Health, Safety and Handling

THE EVOLVEMENT OF SOLIDS HANDLING TECHNOLOGY

Lyn Bates
Ajax Equipment Ltd
Milton Works Mule
Street Bolton BL2 2AR

1 INTRODUCTION

Solids Handling Technology is a multi-discipline science, with roots in civil and mechanical engineering, with stimulus from structural and chemical engineering. Its emergence as a single, comprehensive field of study followed a long gestation period in soil mechanics and a belated recognition in the bulk handling industry that particulate solids represented a fourth state of matter. The technology combines features from liquids, solids and gases, but embraces infinitely greater variation due to the interaction of a host of factors. The effect of mechanical, chemical, thermal, electrostatic and molecular features in a two or three phase media brings in further complications and the situation is made even more complex by the influence of scale, ambient conditions, industrial equipment factors and operating sequences. No wonder that the accumulation of technical knowledge took a long time to mature in order to bring a coherent structure to the subject. Any attack on a subject of this complexity must involve simplifying assumptions. Treating the media as a continuum is appropriate under certain conditions and this step allowed advances to be made.

2 HISTORY

Bulk solids have been handled and stored for thousands of years. One famous engineer from ancient times, Archimedes, calculated the number of grains of sand that would fill the then known universe as 1051, displaying a basic understanding of the packing characteristics of bulk solids. His perception of density being mass divided by displacement, regardless of container shape, is also applicable to particulate solids and their variable voidage condition.

His invention of using helical screws to pump water evolved to ubiquitous applications for handling of bulk solids. Evidence of grain stores in Babylon, ancient Egypt and through the Roman Empire show that relative large volumes of bulk materials have been shipped, stored and handled from antiquity. However, it is only in recent times, with the growth of cities and the industrial revolution, that concentrations of a wide range of loose solids have been held in large gravity flow structures other than by manual handling. Whereas grain is comparatively free flowing, many other mineral and processed products display difficult flow and handling characteristics. Solids handling remains a mature industry with an immature technology.

Modern solids handling technology has its roots in soil mechanics. The stability of bridges, buildings, earthworks, dams and military fortifications has attracted the attention of engineers for centuries. The work of Coulomb and Rankine on friction and Reynolds, who observed the dilatancy effects on sand during deformation are especially relevant to bulk solids flow. The one and only known paper of an obscure German engineer in Hamburg in 1895 was a landmark in solids handling technology, developing a theory to explain the findings of an English engineer, Isaacs Roberts of the effect of wall friction on silo wall pressures. By way of wood and glass models and simple calculus Janssen developed a theory of pressure distribution in grain silos that remains today the most widely used method of assessing forces on silo walls. One might question how many papers published in this year of high technology, will be widely quoted a hundred years from now?

The maturity of centuries of bulk technology progressed at a snail pace over the first half of the next century. Verification and refinements to Janssen followed, mainly being concerned with structural aspects of silo construction. Airy, Prante Toltz, Ketchum, Jamieson, Lufft, Pliessner, Bovey and many others, constructing experiments and calculations. Interestingly, the findings of Janssen were re-discovered by Shaxby in a joint paper with Evans, during experiments with powders. Hvorslev, examining the stability of cohesive soils, introduced the important concept of 'critical state' to the study of the failure characteristics of bulk material. He showed the peak stress at failure to be a function of the effective normal stress and the void ratio, and independent of the stress history of the material. The void ratio has a direct correlation with bulk density and hence provides a basis for specifying the fundamental strength potential of a compact. This concept is particularly important to an understanding of the mechanism of consolidation and flow of solids. Meanwhile, the concentration of industry, increase in the scale of production, and growing automation, highlighted the shortfall in progress in solids handling technology. The growth of mess and pollution, a thriving 'flow aids' industry and the increasing number and scale of silo failures reported by Theimer all pointed to remaining deficiencies in the technology.

3 THE BREAK THROUGH

By a quirk of history a young, Polish army officer stood on a hill in 1939, with Germans advancing up one side and Russian forces up the other. It was time to pack up fighting and seek refuge in England. Andrew Jenike studied for his engineering degree in London after the war and later settled in America, working for US steel. He decided to examine a range of industrial problems and collected all the information available on about thirty subjects, collecting all the information in separate boxes that he rooted through systematically. One day he suddenly made the crucial decision that the flow of solids was one of the most important problems of the day and, being a man of very positive action, immediately scrapped all the boxes of papers collected on other topics. He approached various universities before agreeing with Utah University to research on bulk solids flow for one dollar per year.

