Gila River Indian Community (GRIC) TIP Development

Gila River Indian Community (GRIC) TIP Development

Source Specific Emission Limitations & Standards

Technical Support Document

February 23, 2001 Version

Section VII. Rule E – Nonmetallic Mineral Mining and Processing

(1)  Nonmetallic Mineral Mining and Processing

(2)  Concrete Batch Plants

(3)  Hot Mix Asphalt Plants

(4)  Vermiculite and Perlite Processing

1.0  Introduction & Source Description

Nonmetallic mineral mining and processing is a general source category including one or more of the following components:

§  Sand and gravel mining and processing (including crushed stone processing)

§  Concrete batching

§  Hot mix asphalt production

§  Vermiculite / perlite processing

Because of common raw materials, processing equipment, emission mechanisms, and control techniques, a single emission standard is proposed for nonmetallic mineral mining and processing with subsections to address specific processes as identified above. Several facilities currently located and operating on GRIC land comprise this source category. All subcategories are currently represented and expected to be potential candidates for expansion and/or future new source siting.

1.1  Sand and Gravel Processing1,2,3

Deposits of sand and gravel are generally found in near surface alluvial deposits and in subterranean and sub aqueous beds. Such deposits are common on GRIC Land. Sand and gravel are typically 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. 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, earthmover, truck, belt conveyors, or other means.

Sand and gravel may be used for fill, bedding, sub base, and base course without processing, but all current GRIC facilities include further processing for concrete and or asphalt production. 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. A process flow diagram for construction sand and gravel processing is included in Appendix A of this document.

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 directed to a crusher for size reduction, to produce crushed aggregate. 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. After processing, the sand is transported to storage bins or stockpiles by belt conveyors, bucket elevators, or screw conveyors.

1.2  Concrete Batching3,4

Concrete is composed essentially of water, cement, sand (fine aggregate) and coarse aggregate. Coarse aggregate may consist of gravel, crushed stone or iron blast furnace slag. Some specialty aggregate products could be either heavyweight aggregate (of barite, magnetite, limonite, ilmenite, iron or steel) or lightweight aggregate (with sintered clay, shale, slate, diatomaceous shale, perlite, vermiculite, slag, pumice, cinders, or sintered fly ash). Concrete batching plants store, convey, measure and discharge these constituents into trucks for transport to a job site. In some cases, concrete is prepared at a building construction site or for the manufacture of concrete products such as pipes and prefabricated construction parts. A generalized process diagram for concrete batching is included in Appendix A of this document.

The raw materials can be delivered to a plant by rail or truck, but in the case of existing sources on GRIC land, they are collocated with nonmetallic sand/gravel mining and processing facilities. The cement is transferred to elevated storage silos pneumatically or by bucket elevator. The sand and coarse aggregate are transferred to elevated bins by front-end loader, clamshell crane, belt conveyor, or bucket elevator. From these elevated bins, the constituents are fed by gravity or screw conveyor to weigh hoppers, which combine the proper amounts of each material.

Truck mixed (transit mixed) concrete is the process practiced at one of the existing GRIC concrete batch plants. At these plants, sand, aggregate, cement and water are all gravity fed from the weigh hopper into the mixer trucks. The concrete is mixed on the way to the site where the concrete is to be poured. Central mix facilities (including shrink mixed) constitute another concrete production process in use on GRIC lands. With these, concrete is mixed and then transferred to either an open bed dump truck or an agitator truck for transport to the job site. Shrink mixed concrete is concrete that is partially mixed at the central mix plant and then completely mixed in a truck mixer on the way to the job site. Dry batching, with concrete mixed and hauled to the construction site in dry form, is seldom, if ever, used.

1.3 Hot Mix Asphalt3,5

Hot mix asphalt (HMA) paving materials are a mixture of well graded, high quality aggregate (which can include reclaimed asphalt pavement [RAP]), and liquid asphalt cement, which is heated and mixed in measured quantities to produce HMA. Aggregate and RAP (if used) constitute over 92 percent by weight of the total mixture. Aside from the amount and grade of asphalt cement used, mix characteristics are determined by the relative amounts and types of aggregate and RAP used. A certain percentage of fine aggregate (less than 74 micrometers [µm] in physical diameter) is required for the production of good quality HMA. Hot mix asphalt paving materials can be manufactured by: (1) batch mix plants, (2) continuous mix (mix outside drum) plants, (3) parallel flow drum mix plants, and (4) counterflow drum mix plants. This order of listing generally reflects the chronological order of development and use within the HMA industry. An HMA plant can be constructed as a permanent plant, a skid-mounted (easily relocated) plant, or a portable plant. All plants can have RAP processing capabilities. Virtually all plants being manufactured today have RAP processing capability. A generalized process diagram for hot mix asphalt is included in Appendix A of this document

1.4  Vermiculite and Perlite Processing6,7

Vermiculite –

Vermiculite is the geological name given to a group of hydrated laminar minerals that are

aluminum-iron-magnesium silicates and that resemble mica in appearance. The chemical formula for vermiculite is (Mg,Ca,K,Fe ) (Si,Al,Fe ) O (OH) 4H O. When subjected to heat, vermiculite has the unusual property of exfoliating, or expanding, due to the interlaminar generation of steam. Uses of unexpanded vermiculite include muds for oil-well drilling and fillers in fire-resistant wallboard. Vermiculite ore is mined using open-pit methods. Beneficiation includes screening, flotation, drying in rotary or fluid bed dryers, and expansion by exposure to high heat. All mined vermiculite is dried and sized at the mine site prior to exfoliation. A generalized process diagram for vermiculite processing is included in Appendix A of this document.

