The processing of minerals entails a number of procedural steps; such as crushing, sizing, classifying, and at times the need for drying. In itself, removing moisture from a given material is relatively basic; just introduce hot air. Although this can be calculated with a high level of accuracy, the drying step presents its own unique and sometimes difficult challenges; high diversity in the raw materials, changing material characteristics during the drying process and the true amount of time that is needed in order achieve our desired goal. Achieving the optimum solution takes know-how, years of proven experience and the correct equipment technology. With that in mind; HAZEMAG delivers solutions!
The decision to dry material or alternatively dry and grind material is driven by the final product goal; what is truly needed? Each industry presents a potential need and opportunity for drying technology. For example:
In this case, the drying process should be understood as the thermal removal of water from the wet material; achieved by convection. In this case, water is evaporated when the hot gas / hot air comes into contact with the wet material. The hot air is normally produced by the flue gases coming from a hot gas generator; however in many cases this can be facilitated by the use of available, on site exhaust gases. The fuel source for the gas generator is normally supplied by natural gas, heating oil or coal fines (dust).
In order to determine the optimum drying solution, the following important points, among others, should be considered and evaluated.
It is important to know and understand the condition of the material in its “wet state”; thus providing the needed knowledge for the design and selection of the feed equipment. Further, it is also important to know and understand the condition of the material in its “dry state”, thus providing the needed knowledge for the design and selection of the product discharge equipment.
The drying process requires careful planning; taking into consideration the material handling steps that precede drying and the subsequent steps that will following drying. The complete process must be synchronized; ensuring a smooth, efficient and un-interrupted flow of material throughout the plant.
The selection of the drying system is not possible without a full understanding in regard to the material that must be dried. For example:
Knowing these properties is essential in determining and selecting the optimum drying solution.
In many cases, the need for the drying process is hindered by the available amount space within the processing plant. Overcoming this is achieved by experience, a range of processing solutions and an industry proven partner.
Knowing the desired or needed mode of operation also plays an important role in determining the selection and design of the drying system, thus ensuring that the drying plant models the function and material flow as needed.
This aspect is especially to be considered at the determination of the hot gas and exhaust gas flow rate.
The Drying Plant: Key Component Breakdown
Knowledge of the process parameters in respect of the drying process is also a basic requirement. But also any physical and/or chemical changes in the material to be dried have to be taken into account.
The rapid dryer HRD is a field-proven type of dryer and combines the advantages of short-time drying and uniflow drying, as a result of which high efficiency is achieved. With the mechanical whirling of the material to be dried with agitator paddles and simultaneous flow of hot gas through the material, gentle thermal drying is possible in a short time.
With the principle of uniflow drying, the material to be dried and the drying gas pass through the dryer in the same direction. This offers the advantage that high hot gas temperatures can be used for drying in the entry zone without the material being damaged. When the hot gases meet the wet material, intensive water evaporation results with simultaneous decrease of temperature of the drying gas.
This operating principle of the rapid dryer is especially advantageous for the drying of moist, plastic clay and difficult-to-dry filter cake that has been previously mechanically dewatered on belt filters, filter presses, drum filters or centrifuges, e.g. FGD gypsum, chemical gypsum, chromium hydroxide, etc. The material to be dried is de-agglomerated. This results in a very large surface area and a whirling of the material particles, which enables intensive material and heat exchange. Thanks to its compact design, only a relatively small space is required.
For the simultaneous grinding and drying of materials, HAZEMAG Hammermills type HUM and HNM are used. In this combined process, grinding and drying complement each other very well and mutually support each other. The thermal stresses promote the comminution process. The larger product surface area produced by grinding is very conductive to the drying process.
Combined grinding and drying units are also short resistance time dryers and are used when fine grinding and drying are required at the same time. Typical applications are limestone, clays, chalk, dolomite, gypsum and anhydrite. Drying is performed according to the uniflow principle, the material being heated to a limited extent.