This must have been the best scientific bargain of the century. With the aid of a young student named Jerry Johanson they developed a theory, powder property measuring instrument and design methodology that offered a solution to the age old problem of designing a silo that would guarantee the reliable discharge of non-free flowing materials. His knowledge of Russian was fortunate in that he came across a publication by a little known engineer called Sokolovskii, that provided the key mathematical tool for analyzing converging flow of a plastic medium. His classical Utah Engineering Experimental Station thesis' published in 1964, burst upon the academic world and suddenly changed the subject of bulk solids from a black art to a respectable topic of study. The influence of charge and discharge sequences, definitive flow patterns, transient and switch stresses all fell into an understandable and predictable structure. Problems of handling loose solids, long the scourge of industry, appeared to be banished to history.

Australia instituted a combined attack on solving bulk solids storage and handling problems with the Universities of New South Wales at Sydney, Newcastle and Wollongong combining to form a centre of Bulk Solids excellence. Through the prolific words and deeds of Alan Roberts and Peter Arnold they made a strong contribution to performance of the Continent's large-scale iron and steel export industry, and spread the word internationally. Robert, Arnold and Mclean's publication of a more intelligible interpretation of Jenike's work, put the theory within reach of industry.

4 THE GOLDEN YEARS

In the UK in the late 60's this science formed a part of Harold Wilson's 'White Heat of technology' that was going to transform British industry. The work of Roscoe, Schofield and Wroth on critical state soil mechanics at Cambridge University' fell into place. The Government set up Warren Spring Laboratories, with a major section under Dr. Fred Valentin devoted to bulk technology development. Bradford University formed a School of Powder Technology, headed Dr. John Willams. Prof. Scarlet formed a School of Particle Technology at Loughborough University. The exciting days of theory, testing and development flowed fast and furious. Devices were designed at Warren Spring Laboratories for measuring the tensile and cohesive strength of compacted bulk materials and they conducted much research into powder and paste behavior. The use of injected air for powder state control in flow was put on a scientific basis and the work broadened into investigating various forms of solids handling equipment. Walker, at the South East Electricity Generating Board developed the more user friendly Annular Shear Cell and supporting theory, and Williams and Birks introduced the Unconfined Failure Tester, surely the ultimate tool for measuring arching potential.

The inter-disciplinary nature of the field of bulk solids was firmly established when Abraham Goldburg organised the first PowTech exhibition, which was quickly followed by a proliferation of similar events in UK and internationally. Specialised trade journals for the bulk solids industry were started, and more continue to be introduced. These stirrings of a coherent industrial discipline have steadily strengthened through the formation of the trade organisation SHAPA, (Solids Handling and Processing Association), which now has 100 members selling equipment to this market.

The British Materials Handling Board was set up by the government to assist the co-ordination of research into powder technology and aid the dissemination of information. Despite sterling work by its secretary Peter Middleton in organising meetings, and publishing reports and books, with limited resources it was only able to scrape the surface of the task. Relative to the scale of the industrial importance of bulk solids, the number of universities and research establishments involved throughout the world has always been surprisingly small, even though their contributions has been very significant.

5 THE STRUGGLE TO APPLY THE TECHNOLOGY

Somehow, however, British Industry never secured the extent of benefits expected. These truly golden years of promise for powder technology are not as yet fulfilled. As a 'new' subject in a consolidated form across all industries, the scale of the training problem was immense and never caught up with the backlog. Bulk technology remained a little taught subject in the syllabus of either mechanical or chemical engineering degree courses, tending to fall between the two stools of the mechanical and chemical engineering professions. This is despite the fact that over half of all the products used or consumed by humans are at some stage in a particulate form and are handled many times from source to use.

Many groups previously active have retracted or become extinct. The School of Powder technology at Bradford University was assimilated into the Chemical Engineering department with the retirement of Williams. The Loughborough School of Particle Technology never reached the same heights when Scarlet departed to Delft. The initial Government backing faded as the 'breakthrough' of efficiency was sluggish, culminating in Michael Heseltine selling the Warren Spring Laboratory site for a car park. This situation partially explains the sad reading of the Rand reports on the lack of improvement in the solids handling industries from the 1960's onwards. The dismal record of efficiencies in plants that handle loose solids is compared with industries that handle gasses and liquids. The real reason for the lack of technical progress is that powder technology is a vast subject, greatly complicated by the host of interacting variables that influence the behavior of a mass of particulate solids.

