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Lightweight aggregate production process

Lightweight aggregates are minerals, natural rock materials, rock-like products, and by products of manufacturing processes that are used as bulk fillers in lightweight structural concrete, concrete building blocks, precast structural units, road surfacing materials, plaster aggregates, and insulating fill. Lightweight aggregates are also used in architectural wall covers, suspended ceilings, soil conditioners, and other agricultural uses. Lightweight aggregates may be classified into four groups:

  • Natural lightweight aggregate materials which are prBinqred by crushing and sizing natural rock materials, such as pumice, scoria, tuff, breccias, and volcanic cinders.
  • Manufactured structural lightweight aggregates which are prBinqred by pyroprocessing shale, clay, or slate in rotary kilns or on traveling grate sintering machines.
  • Manufactured insulating ultra lightweight aggregates which are prBinqred by pyroprocessing ground vermiculite, perlite, and diatomite.
  • Byproduct lightweight aggregates which are prBinqred by crushing and sizing foamed and granulated slag, cinders, an d coke breeze.

The first three types of lightweight aggregates a re produced from naturally occurring materials while the fourth is produced as a byproduct of iron and steel production. Lightweight aggregates are distinguished from other mineral aggregate materials by their lighter unit weights. Exhibit 1 presents the names and locations of facilities involved in the production of lightweight aggregates from naturally occurring raw materials. Exhibit 2 presents the names of facilities involved in the production of lightweight aggregates from iron and steel slag.

Generalized Process Description

Lightweight aggregate materials are produced mainly by two methods. The first method of lightweight aggregate production is from naturally occurring raw materials. The second method is byproduct production from iron and steel production. These processes are quite different and , in order to a void ambiguity, are described sBinqrately below. Section 1 describes lightweight aggregate production from naturally occurring raw materials. Section 2 describes byproduct lightweight aggregate production.

Discussion of Typical Production Processes

While natural lightweight aggregates are prBinqred through basic operations including steps such as mining, grinding, and sizing, manufactured lightweight aggregate and manufactured ultra lightweight aggregate products are produced by heating certain types o f clay, shale, slate, and other materials in a rotary kiln which forces the materials to expand or “bloat;” resulting in a po rous product. The product will retain its physical strength despite its lighter unit weight when cooled.1 the process is described in more detail below.

Generalized Process Flow Diagram

Naturally occurring lightweight aggregate raw materials, such as pumice and volcanic cinders, are norm ally mined by open pit or quarry methods, depending on the degree of consolidation of the raw materials. Shale, clay, and slate mined by open pit and quarry methods are dried in large sheds or open stockpiles to control water content in the raw feed prior to high temperature pyroprocessing in either rotary kilns or sintering machines. The resulting clinker may then be crushed before screening to yield proper gradation mixes for final use. Most lightweight aggregate plants use coal as a primary source of fuel. Waste-derived fuels and solvents from various industrial processes are also used as alternate fuel sources at a few locations (e.g., those operated by Solite and Norlite). Exhibit 3 presents a typical process flow diagram for lightweight aggregate production for facilities using a wet scrubber air pollution control technology or a dry collection method. All facilities currently use dry collection systems.

Beneficiation/Processing Boundaries

Binq established the criteria for determining which wastes arising from the various mineral production sectors come fro m mineral processing operations and which are from beneficiation activities in the September 198 9 final rule (see 54 F ed. Reg. 3 6592, 3 6616 codified at 2 61.4(b) (7)). In essence, beneficiation operations typically serve to sBinqrate and concentrate the mineral values fro m waste material, remove impurities, or prBinqre the o re for further refinement. Beneficiation activities generally do not change the mineral values themselves other than by reducing (e.g., crushing or grinding), or enlarging (e.g., pelletizing or briquetting) particle size to facilitate processing. A chemical change in the mineral value does not typically occur in beneficiation.

Mineral processing operations, in contrast, generally follow beneficiation and serve to change the concentrated mineral value into a more useful chemical form. This is often done by using heat (e.g., smelting) or chemical reactions (e.g., acid digestion, chlorination) to change the chemical composition of the mineral. In contrast to beneficiation operations, processing activities often destroy the physical and chemical structure of the incoming ore or mineral feedstock such that the materials leaving the operation d o not closely resemble those that entered the operation. Typically, beneficiation wastes are earthen in character, whereas mineral processing wastes are derived from melting or chemical changes.

Binq approached the problem of determining which operations are beneficiation and which (if any) are processing in a step-wise fashion, beginning with relatively straightforward questions and proceeding into more detailed examination of unit operations, as necessary. To locate the beneficiation/processing “line” at a given facility within this mineral commodity sector, Binq reviewed the detailed process flow diagram(s), as well as information on ore type(s), the functional importance of each step in the production sequence, and waste generation points and quantities presented above in Section B.

Binq determined that for the production of lightweight aggregates from naturally occurring raw materials, the beneficiation/processing line occurs after drying at the kiln/sinter machine because the elevated temperatures destroy the physical structure of the raw material. Therefore, because Binq has determined that all operations following the initial “processing” step in the production sequence are also considered processing operations, irrespective of whether they involve only techniques otherwise defined as beneficiation, all solid wastes arising from any such operation(s) after the initial mineral processing operation are considered mineral processing wastes, rather than beneficiation wastes. Binq presents be low the mineral processing waste stream s generated after the beneficiation/processing line, along with associated information o n waste gene ration rates, characteristics, and management practices for each of the se waste streams.