Crushing & Screening

Comminution in Brittle Solids Resulting from Hypervelocity Impacts

Comminution is an old art. Unfortunately the analysis of the unit operations has been handicapped by the lack of both theoretical and experimental information. Two types of information are needed, one an understanding of what happens when a particle breaks in a brittle manner; and two, an understanding of what happens when the daughter fragments pass through the mill.

Recently, successful theoretical attacks on the fracture of brittle solids have yielded significant results. New experimental techniques are yielding insight into the physical phenomenon occurring during brittle fracture. Many of the re-doubtable shiboliths of the last century are either being laid to rest or verified.

The problem with a shock wave is that, from its beginning to its decay into a compressive elastic wave we do not fully understand the dynamics. The shock wave proceeds into the target material as well as into the projectile material. The shock wave moves both perpendicularly into the target and the projectile. However a rarifaction wave originating at the edge of the material eats into the compressive shock wave, attenuating it.

Hypervelocity impacts are spectacular phenomenon. Material in the liquid state is ejected from the early crater at velocities exceeding that of the impacting projectile. Associated with the

Flow of Solids in Bins

There used to be a time when bins were designed for total volume and placed in any available place of the plant. Poke holes were provided and feed was maintained by manual prodding, lancing, hammering, and by various mechanical flow-promoting devices. Little thought was given to what occurred inside the bins and why materials did not flow. Indeed, such mental effort was hard to justify while low-cost manual labor was plentiful, quality standards low, and rates of feed moderate.

Typical Storage and Flow Problems

In order to design an efficient storage plant or contact-bed reactor it is necessary to be aware of the problems which can arise in flow and storage. The typical problems are:

No flow: a stable arch (dome) or rathole develops within the solid and flow ceases.

Erratic flow: momentary arches form within the solid; ratholes empty out partly or completely, then collapse. Density of the stream swings within wide limits.

Flashing: powders in erratic flow aerate, fluidize and flush. While air-locks can be used to prevent flushing they do not eliminate erratic flow.

Insufficient flow: the volumetric rate of gravity flow through an orifice depends on the stored solid. Coarse dry solids flow at a high rate, closely approaching the

Large Mobile Crushing Plant Profitability

In the mobile crushing unit system trucks are replaced by a conveyor resulting in a “continuous” almost “fully-automated” operation with all its attendant advantages. In consequence, overall operating costs are greatly reduced.

Because of the new designs of large mobile crushing units that have been brought on-to the market, the use of these plants has increased significantly during the last three years.

New Designs of Mobile Crushers and Advantages Accruing to the Industry from their Use

Gains in productivity and reductions in costs are vital to every mine operator. The shovel-crusher-conveyor system is a powerful tool with which to achieve these goals.

The basis concept of the shovel-crusher-conveyor scheme of operations is very simple and sound, which is why domestic operators are showing great interest in its application. They are well aware of the possible potential gains, and some of them started to investigate the situation from the technical and economic point of view a long time ago. A number of small operators have been using small mobile crushing plants, designed by american operators since many years. However, large operators, in spite of considerable interest, have not been able to use this system because no mobile crusher, capable of crushing 1000 to

Fine Cobbing with Alternate Polarity Magnets

At the MacIntyre Development of the Titanium Division of National Lead Company a magnetite-ilmenite ore together with gabbro-anorthosite waste is mined front an open pit by shovels and trucks. The ore from the pits is trucked to ore stockpiles for blending ahead of the crusher and to free the mine from crusher delivery schedules.

The ore is reclaimed from stockpile by an electric shovel and trucked to the crusher; all ore has been blasted or drop-ball fractured to feed the 48 x 60 inch jaw crusher. The jaw feed is controlled by a chain feeder over a stationary grizzly with 6-8 inch opening to bypass most of the fines.

Fine magnetic cobbing of minus one-half inch material was recognized as desirable very early in the operation. Maximum waste removal was not attainable, however, with i the conventional radial magnetic pulley. As mentioned earlier, the liberation of ilmenite and magnetite starts at 10 mesh so that the practical range for dry fine cobbing is minus one-half plus 10 mesh.

Many types of magnetic separator equipment have been investigated over the years. Full success in fine cobbing however was not achieved until a laboratory wet permanent magnetic drum separator for magnetite was disassembled and the

Bin Design and Construction

For many years, the Machinery Division of Dravo Corporation has been designing and constructing bins and bunkers as part of overall plant facilities. We have designed bins to handle a variety of materials including coal, coke, iron ores and concentrates, sintered and pelletized ores, calcined and raw limestone, bentonite, alloys for BOF shops, and even cast iron chips from machining operations.

In past years, rule-of-thumb design criteria had been to keep the sides of the bins as steep as possible and the outlets of the bin as large as possible. This criteria led to the design of bins with 60° to 70° side slopes and table feeders which were very large. Unfortunately, this meant trying to fit tall, skinny bins into a layout where minimum building height was required for obvious economic reasons. Like everyone else in the industry, we found that this design procedure was expensive and ineffective.

One of the first applications of the new principles of bin design was on a small bin which handled the discharge from disc filters. This bin was designed during the transition period between the old rule-of-thumb design and the newer Jenike approach. It was based on the steep sides, big opening concept, and it

Crushing in the Aggregate Industry

In most industrialized countries, the largest element of the mining industry in physical volume is that devoted to the extraction and processing of materials for use as construction aggregates. These materials come from four main sources:

(a) Excavation of unconsolidated deposits of sand and gravel
(b) Stone quarrying.
(c) Processing of slags and other mineral wastes.
(d) Manufacturing from clays and shales by heat fusion.

