Sand and Gravel Processing

Sand and Gravel Processing

Deposits of sand and gravel, the unconsolidated granular materials resulting from the natural disintegration of rock or stone, are generally found in near-surface alluvial deposits and in subterranean and subaqueous beds. Sand and gravel are siliceous and calcareous products of the weathering of rocks and unconsolidated or poorly consolidated materials. Such deposits are common throughout the country.

Construction Sand and Gravel

Sand and gravel typically are mined in a moist or wet condition by open pit excavation or by dredging. 

Open pit excavation is carried out with power shovels, draglines, front end loaders, and bucket wheel excavators. In rare situations, light charge blasting is done to loosen the deposit. Mining by dredging involves mounting the equipment on boats or barges and removing the sand and gravel from the bottom of the body of water by suction or bucket-type dredges. After mining, the materials are transported to the processing plant by suction pump, earth mover, barge, truck, belt conveyors, or other means.

Although significant amounts of sand and gravel are used for fill, bedding, subbase, and basecourse without processing, most domestic sand and gravel are processed prior to use. The processing of sand and gravel for a specific market involves the use of different combinations of washers, screens, and classifiers to segregate particle sizes; crushers to reduce oversized material; and storage and loading facilities.

After being transported to the processing plant, the wet sand and gravel raw feed is stockpiled or emptied directly into a hopper, which typically is covered with a "grizzly" of parallel bars to screen out large cobbles and boulders. From the hopper, the material is transported to fixed or vibrating scalping screens by gravity, belt conveyors, hydraulic pump, or bucket elevators. The scalping screens separate the oversize material from the smaller, marketable sizes. Oversize material may be used for erosion control, reclamation, or other uses, or it may be directed to a crusher for size reduction, to produce crushed aggregate, or to produce manufactured sands. Crushing generally is carried out in one or two stages, although three-stage crushing may also be performed. Following crushing, the material is returned to the screening operation for sizing.

The material that passes through the scalping screen is fed into a battery of sizing screens, which generally consists of either horizontal or sloped, and either single or multideck, vibrating
screens. Rotating trommel screens with water sprays are also used to process and wash wet sand and gravel. Screening separates the sand and gravel into different size ranges. Water is sprayed onto the material throughout the screening process. After screening, the sized gravel is transported to stockpiles, storage bins, or, in some cases, to crushers by belt conveyors, bucket elevators, or screw conveyors.

The sand is freed from clay and organic impurities by log washers or rotary scrubbers. After scrubbing, the sand typically is sized by water classification. Wet and dry screening is rarely used to size the sand. After classification, the sand is dewatered using screws, separatory cones, or hydroseparators. Material may also be rodmilled to produce smaller sized fractions, although this practice is not common in the industry. After processing, the sand is transported to storage bins or stockpiles by belt conveyors, bucket elevators, or screw conveyors.

Industrial Sand and Gravel

Industrial sand and gravel typically are mined from open pits of naturally occurring quartz-rich sand and sandstone. 

Mining methods depend primarily on the degree of cementation of the rock. In some deposits, blasting is required to loosen the material prior to processing. The material may undergo primary crushing at the mine site before being transported to the processing plant.

The mined rock is transported to the processing site and stockpiled. The material then is crushed. Depending on the degree of cementation, several stages of crushing may be required to achieve the desired size reduction. Gyratory crushers, jaw crushers, roll crushers, and impact mills are used for primary and secondary crushing. After crushing, the size of the material is further reduced to 50 micrometers or smaller by grinding, using smooth rolls, media mills, autogenous mills, hammer mills, or jet mills. The ground material then is classified by wet screening, dry screening, or air classification. At some plants, after initial crushing and screening, a portion of the sand may be diverted to construction sand use.

After initial crushing and screening, industrial sand and gravel are washed to remove unwanted dust and debris and are then screened and classified again. The sand (now containing 25 to 30 percent moisture) or gravel then goes to an attrition scrubbing system that removes surface stains from the material by rubbing in an agitated, high-density pulp. The scrubbed sand or gravel is diluted with water to 25 to 30 percent solids and is pumped to a set of cyclones for further desliming. If the deslimed sand or gravel contains mica, feldspar, and iron bearing minerals, it enters a froth flotation process to which sodium silicate and sulfuric acid are added. The mixture then enters a series of spiral classifiers where the impurities are floated in a froth and diverted to waste. The purified sand, which has a moisture content of 15 to 25 percent, is conveyed to drainage bins where the moisture content is reduced to about 6 percent. The material is then dried in rotary or fluidized bed dryers to a moisture content of less than 0.5 percent. The dryers generally are fired with natural gas or oil, although other fuels such as propane or diesel also may be used. After drying, the material is cooled and then undergoes final screening and classification prior to being stored and packaged for shipment.

