Down To Size

Chris Smith (CS): For crushing concrete into base material, there is a pretty strict spec we like to follow to maintain a quality product for our customers and for internal company use. The use of primary jaw crushers and secondary cones help ensure that spec. Also, primary screeners and product screening plants will be used. There is [also] a demand for crushed asphalt, but it is not normally made into a course base. But if it were needed, impactors will crush asphalt to a better gradation compared to a cone crusher. It’s not that the cone will not crush it, but with asphalt, impactors give us the production scale we prefer.

CDR: What types of additions or modifications are needed to meet stricter requirements to produce a recycled aggregate to be used in a paved surface mix?

CS: With the larger size crusher like we operate for recycling we can get any gradation with just a few minor adjustments just like any quarry operation. To produce the types of stone for hot mix designs, keep the plant running at full efficiency to ensure the full ability of crushers. Next, adjust the primary crusher for desired size. From there, move onto the secondary system, which would contain the cone crusher, which would have to be set pretty precisely depending on type of gradation needed. Then we would move on to the product screener, which contains certain screens sizes for each type of stone. On this product screener it contains two decks with high frequency motors to fully screen the product with no carry over of any other smaller unwanted materials. Dykes Paving has crushed 67 stone and 89 stone that passed gradation for the state Department of Transportation (DOT). This stone was crushed for our asphalt hot mix designs and cold mix designs.

CDR: Are there certain equipment considerations unique to crushing asphalt?

CS: When getting into asphalt recycling, the majority of the end product will be a finer gradation, and this would be for asphalt plants—RAP (recycled asphalt pavement) for reapplying into hot mix asphalt. Crushing to a smaller gradation means more wear on the impact crusher, which will have a higher production total compared to a cone when crushing down asphalt. When running an impact plant it would be set up to the type of end product required. For finer gradations like ¾-inch minus, in my opinion the plant would need to be set up with a single screen box of some sort with a closed-circuit setup. That would mean all the over-size material coming off the screen would return into the crusher until it was crushed down to the desired size. The important thing with running these closed-circuit systems is that the impactor must stay in a tight adjustment so the material doesn’t just re-circulate through the plant, minimizing production. Even though impactors have a higher production scale crushing asphalt, they also show a lot of wear, especially when producing a finer gradation of material. They require tremendous amounts of adjusting to maintain spec and they also require maintenance, including replacing wear liners and apron liners. The blow bars also need to be maintained and flipped or replaced after so many tons. This can be three times a month—changing of turning blow bars that are almost $400 depending on the type of bar used. Ceramic blow bars are ideal for asphalt crushing but not the best for concrete.

CDR: Are impact crushers better suited to making any certain kinds of end products? What about jaws and cones?

CS: The impacts can produce a tight spec material, but they require daily inspection because of how fast the impact wear parts can wear down. Before the end of a shift of crushing a product that requires a tight spec, you will have adjusted the impactor two, maybe three times. However, the impactor can produce many types of products, but it’s the upkeep that makes them the challenge and the costs are twice the amount (in a five to 10 year outlook) of running a cone with a primary jaw. The operating cost of the jaw and cone is 20 to 30 percent (if not more) the cost of an impact plant—that’s why they can be more appealing.

When it comes to running a quality product that meets gradations needed by DOT or other customers the jaw/cone plants maintain that with less cost in a 10-year span compared to the impact. The other thing with the jaw/cone setup is that maintenance is much easier and replacing liners only comes once every eight months or so running almost 1.5 million tons of concrete. The cost of a new portable cone crusher like the one Dykes Paving operates would cost $400,000 but when it comes to wear, changing plant settings for different types of products and maintaining a setting for a certain products, the jaw and cone are the best combination for better concrete products and a very high production scale.

CDR: For more tightly specified products, is it more efficient to run material through the same machine multiple times or to go through a series of crushers to obtain the desired size/spec?

