- Created: Monday, 01 September 2008 08:00
- Published: Monday, 01 September 2008 08:00
To increase throughput and maintain profit margins, quarry managers should focus on the entire production process, according to industry experts interviewed by Rock Products. Efficiency starts with the quality of feed material but extends to screening, washing and crushing as processes with which minimal expense can yield maximum profitability.
FEED MATERIAL SETS THE STAGE
Maintaining proper material size and shape at the blasting stage takes a great deal of stress off of equipment and processes further downstream, remarks Troy Kutz, vice president at Rockford Sand & Gravel, Rockford, Ill. This task is so critical, in fact, that the company outsourced it to Miller's Blasting Service, Chana, Ill., six years ago. ìThe feed material makes or breaks your entire operation,î notes Kutz. ìThe cheapest place to break your rock is at the shot.î
Kevin Empen, president of Preston Ready Mix & Quarry, Preston, Iowa, says outsourcing the blasting and drilling functions also has been the right approach for his company. Feed material with a break pattern that takes stress off of Preston's impact crushers ó and produces minimal fines ó reduces the company's operating costs in the long run, he says.
ìBlasting has changed so much in the past 10 years,î he says, referring to technology such as programmable, electronic, blast-initiation systems that allow blasters to precisely control the feed material break pattern. ìYou get better breakage and the cost [of processing the aggregate] actually decreases, but it doesn't actually look like that at the beginning. Quarry people get into the niche of crushing a certain way. They figure, it works this way; why change?î
Along with blasting, Preston also outsourced drilling to a local contractor. The result is that ìwe reduce the fines, but we get good breakage,î Empen says. ìThe rock is fractured from the blasting ó it doesn't look like it is, but it is.î
MAXIMIZING THROUGHPUT, MINIMIZING FINES
The fracture pattern of the feed material allows Preston to feed its impact crushers with larger feed material, yet the material breaks more easily, yielding increased throughput.
ìWe oversize the impact motor and actually slow it down, so we burn less fuel,î Empen adds. ìYou don't want to push a crusher right to the limit. If you oversize, it doesn't have to work so hard and you get a better product.î
Assuming that the feed material has the most brittle break pattern possible, the experts add that running crushers as slowly as possible, yet keeping them full, provides both high efficiency and high crushing profitability. Kutz says that his company mostly uses impact crushers to maintain consistency in the limestone that it crushes. ìFor us, trying to run them as slowly as possible helps reduce the amount of fines we get while still maintaining the throughput,î he says.
To Mark Krause, process optimization specialist with Lafarge North America, increasing crushing efficiency might necessitate a shorter-term view of the operational life of crushers, although some adjustments can be made to extend their lives and maximize their throughput.
ìWe're asking a lot more out of our equipment,î he says. ìWe're asking it to be more efficient, and we're asking smaller packages to do more. Do we keep in mind that when we do that, we're more than likely shortening the life expectancy of that equipment? For instance, we probably expect a cone crusher to last 25 to 40 years. Today's new cone crushers are probably 10- to 15-year machines. There's nothing wrong with that ó it's just a different mind- set.î Krause says he is spending less up front because he is buying a package that's more efficient, but it might not last as long as the old one.
