Quality-Driven Production of Aggregates in Crushing Plants

Doctoral thesis, 2009

Aggregate quality is of great importance, and in this context it is essential to understand how various process parameters influence the product quality characteristics. Thus, there is a need for aggregate producers to improve their process knowledge. Research has led to an improved understanding of how different process parameters affect the outcomes of specific comminution processes. In aggregate production, breakage models are one example of such a research result. These models have been refined over the years. The breakage models that have been presented to date can accurately predict the size distribution of aggregates. On the other hand, they cannot normally predict any other material characteristics such as shape and strength. This thesis outlines the research to date, which has focused on a frequently discussed quality parameter, namely the shape of the product. Knowledge of various methodologies is required to measure this parameter. A novel method for measuring the shape of fine aggregates is presented. The rheology test, the methodology of which was originally based on a New Zealand standard, can be correlated with other data to measure particle shape in accordance with the F-aspect method. The correlation between these methods is very good, and since the rheology test is easy to use, it is suitable for a quarry or a basic test facility. The cone crusher is the most commonly used tertiary stage crusher today. One of its great advantages is that it produces a relatively low amount of fines. However, it only produces acceptable particle shapes for certain fraction sizes. The vertical shaft impact crusher (VSI) creates particles of a very satisfactory shape across all fractions, including fine aggregate, as confirmed by studies using the aforementioned rheology test. This thesis presents two models for predicting shape in crushed aggregate from a cone crusher. The first model is empirical, and my conclusion is that there are four dominating parameters affecting shape: the average feed size, the closed side setting (CSS), throw and eccentric speed. To further investigate how compressive breakage affects particle shape, an inter-particle breakage model is introduced. This model simulates a specific crushing stage inside the crusher. Simulation results show that the outcome from a crushing event can be predicted with good accuracy. A VSI model for describing particle size and shape is presented in this work. It is proposed that the dominating breakage mechanism in a VSI equipped with a rock box is of abrasive character. Abrasive breakage can appropriately shape all particle sizes, assuming the crusher is operated at a proper velocity. The empirical cone crusher model and the VSI models are combined in a mathematical simulation in order to determine how a crushing plant should be operated to maximise both yield and aggregate quality. The results show that there exists a set of rules to determine the crusher and screen settings that will achieve the best performance in terms of product yield and quality requirements.