CS: As previously mentioned, it can be efficient to have a closed-circuit crushing plant configuration if the material makes no more than two passes through the crusher, or if there is so much material re-circulating that the operator can only put a little raw material in the crusher at a time to keep the plant from overloading. The configuration of the concrete recycling plants operated by Dykes Paving are set up several different ways in which the plant has material introduced to the cone crusher a few times but is only crushed through the jaw one time, which is the desired size for the cone. It is not inefficient to let material go through the crusher a few times to get a certain size aggregate, but it is not productive putting very little raw material at a time because the plant is full of material re-circulating through the crusher, so maintaining the adjustments on the plant will give the ability to keep a constant feed of raw material and re-crush material that needs to be sized down more for spec.

CDR: Do the requirements for additional screening and conveying equipment differ depending on the end product?

CS: It definitely comes into consideration when different types of material are being produced. Screen boxes and screening plants are now designed to perform different functions such as screening out three products. When you get into closed-circuit plant configurations you have to convey the over-size from the screen back to the cone feed. For the most part it is pretty standard to have a product conveyor that stacks material coming out of the screen. Some of the jaw plants will need a conveyor for dirt that is pulled from the raw material through the grizzly bars. This is done before the raw material goes through any part of the crushing plant. In some cases there is only one screen box that pulls the product and sends the over-size back into the cone crusher, but at Dykes Paving we use the primary, which pulls fines out of the material before it enters the cone. This helps in a big way to keep the cone full of material that needs to be crushed and not a lot of fines. Then it goes into the secondary screening plant, which really ensures a clean in spec material. This step was done after the plant was set up because of the need to remove almost all fines from out of the course stones, but it is just an example of how as a operation matures, you learn that some things will cost a little more money, but it will make a product that customers can count on to be in spec and of the best quality.

CDR: At what point does an operator assess whether any dust control or noise control modifications will be needed?

CS: Dust control is very important issue and must always be addressed to stay in compliance with local and state officials as well as to be a good neighbor to the surrounding properties and companies. It’s not a hard thing to install or maintain, but it means a lot to the environment and the employees that are exposed to the day-in and day-out operation of crushing. Simple means of dust control include a water hose with an adjustable nozzle set to lightly spray down into or the discharge of the crushing plants. Then you could get into full spray bars that will lightly spray the whole width of the conveyor belt. More water applied into the products before going into stockpiles will help eliminate dust kicking up in the wind and when loading out. Sound exposure is determined by what is creating the sound. The plants are powered by 3412 Cat diesel engines that are enclosed inside box trailers. These areas are not allowed to be entered without sound protection, but this is the only area that sound protection is required—the rest of the facility is not exposed to any very loud decibels.

CDR: What factors can limit a crushing plant’s rated production tonnage? How should this figure into a recycler’s decision when purchasing a plant?

CS: There are many factors to take into consideration when shopping for a crushing plant. When Dykes Paving went into crushing, we went in with production as one of our main goals and the ability to crush spec material with a certain production scale to be met. These plants did get our price tags up into the millions, but in the long run it meets the demand of aggregate and has been a lot more efficient to crush 5,000 tons a day than get a cheaper crusher that might not be as durable to the amounts needed and only get maybe 1,000 tons. The things to shop for would be types of concrete that will be crushed, demo or slabs and there are other type of concrete that can be taken in at a site and processed. Once that is determined [next comes] your market value of the crushed stone and what kind of demand will have to be kept up with. When these things are established the design of the plant begins. This process will be the configuration of the plant such as portable plant or stationary, but it all goes back to what type of operation the plant will be set up in. When you start the design of the plant, you will need to determine what products will be made and what kind of gradation will it be based off of. These plants are not complex, but they become complex when you start adding elements and additional functions. When chosing a hydraulic stabilizer over a manual one, the cost might be a fair amount more but it all depends if the crusher will be mobile or will sit in one spot for a while. Many factors play into these plants, and each one is different because of location, process and end use of the concrete. In crushing it’s not always "you pay for what you get."