The equipment and operator sensitivity of the apparatus, the time consuming process and the expertise needed to interpret the results of the Jenike method tended to confine application to major installations. It did not help that the theory was complex and written in a style difficult to understand. The academic world pursued even more complicated mechanical devices to reflect 'true' shear conditions, as the basic theory was considered solved, taking the subject further from widespread industrial utility. It took about ten years to develop a written procedure and provide a reference test material to give a basis for achieving consistent results, and it is still a very dubious question as to whether that objective has been achieved. No wonder that the technology virtually ground to a halt, with limited shining exceptions such as at British Steel under Herbert Wilkinson and Harold Wright. The technique remains the only internationally recognised method of establishing the critical arching dimension of a bulk material in a mass flow silo to determine a 'safe' outlet size. As an example of the slow progress in the industry, the adoption of the Jenike method as a standard for ASTM in the USA, the country of its origin, has only finally gained approval during 2000.

The greatest shortcoming in the promotion of the technology was that it was in few people's interest to publicise that the value of wall friction is arguably the most important feature for design requirements. This is a simple measurement to secure. Neither was it widely promoted that static stresses are crucially important to problems of initiating flow, as emphasis on dynamic stresses for flow channel analysis tended to dominate theoretical considerations. The pragmatic requirements of industry came a poor second to the intellectual challenge of understanding the failure complexities of particulate solids in different stress conditions.

J. Schwedes, Brian Scarlet, and Geisle Enstad of TelTech in Norway, separately sought the Holy Grail of powder testing through the development of elaborate bi-axial test devices. Peichel and Schultz market versions of an annular shear tester for automatic conduction of shear tests. Michael Rotter, at Edinburgh University re-invented the Uni-axial tester for coal applications, countering the wall friction problem by double-ended compaction in the preparation cylinder by use of an elastic support for the walls. Alan Roberts in Australia and Andy Matchett of Teeside University have studied the effect of vibration on wall friction. An approach to the failure properties of cohesive powders has been made by Molerus in the light of fluid dynamics and Enstad has proposed a new theory of arching in mass flow hoppers. Chris Thornton at Aston University is pursuing the interaction of hundreds of thousands of particles in two dimensions, by means of computer simulations. The task of replicating this analysis in three dimensions is many orders of magnitude more difficult, As a spoonful of micron size particles contains in the region of 5,000,000,000 particles, and individual interactions are significantly more complicated than the simple models used, realistic replication of bulk material behavior on these lines may be expected to lie far in the future.

6 WHERE DOES THAT LEAVE US TODAY IN TERMS OF SOLVING CURRENT PROBLEMS?

Researchers are using powerful and sophisticated techniques, such as positron tracking, Discrete Element Modelling (DEM) simulation, mathematical models and finite element analysis, to examine the behaviour of loose solids. In the UK two major bulk solids centres, based respectively at Greenwich University and the Glasgow Caledonian University, provide consultation service and training for industry. Teeside, Cambridge, Surrey, Edinburgh, Birmingham, Bristol, Bath, and a few other Universities have sections active in different areas of bulk technology.

Jerry Johanson is back on the scene with a set of instruments to measure wall friction, bulk porosity and a form of shear strength. This latter device is presented with limited underlying theory and hence is not intellectually acceptable to the scientific community. His vast experience has brought him to the conclusion that the Jenike method is not appropriate to industry and a simpler approach is needed. This form of 'vertical shear cell' has been used by the author for some years as a simple tool to measure static failure conditions of a uni-axially loaded compact, a formation and failure condition reflecting incipient collapse over an opening in a non-mass flow container. An elementary comparison of the mass stimulating shear against the strength opposing failure leads to the prediction of a critical size of opening at which self weight collapse will take place. Not an elegant solution, but a conservative guide to the size of outlet needed to commence flow compared with a situation of wall slip and of re-developing flow after a dynamic stress field has been established. A wall friction device and this tool suffice to address many solids handling problems