Process Systems & Design are experts in the processing of sand and gravel. Visit http://www.processsystemsdesign.com or call (410) 861-6437 with any inquiries or questions you may have.

Safe, Efficient, and Clean Operation of Belt Conveyors

Belt conveyors are among the most commonly used piece of equipment at mineral processing operations. A conveyor, and the associated transfer points, can generate significant quantities of hazardous debris and respirable dust. Operations must control these emissions by containing, suppressing, or collecting the dust and debris mechanically, either before or after it spills or becomes airborne, giving special attention to transfer points.

A conveyor belt consists of many different parts as seen in Figure 1.

Figure 1 - Basic components of a conveyor belt. 

There are three primary root causes for hazardous debris or fugitive dust emissions associated with conveyor belts: spillage, carryback, and airborne dust (Figure 2). Control of all three primary dust sources is necessary to eliminate hazardous debris and fugitive dust emissions.

Fig. 2 - Types of fugitive dust emissions and debris from conveyor belts 

Controlling Material Spillage

Material spillage from a conveyor belt is caused by a lack of material control, either at a transfer point or along the transfer route. Spillage along the transfer route is generally associated with carryback.

Carryback

Material that sticks or clings to a conveyor belt after passing over the head pulley is called carryback. Carryback tends to fall from the belt as it passes over return idlers. This creates piles of material that require clean-up, which can increase worker dust exposure. Also, respirable portions of carryback can become airborne and increase fugitive dust exposure levels. The goal is to remove carryback before it is released into the air and becomes a source of contamination to the workers or creates piles of material that require clean-up.

The primary means of controlling carryback is to clean the belt as it passes over or past the head pulley (i.e. shortly after material is discharged from the belt). The two most common means of cleaning a conveyor belt of carryback are to mechanically "scrape" the belt via scrapers or brushes or to wash the belt.

Water Sprays for Prevention of Airborne Dust

Wet spray systems, the use of water to control dust, may be classified into prevention applications and suppression applications. When properly designed and installed, water sprays are a cost-effective method of controlling dust from conveyors. The most common and effective practice for conveyor sprays is to wet the entire width of product on the belt.

To discuss this topic in greater detail, contact the experts at Process Systems & Design. They'll be happy to share their decades of experience and knowledge with you.

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The Cement Manufacturing Process

 

Cement is a finely ground powder which, when mixed with water, forms a hardening paste of calcium silicate hydrates and calcium aluminate hydrates. Cement is used in mortar (to bind together bricks or stones) and concrete (bulk rock-like building material made from cement, aggregate, sand, and water). By modifying the raw material mix and the temperatures utilized in manufacturing, compositional variations can be achieved to produce cements with different properties. In the U.S., the different varieties of cement are denoted per the American Society for Testing and Materials (ASTM) Specification C-150.

Cement is produced from raw materials such as limestone, chalk, shale, clay, and sand. These raw materials are quarried, crushed, finely ground, and blended to the correct chemical composition. Small quantities of iron ore, alumina, and other minerals may be added to adjust the raw material composition. The fine raw material is fed into a large rotary kiln (cylindrical furnace) which rotates while the contents are heated to extremely high temperatures. The high temperature causes the raw material to react and form a hard nodular material called “clinker”. Clinker is cooled and ground with approximately 5 percent gypsum and other minor additives to produce Portland cement.

The heart of clinker production is the rotary kiln where the pyroprocessing stage occurs. The rotary kiln is approximately 20 to 25 feet (ft) in diameter and from 150 ft to well over 300 ft long; the kiln is set at a slight incline and rotates one to three times per minute. The kiln is most often fired at the lower end (sometimes, mid-kiln firing is used and new units incorporate preheating as well as precalcining), and the raw materials are loaded at the upper end and move toward the flame as the kiln rotates. The materials reach temperatures of 2500°F to well above 3000°F in the kiln. Rotary kilns are divided into two groups, dry-process and wet-process, depending on how the raw materials are prepared.