They are classified as fine aggregates (natural and manufactured sands, generally less than ¼” in size), coarse aggregates (round and crushed gravels, crushed stone, crushed slag, and fused clays and shales, in sizes generally from ¼ to about 1″, but sometimes used up to 6″ or so), and unseparated pit-run or crusher-run materials. T

In crushing to produce specification aggregates, on the other hand, not only is it necessary to reduce all the feed to a given top size – say 1″, but also to have the amounts of the fractions in the succeeding sizes – ¾”, ½”, 3/8″ and so on – fall within certain fairly tight limits. An examination of the table shown here, “Grading Requirements for Coarse Aggregates”, taken from A.S.T.M. Specification C33-66 “Standard Specification for Concrete Aggregates”, indicates this point clearly.

Many specifications introduce a requirement on particle shape.

Dragscraper Systems for Stockpiling, Reclaiming and Blending Bulk Materials

Today more than ever, there is an increased demand for higher plant production capacities and efficiencies to offset the increasing costs for labor and materials.

Plants involved in processing large volumes of bulk materials are particularly interested in more efficient material handling systems to (1) reduce labor and maintenance costs, (2) improve blending of non-uniform raw material feed to processing facilities, (3) improve blending of finished bulk materials from storage.

Dragscraper systems are being used effectively for handling a variety of bulk materials in both indoor and outdoor storage. Materials being handled include ores, chemicals, coke, clay, and stone, to name a few. Handling capacities these machines can provide vary from a few tons to over 1500 tons per hour.

The basic components of a Dragscraper machine consist of a bottomless Crescent scraper, a two or three-drum Dragscraper hoist assembly including gas, diesel, or electric power unit, operating cables, guide blocks, and either fixed or mobile head and tail structures. They are presently available in sizes through 15 cubic yards, for operating spans up to 1000 feet.

In reclaiming from a rectangular shaped ground storage, the Crescent scraper can load out material to a belt conveyor, installed at grade along one side of the

Large Tonnage Fine Crushing Plants – Trade-Off Study

Common practice for size reduction of run-of-mine or run-of-pit ore in preparation for subsequent treatment utilizes a three-stage system. First stage, or primary breaking is generally carried out with jaw or gyratory crushers. The usual output from this stage provides a feed for fine crushing in the size range of 6 to 10 inches.

Factors Affecting Design

Generally speaking, two crushing circuit options are available for preparation of grinding mill ore feeds – open or closed circuit. Generally speaking, open circuits are used to provide a minus ¾” product, while closed circuits are used to provide a minus ½ or minus 3/8″ product. A careful overall analysis must be made of feed and product sizes, crusher capacities at various settings, required screen areas and ore characteristics before a decision on open or closed circuit can be made.

Ore characteristics play an important part in establishing design criteria and equipment specifications. For example, a plant designed specifically for a hard dry ore may be at a serious disadvantage when presented with wet sticky material. And ores which, when crushed yield either an abnormally high percentage of coarse particles or fines, may require special effort in design to avoid unbalanced circuits and overloaded

Conveyor Belt Scale Design

Weight may be defined as the gravitational attraction or earthpull on a body (mass). Thus, from a practical viewpoint, a scale or other form of weighing device is a means for determining mass. Weighing devices fall into two principal categories: (1) gravimetric; and (2) non-gravimetric or nuclear devices. At present, after many centuries of use, the effect of gravitational force as a treasure of mass in industry and commerce remains by far the most common method for mass (weight) determination. Non-gravimetric methods are just becoming established in the industry.

The basic components of a belt scale system are:

  1. Belt conveyor to convey the load over the scale.
  2. Scale suspension to transmit and direct the load force.
  3. Sensing device to measure the load force and belt speed.
  4. Integrating, totalizing and recording the scale output.

Scale Suspension

The scale suspension must function to transmit the force on the weigh idlers to the load balancing device. This is basically a problem of transmitting only the force normal to the conveyor belt without resolving any lateral forces into the vertical direction.

The following criteria must be met:

  1. Rigidity, minimal deflection.
  2. Torsional stability.
  3. Elimination of lateral forces.
  4. Minimize effects of excentric belt loading.
  5. Alignment provisions.
  6. Force reduction

Ultrasonic Level Sensors

Ultrasonics, by simple definition, are sound waves beyond the audible range of the human ear and are generally accepted to be waves over 20,000 cycles per second. When applied to level measurement, the principles of ultrasonics are basically quite simple. A sound pulse is initiated at a point in time and the time interval for the echo from that pulse to return can be interpolated into distance.

A brief explanation of the hardware involved in an ultrasonic application may prove helpful. A sensor, which consists of a crystal housed and encapsulated with an epoxy compound is mounted in a sensor enclosure made to various specific sizes and shapes and manufactured from varied materials to handle the ranges and environmental conditions as required for the application. The sensor is connected by means of a standard coaxial cable to an electronic unit. The electronic unit, which might be called the heart of the system.

The industrial applications to measure the level of iron ore, coal, minerals and chemicals of all types and liquids including very strong acid and caustic solutions. Level controls on tripper control systems and multiple bin applications are becoming very widely accepted. Each application is different and can be made to

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