes. Any material fed to the crusher must be presized for that particular crusher, usually 8 inches or less. In addition, the cone crusher will not produce a consistent cubicle product, a requirement in many material specifications. Finally, these units, like the jaw crushers, are not suitable for handling clay and metal mixed in with the concrete, rock or minerals.
Designed to fill the gap left in the market by cone and jaw crushers, horizontal impact crushers are ideally suited for contaminated materials, easily handling clay, dirt and metal mixed in with the material to crush.
The units are designed to process metals such as rebar, wire mesh and dowel pins. Horizontal shaft impactors (HSIs) produce a uniform cubicle product and can tackle high reduction ratios, allowing them to accept all infeed materials. This helps HSIs to provide maximum productivity in both primary and secondary applications. While HSI units can handle hard material, they are ideally suited for soft to medium rock.
HSI crushers consist of a rotor attached to a horizontal rotating shaft. (See diagram at left.) The shaft is placed inside a chamber, which is lined with replaceable liner plates, anvils and blow bars. The material is fed into the top of the chamber where it impacts the rotor. The rotor then throws the material outward against the curtain anvils. Repeated impact against the curtain anvil by the blow bars reduces the material until the desired size is produced. Material sizing is achieved by rotor rotational speeds and adjustments of the clearance between the blow bars and curtains.
The nature of HSI crushing can lead to slightly higher operating costs, the cost incurred for greater versatility and uniform product sizing.
Machine Size and Output
Determining the desired tons per hour (tph) for a plant also is essential in the selection process. Companies should be able to calculate what their projected sales are for a year and break the number down to the desired monthly tonnages. From there, calculating the number of days per week the machine will be in operation will determine tonnage outputs.
|Abrasion-resistant wear bars allow crushed material to build up and act as a wear liner.|
For example, if a company can sell 500,000 tons per year, its crusher need to produce just short of 42,000 tons each month. If the crusher is set up to run three days per week (approximately 13 days per month) for eight hours per day, the operation will require a machine capable of processing 400 tph.
Proper production capacities are critical to the success of a business. A crusher that is too small fails to produce the desired tonnages, limiting yields and profits and capping the growth potential of the organization. A machine that’s too large carries extra expense with no added value—especially considering crushers work best when choke fed and under pressure.
Each crusher’s maximum tph output is determined when the machine is at or close to full capacity during its peak performance. Failing to maintain a full load results in improper wear and inefficient crusher operation. Using a machine capable of processing 450 tph to perform at a much lower rate, such as 50 tph, will result in visible signs of irregular wear patterns, leading to costly repairs to the crusher. That being said, it is important to review the business plans, calculate short-term growth initiatives and buy a machine sized to allow for that growth while still maximizing efficiencies.
While tons per hour are critical to calculate, another production factor that significantly impacts the crusher selection process is the number of end product specifications required. Is the operation producing and selling just one size of product, or are two, three or even four different specs required?
The number of screens available on a crusher unit will dictate the number of different sized products a machine can produce in one pass.
Closed-circuit machines equipped with a two-deck screen offer the ability to produce up to three different product sizes. The top deck might be screening materials of 2-inch minus; the bottom, 1-inch minus and the oversized material can either be fed back to the hopper for additional crushing or, with the installation of flop gates, can be kept for a third product for sale. Similarly, three-deck plants feature three screens and are required when production specs require four different size classifications.
To Move or not to Move
Some operations do not require mobility. Since the projected life of the quarry will be decades, stationary crushers are put in place offering a custom, heavy-duty design. However, most contractors require highly mobile machines that can be dismantled and moved from one site or job to another.
Mobile crushers allow operators to move the crusher unit closer to the face of the crushing activity. This effectively reduces the unnecessary expense and hassle that can often be associated with loading and hauling large materials across a jobsite.
The author is national sales manager of Irock Crushers, Oakwood Village, Ohio. More information is available at www.irockcrushers.com.