MAINTENANCE A KEY
ìThe other thing about crushers is, whether it's a primary or secondary impactor, make sure you're using modern technology for wear parts as well,î Krause continues. ìThere's been a lot of work done in the past few years on ceramic and carbide-embedded wear parts. The longer you can let those parts run, the more efficiently that crusher is going to run.î
Increasing the speed or replacing the liner are other simple operational changes. Ensuring that the crusher's surge point remains full also costs less than purchasing a new crusher. ìIf you're going to get the most out of any tool, you need to keep it full,î says Krause. ìSpend the money on the surge point and see if you get your desired outcome that way; if you don't, you can worry about buying a new crusher. But you've already spent the money that you're going to have to spend on a new surge point anyway.î
Krause says that adding a surge bin is a low-cost way to keep the surge point full. ìThey're not big bins ó maybe 40 to 50 tons. Some operators might use a 20- to 25-ton bin and a belt or vibratory feeder.î The entire cost is probably around $50,000, he estimates. ìSometimes you'll find that you can't put the bin above the crusher, so you need to locate it a conveyor length away.î
Empen and Preston Ready Mix & Quarry installed a surge bin equipped with a metal detector and a belt scale between the company's primary and secondary crushers. The detector is set for a 1-inch particle size; if something that large is detected, the operator shuts down the belt and manually removes the particle. ìWe run a lot of belt scales just to see what we can adjust in the plant according to the belt scale,î says Empen. ìThe surge bin that feeds our secondary cone plant is very important because we can keep that cone at full production all the time.î
Empen made a large capital investment in an impact crusher to replace a jaw crusher for primary reduction. ìThe big thing about oversizing the crusher is if you oversize a little bit, your wear costs go down,î he says. ìWe went from a jaw to an impact for primary and that was just so we could adjust our particle size coming out a lot better. But you've got to keep your wear costs down if you can. Some people never look at the long term.î
Perhaps the best option for reducing wear is to optimize the utilization of a cone crusher for secondary reduction. ìA cone is, by far, the cheapest thing to run for wear costs because you're using a lot of rock on rock.î
Mark White, regional sales applications engineer for Paschal Associate Sales, an Asheboro, N.C.-based equipment dealer and engineering solutions provider, says that, assuming the feed materials are suitable for a primary impact crusher, adjustments can be made to reduce both wear and the amount of fines. Because impact crushers usually have a high reduction ratio to begin with, White recommends running them at a ìmoderately slow speed,î as long as there is no adverse effect on the inertia of the rotor or impeller table. Also, like cone crushers, an impactor will be more efficient if it is fed full, he adds. This results in a better wear profile on the casting.
White also stresses the importance of choke-feeding a cone crusher. This is particularly important if excessive ring bounce is a concern, he says. The challenge is segregation of the feed material into the cone cavity with the coarse feed going to one side and fines to the other. This can be a real challenge if tight crusher settings are required; so he recommends using a rotating feed distributor or other device to minimize or eliminate segregation. While less wear and tear on the crusher is the goal, he adds, better product consistency also can result.
SCREENING: A MAJOR BOTTLENECK
A major production bottleneck at most plants is the screening process. Using newer screen media and using more weight are two ways to eliminate the biggest cause of this bottleneck: blinding and plugging of screen media.
White stresses that it's important to ensure even distribution of material onto the entire area of screens. Also, a dynamic speed and stroke combination should be used, if possible, to generate significant G-forces. He notes that a huge improvement in screening during the past 10 years has been the evolution of ìhigh open areaî modular synthetic screens that reach up to 40% to 50% open area in some designs. Also, new designs of anti-blinding synthetic screens and stainless-steel live wire also have reduced or eliminated blinding.
Improving screening efficiency might make the biggest impact on plant throughput in most operations, Krause indicates. ìThere are a lot of screens that were installed 10 years ago for certain applications; the application has changed, but they haven't gone in and changed the mechanical setup of the machine,î he explains. ìThey're still using the same speed, same stroke, same everything.î
Changing speed is easy, but adjusting the stroke is more difficult. ìWe have to be cautious that we don't go outside the bounds of what the manufacturer suggests the screen can do. You can't have high speed and high stroke together or you might end up destroying the screen.î Newer machines make it easier to add weights, allowing producers to adjust to different material often.
Krause says the media has changed dramatically in the world of screening. ìThere are all types of live wire screen deck today that incorporate rubber and wire or urethane and wire in combination on a deck where, traditionally, we once used wire cloth. Then rubber decks and urethane decks came along: rubber for dry applications, urethane for wet applications,î he notes.