In wet-process kilns, raw materials are fed into the kiln as a slurry with a moisture content of 30 to 40 percent. To evaporate the water contained in the feedstock, a wet-process kiln requires additional length (in comparison to a dry kiln). Additionally, to evaporate the water contained in the slurry, a wet kiln consumes nearly 33 percent more kiln energy when compared to a dry kiln. Wet-process kilns tend to be older operations as compared to dry-processes where raw materials are fed into the process as a dry powder. There are three major variations of dry- process kilns in operation in the U.S.: long dry (LD) kilns, preheater (PH) kilns, and preheater/precalciner (PH/PC) kilns. In PH kilns and PH/PC kilns, the early stages of pyroprocessing occur before the materials enter the rotary kiln. PH and PH/PC kilns tend to have higher production capacities and greater fuel efficiency compared to other types of cement kilns.

Three important processes occur with the raw material mixture during pyroprocessing. First, all moisture is driven from the materials. Second, the calcium carbonate in limestone dissociates into CO2 and calcium oxide (free lime); this process is called calcination. Third, the lime and other minerals in the raw materials react to form calcium silicates and calcium aluminates, which are the main components of clinker. This third step is known as clinkering or sintering. The formation of clinker concludes the pyroprocessing stage.

Once the clinker is formed in the rotary kiln, it is cooled rapidly to minimize the formation of a glass phase and ensure the maximum yield of alite (tricalcium silicate) formation, an important component for the hardening properties of cement. The main cooling technologies are either the grate cooler or the tube or planetary cooler. In the grate cooler, the clinker is transported over a reciprocating grate through which air flows perpendicular to the flow of clinker. In the planetary cooler (a series of tubes surrounding the discharge end of the rotary kiln), the clinker is cooled in a counter-current air stream. Reciprocating type grate coolers can also be used to cool the clinker. The cooling air is used as secondary combustion air for the kiln to improve efficiency since the cooling air has been preheated during the process of cooling the clinker.

After cooling, the clinker can be stored in the clinker dome, silos, bins, or outside in storage piles. The material handling equipment used to transport clinker from the clinker coolers to storage and then to the finish mill is similar to that used to transport raw materials (e.g. belt conveyors, deep bucket conveyors, and bucket elevators). To produce powdered cement, the nodules of clinker are ground to the consistency of powder. Grinding of clinker, together with additions of approximately 5 percent gypsum to control the setting properties of the cement can be done in ball mills, ball mills in combination with roller presses, roller mills, or roller presses. While vertical roller mills are feasible, they have not found wide acceptance in the U.S. Coarse material is separated in a classifier that is re-circulated and returned to the mill for additional grinding to ensure a uniform surface area of the final product.

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The Ethanol Production Process

Ethanol Plant

Though new technology may eventually blur the distinction between them, ethanol is produced by one of two processes: wet milling and dry milling. Wet mills are more expensive to build, are more versatile in terms of the products they can produce, yield slightly less ethanol per bushel, and have more valuable co-products. Wet milling initially accounted for most of the ethanol fuel production in the United States, but new construction has shifted to dry mills, partly because dry mills cost less to build.

Dry-milling plants have higher yields of ethanol. The wet mill is more versatile, though, because the starch stream, being nearly pure, can be converted into other products (for instance, high-fructose corn syrup (HFCS)). Co-product output from the wet mill is also more valuable.

In each process, the corn is cleaned before it enters the mill. In the dry mill, the milling step consists of grinding the corn and adding water to form the mash. In the wet mill, milling and processing are more elaborate because the grain must be separated into its components. First, the corn is steeped in a solution of water and sulfur dioxide (SO2) to loosen the germ and hull fiber. This 30- to 40-hour extra soaking step requires additional tanks that contribute to the higher construction costs. Then the germ is removed from the kernel, and corn oil is extracted from the germ. The remaining germ meal is added to the hulls and fiber to form the corn gluten feed (CGF) stream. Gluten, a high-protein portion of the kernel, is also separated and becomes corn gluten meal (CGM), a high-value, high-protein (60 percent) animal feed. The corn oil, CGF, CGM, and other products that result from the production of ethanol are termed co-products.

Unlike in dry milling, where the entire mash is fermented, in wet milling only the starch is fermented. The starch is then cooked, or liquefied, and an enzyme added to hydrolyze, or segment, the long starch chains. In dry milling, the mash, which still contains all the feed co-products, is cooked and an enzyme added. In both systems a second enzyme is added to turn the starch into a simple sugar, glucose, in a process called saccharification. Saccharification in a wet mill may take up to 48 hours, though it usually requires less time, depending on the amount of enzyme used. In modern dry mills, saccharification has been combined with the fermentation step in a process called simultaneous saccharification and fermentation (SSF).

