Good vibrations: selecting the right vibrating scalper can streamline production and decrease costs

Pit & Quarry, March, 2004 by Leon Luadzers

This is the first in a two-part article on vibrating scalpers. This is an update of an article that originally appeared in Pit & Quarry 16 years ago.--Ed.

Since my original article on this subject, vibrating technology has given us new products and there are new scalping applications. My sizing formula has been incorporated into the most popular aggregate-flow computer program, making it easier to size vibrating scalping screens. With these new developments, I thought it was timely to update this subject for the benefit of our industry.

I originally did research on this subject about 17 years ago as an applications engineer. At the time, there was no definitive data or formula on sizing vibrating scalping screens. I researched actual field data on vibrating scalpers, comparing actual capacities with deck openings and created a formula based on this data. The formula has been widely used since then and has worked well.

Vibrating scalping screens continue to serve important functions in several industries, including aggregates and mining. Scalpers also perform valuable service in concrete and asphalt recycling, soil processing and various environmental processing applications.

A scalper can be described as an extra-heavy-duty vibrating screen typically designed with grizzly bars or heavy-duty perforated plate. Scalpers typically separate materials with large openings and accept feed of larger feed size (typically 10 in.).

The function of scalping is usually described as the separation of oversize material from a feed of predominately finer feed, however, this is not the situation in every application. Neither of these descriptions are always accurate, because scalpers can be equipped with various decking materials and openings and can perform various functions in a variety of applications.

Scalper applications

* Scalping ahead of a crusher to remove fines: Fines can cause a variety of problems in primary, secondary, tertiary and quaternary crushing. Fines can create a packing condition, reducing crusher efficiency and capacity and possibly crusher damage. Eliminating fines increases the capacity of the crusher to do work on oversize feed. Fewer fines going through the crusher reduces wear in the crushing chamber, thus reducing downtime. The scalper can reduce circulating loads in closed circuit applications.

* Scalping before finish screening: A scalper can increase final screen efficiency and reduce required screen area by decreasing material bed depths, thus allowing quicker stratification.

* Replace stationary grizzly bars: The vibrating grizzly is more effective than stationary bars and has less tendency to plug (lodging of material between the bars and supports). The vibrating action of the scalper causes the material to stratify and separate.

* Creating a bed of fines on a conveyor belt: A scalper can put a layer of protective fines on expensive conveyor belts and reduce the impact of the large lumps.

* Production of riprap: Scalpers can be designed to produce various sizes of riprap for dam projects, river banks and shorelines, etc.

* Site prep, overburden screening: Separating oversize stone or other material from soil for use in large projects such as airport, residential, commercial and dam projects.

* Recycling prescreening: Prescreening or scalping recycled concrete, asphalt and other materials reduces processing and crushing cost.

* Environmental, landfill, topsoil processing: Prescreening in waste and landfill recycling, and soil recycling and reclaiming to create useable products and reduce processing cost.

Sizing the scalper

The first step in sizing and selecting the scalper is to review the application factors and to analyze how the scalper fits into the process flow. It is important to identify the scalper's duty and determine what affect it has on other equipment. The scalper's efficiency will affect the efficiency and capacity of crushers and other vibrating screens.

Once all the application criteria, such as feed rates, material gradation, deck opening and efficiency rate are determined, then special application factors are applied to the sizing formula to determine the required deck area.

My vibrating scalper sizing formula determines the square footage required for a desired tonnage rate. As mentioned, this formula was developed mainly from field data combined with empirical values corresponding to specific elements of the application.

This formula has worked well in many applications during the last 16 years. This formula was conceived as a guide to scalper size selection and the final selection should be based on field experience and physical installation factors as well.

The basic formula, with empirically derived capacity and modifying factors, is shown in Table 1.

The material bed depth should not exceed two times the average particle size going over the deck, particularly at the discharge end. If material bed depth is excessive, fines will have greater difficulty sifting through to the bottom of the feed bed where they have an opportunity to fall through the deck openings. The width of the scalper controls the material bed depth and should be chosen carefully.