Crude ore from open-pit mines is brought to the mill by truck and is loaded onto outdoor stockpiles. Primary processing consists of screening the raw material to remove the waste rock greater than 5/8 inch and returning the raw ore to stockpiles. Blending is accomplished as material is removed from stockpiles and conveyed to the mill feed bin. The blended ore is fed to the mill, where it is separated into fractions by wet screening and then concentrated by gravity. All concentrates are collected, dewatered, and dried in either a fluidized bed or rotary dryer. Drying reduces the moisture content of the vermiculite concentrate from approximately 15 to 20 percent to approximately 2 to 6 percent. At most facilities, the dryer products are transported by bucket elevators to vibrating screens, where the material is classified. The dryer exhaust generally is ducted to a cyclone for recovering the finer grades of vermiculite concentrate. The classified concentrate then is stored in bins or silos for later shipment or exfoliation. The rotary dryer is the more common dryer type used in the industry, although fluidized bed dryers also are used. Drying temperatures are 250 to 900 F, and fuel oil is the most commonly used fuel. Natural gas and propane also are used to fuel dryers.

After being transported to the exfoliation plant, the vermiculite concentrate is stored. The ore concentrate then is conveyed by bucket elevator or other means and is dropped continuously through a gas- or oil-fired vertical furnace. Exfoliation occurs after a residence time of less than 8 seconds in the furnace, and immediate removal of the expanded material from the furnace prevents damage to the structure of the vermiculite particle. Flame temperatures of more than 1000 F are used for exfoliation. Proper exfoliation requires both a high rate of heat transfer and a rapid generation of steam within the vermiculite particles. The expanded product falls through the furnace and is air conveyed to a classifier system, which collects the vermiculite product and removes excessive fines. The furnace exhaust generally is ducted through a product recovery cyclone, followed by an emission control device. At some facilities, the exfoliated material is ground in a pulverizer prior to being classified. Finally, the material is packaged and stored for shipment.

Perlite –

Perlite is a glassy volcanic rock with a pearl-like luster. It usually exhibits numerous concentric cracks that cause it to resemble an onion skin. A typical perlite sample is composed of 71 to 75 percent silicon dioxide, 12.5 to 18.0 percent alumina, 4 to 5 percent potassium oxide, 1 to 4 percent sodium and calcium oxides, and trace amounts of metal oxides. Crude perlite ore is mined, crushed, dried in a rotary dryer, ground, screened, and shipped to expansion plants. Horizontal rotary or vertical stationary expansion furnaces are used to expand the processed perlite ore. Crude perlite is mined using open-pit methods and then is moved to the plant site where it is stockpiled. A generalized process diagram for perlite processing is included in Appendix A of this document

The first processing step is to reduce the diameter of the ore to approximately 0.6 inch in a primary jaw crusher. The crude ore is then passed through a rotary dryer, which reduces the moisture content from between 4 and 10 percent to less than 1 percent. After drying, secondary grinding takes place in a closed-circuit system using screens, air classifiers, hammer mills, and rod mills. Oversized material produced from the secondary circuit is returned to the primary crusher. Large quantities of fines, produced throughout the processing stages, are removed by air classification at designated stages. The desired size processed perlite ore is stored until it is shipped to an expansion plant. At the expansion plants, the processed ore is either preheated or fed directly to the furnace. Preheating the material to approximately 800 F reduces the amount of fines produced in the expansion process, which increases usable output and controls the uniformity of product density.

In the furnace, the perlite ore reaches a temperature of 1400 to 1800 F, at which point it begins to soften to a plastic state where the entrapped combined water is released as steam. This causes the hot perlite particles to expand 4 to 20 times their original size. A suction fan draws the expanded particles out of the furnace and transports them pneumatically to a cyclone classifier system to be collected. The air-suspended perlite particles are also cooled as they are transported to the collection equipment. The cyclone classifier system collects the expanded perlite, removes the excessive fines, and discharges gases to a baghouse or wet scrubber for air pollution control. The grades of expanded perlite produced can also be adjusted by changing the heating cycle, altering the cutoff points for size collection, and blending various crude ore sizes. All processed products are graded for specific uses and are usually stored before being shipped. Most production rates are less than 2 tons/hr, and expansion furnace temperatures range from 1600 to 1800 F. Natural gas is typically used for fuel, although No. 2 fuel oil and propane are occasionally used.