Glucose is then fermented into ethanol by yeast (the SSF step in most dry- milling facilities). The mash must be cooled to at least 95deg. F before the yeast is added. The yeast converts the glucose into ethanol, carbon dioxide (CO2), and small quantities of other organic compounds during the fermentation process. The yeast, which produces almost as much CO2 as ethanol, ceases fermenting when the concentration of alcohol is between 12 and 18 percent by volume, with the average being about 15 percent. An energy-consuming process, the distillation step, is required to separate the ethanol from the alcohol-water solution. This two-part step consists of primary distillation and dehydration. Primary distillation yields ethanol that is up to 95-percent free of water. Dehydration brings the concentration of ethanol up to 99 percent. Finally, gasoline is added to the ethanol in a step called “denaturing,” making it unfit for human consumption when it leaves the plant.

The co-products from wet milling are corn oil and the animal feeds corn gluten feed (CGF) and corn gluten meal (CGM). Dry milling production leaves, in addition to ethanol, distiller’s dried grains with solubles (DDGS). The feed co-products must be concentrated in large evaporators and then dried. The CO2 may or may not be captured and sold.

Reprinted from USDA publication “New Technologies in Ethanol Production”

Happy Fourth of July from PSD

"We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness. — That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, — That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness."

THOMAS JEFFERSON, Declaration of Independence

Dust Collection Systems in Mineral Processing Plants

Dust collection systems are the most widely used engineering control technique employed by mineral processing plants to control dust and lower workers' respirable dust exposure. A well- integrated dust collection system has multiple benefits, resulting in a dust-free environment that increases productivity and reclaims valuable product.

The most common dust control techniques at mineral processing plants utilize local exhaust ventilation systems (LEVs). These systems capture dust generated by various processes such as crushing, milling, screening, drying, bagging, and loading, and then transport this dust via ductwork to a dust collection filtering device. By capturing the dust at the source, it is prevented from becoming liberated into the processing plant and contaminating the breathing atmosphere of the workers.

LEV systems use a negative pressure exhaust ventilation technique to capture the dust before it escapes from the processing operation. Effective systems typically incorporate a capture device (enclosure, hood, chute, etc.) designed to maximize the collection potential.

As part of a dust collection system, LEVs possess a number of advantages:

• the ability to capture and eliminate very fine particles that are difficult to control using wet suppression techniques;
• the option of reintroducing the material captured back into the production process or discarding the material so that it is not a detriment later in the process; and
• consistent performance in cold weather conditions because of not being greatly impacted by low temperatures, as are wet suppression systems.

In addition, LEVs may be the only dust control option available for some operations whose product is hygroscopic or suffers serious consequences from even small percentages of moisture (e.g., clay or shale operations).

In most cases, dust is generated in obvious ways. Anytime an operation is transporting, refining, or processing a dry material, there is a great likelihood that dust will be generated. It also follows that once the dust is liberated into the plant environment, it produces a dust cloud that may threaten worker health. In addition, high dust levels can impede visibility and thus directly affect the safety of workers.

The five areas that typically produce dust that must be controlled are as follows:

1. The transfer points of conveying systems, where material falls while being transferred to another piece of equipment. Examples include the discharge of one belt conveyor to another belt conveyor, storage bin, or bucket elevator.
2. Specific processes such as crushing, drying, screening, mixing, blending, bag unloading, and truck or railcar loading.
3. Operations involving the displacement of air such as bag filling, palletizing, or pneumatic filling of silos.
4. Outdoor areas where potential dust sources are uncontrolled, such as core and blast hole drilling.
5. Outdoor areas such as haul roads, stockpiles, and miscellaneous unpaved areas where potential dust-generating material is disturbed by various mining-related activities and high-wind events.

While areas 4 and 5 can be significant sources of dust, they are generally not included in plant or mill ventilation systems design because of the vast area encompassed and the unpredictability of conditions. Therefore, dust control by methods alternative to LEVs is required.

Dust control systems involve multiple engineering decisions, including the efficient use of available space, the length of duct runs, the ease of returning collected dust to the process, the necessary electrical requirements, and the selection of optimal filter and control equipment. Further, key decisions must be made about whether a centralized system or multiple systems are best for the circumstances. Critical engineering decisions involve defining the problem, selecting the best equipment for each job, and designing the best dust collection system for the particular needs of an operation.

For more information on dust control systems, contact Process Systems Design by visiting http://processsystemsdesign.com or calling (410) 861-6437.