ìWe adjust our weights all the time,î says Preston Ready Mix & Quarry's Empen, adding that the company also routinely adjusts its impact and cone crusher settings as well as the feed rate, depending on the feed material. ìYou always have to match up the impact with the cone so it's the right size for what you want to make. A lot of people run the impactor at a certain size and never change it. You have to match everything up and adjust it so you get the most production out of it without producing a lot of fines. Get everything adjusted, get your feed rate adjusted, get your cone adjusted. Once you do that, you get the most production at your lowest cost.î
WASHING PRESSURE OVER QUANTITY
Advancements also have been made that help reduce the amount of water that quarries have to use in their washing processes. A high quantity of water may not slow down production, but it may create more environmental concerns and higher treatment costs.
To a large extent, the amount of water needed does depend on the tons per hour and the percentage of silt (minus-50 mesh particles) in the material, White points out. So, again, the quality of feed material is critical. A common rule of thumb, White adds, is 3 to 5 gallons per ton, leaning towards the high end if there is a lot of silt. An important part of the washing process is proper pre-wetting of dry feed to a rinse screen, White adds. About 25% to 30% of the total rinse water should be flooded into a header box with the dry feed, with the remainder of the water distributed through the spray bars. Equipment for water and fines recovery includes separators, de-watering screens, ultrafines recovery systems, clarifiers and belt presses.
ìWashing has been so misunderstood,î Krause argues. ìWe're inefficient for two reasons. One is the concept that more water is more efficient. Then, it's a lack of understanding that there is new technology ó for example, dewatering screens, different pumps, different additives such as flocculants that can greatly reduce the cost of the washing operation.î
To Krause, high water use in washing creates two types of costs. ìWhatever water you put in, you've got to take out and treat,î he says. ìSecondly, we see a lot of waste because water tends to carry particles with it and particles that you wish to retain ó your sand ó are now going somewhere you don't want them to go. There's a cost to going in there and reclaiming that. If that sits in the bottom of the pond, at some point, you've got to clean out your pond.î
Krause sees a big increase in the popularity of dewatering screens today. ìA standard screen will probably put out about 20% liquid to the stockpile. Dewatering screens will lower that percentage of water going out to the pile to about 10%.î
Empen agrees that less is more in regard to washing. ìIn the past, the focus was on volume,î he says. ìToday, the focus is on getting the pressure way up. Instead of just throwing water at something to get it cleaner, let's look at spraying it to get higher pressure.î Increasing pump pressure can blast ultrafines off coarse material while maintaining particle shape, in contrast to log washers that can grind down coarse aggregate particles, Empen adds.
For their ponds, Preston Ready Mix & Quarry also has been looking at portable clarifiers that separate solids from water and allow water recycling. That would go a long way toward reducing the costs of treating process water.
FUEL SAVINGS AND OTHER SUGGESTIONS
The previously listed equipment and processes represent just some of the areas that managers should focus on for increasing throughput and efficiency.
One factor that should not be overlooked is the cost of fuel for loaders and generators. ìFuel has been a big concern with us for years,î says Empen. ìA lot of people don't watch their fuel; we watched it for a long time. There's a lot of technology out there that can save you money, but you've got to research it a little.î In recent years, Preston has addressed fuel prices by purchasing fuel-efficient equipment such as loaders and generators. More recently, the company has begun running its loaders on biodiesel, which partially replaces diesel with oil from soybeans and animal fats. ìWe've found that for every 20% of biodiesel in diesel, we save 5% on loader fuel.î He reports that both of the company's loaders consume about 8 gallons of biodiesel an hour, compared with 8.6 gallons if they ran on straight diesel. That translates to a savings of about $10 a day, which adds up.
ìThe price of fuel has dramatically changed the cost equation for most operations,î Krause says, adding that additives that increase fuel economy should merit strong consideration. ìRight now, everybody should be looking for ways to minimize their fuel costs ó motors, trucks, diesel engines, diesel gensets if they're not on line power.î
Empen says that another way to save on fuel cost is to use portable radial stackers for stockpiling, instead of trucks. ìYou can do it so much cheaper than with trucks and you don't get a lot of segregation,î he explains. ìAlso, going with synthetic oil at your plant cuts down on your downtime, your temperature and your fuel usage. It doesn't look like a lot at the beginning, but it adds up at the end.î
Don Talend of Write Results, West Dundee, Ill., is a communications and publicity consultant specializing in the trade media.