Briquetting Process and Use of Binders
Briquetting Process and Use of Binders
During the various stages of iron and steel production, a number of iron bearing waste by-products are generated. These waste by-products cannot be recycled back as such because of their fine particle size, hence the waste by-products need to be agglomerated. Further, in the present day scenario, stringent statutory regulations restrict the emissions of the industrial pollutants. Also, with the increasing production of crude steel and depletion of rich ores, the costs of primary materials are increasing. These increasing costs have made it imperative that the recycling of the plant waste by-products is maximized. Briquetting of fines is a technically viable, economically attractive, and environmentally safe way of agglomerating iron rich waste fines of iron and steel plant.
Cold bond briquetting (or simply briquetting) is an important process which is used to recycle and utilize the iron and steel plant waste by-products of fine particle size. The briquetting process converts the fine particles into cold bonded briquettes. In this process, various waste by-products can be easily agglomerated using very small amount of the binder. The briquetting process avoids heating up, softening, and melting the fine particles, which can save a lot of fossil energy and decrease the environmental pollution. It is also cost effective than other agglomerating processes because of lesser number of processing steps.
Industrial methods of briquetting date back to the second part the 19th century when large scale development of briquetting has taken place. In 1858, John Piddington received a patent for using 36 pounds of starch along with 8 % water per ton of coal. In 1897, in Pennsylvania, Ellsworth Zwoyer patented a process for briquetting coal fines with a starch binder.
Briquettes are made of different qualities and dimensions depending on the raw materials, die or mould, and technologies applied during production. Briquettes vary a lot in size and form, but normally they are of a cylindrical shape with a diameter of between 25 mm and 100 mm and lengths ranging from 10 mm to 400 mm. Square, rectangular, and polygonal briquettes also exist. Densified material such as briquettes have several advantages, which includes, but are not limited to, increased energy density, ease of handling, transport and storage, improved combustibility, lower particle emission, low volatility, and uniform size, density, and quality.
The cold bonded briquettes provide a suitable feed for the metallurgical furnaces. The charging of the cold bonded briquettes in a metallurgical furnace depends on the physical and mechanical properties of the briquettes. For getting briquettes with sufficient mechanical strength and thermal stability a binder is needed. With the proper selection of a binder the mechanical and metallurgical properties of the briquettes can be influenced.
The quality of cold bonded briquettes and their properties are also dependent on the binder used. The strength of the cold bonded briquette is dependent on the bonding mechanism with the binder. The material used as binder is required to burn off without interfering with the reactions of the metallurgical furnace (organic binders) or are to be compatible with the furnace reactions (inorganic binders).
For briquettes to be considered suitable as feed material for a metallurgical furnace, they are required to have sufficient strength for their handling, transportation, and open storage. Further, they need to have properties which prevent their degradation and excessive swelling. Also, the process of producing briquette is to be robust enough to tolerate a variety of materials in various proportions and is to be economically feasible even for small scale operations without generating any pollutants. Fig 1 shows schematic diagram of briquetting process.
Fig 1 Schematic diagram of briquetting process
Briquettes are to be sufficiently coherent. Briquettes of any desired degree of coherence can be made by varying the amount of binding material used in the briquette and by varying the pressure during the briquetting process. An increase of either the binder or the pressure, of course, represents an added cost in the manufacture of the briquettes. The density of the briquettes is increased by pressure.
The briquetting process compact the loose material under applied pressure (volume reduction) and agglomerates it so that the product briquette remains in the compressed state. The cohesion of the particles, is based on three main mechanisms namely (i) generating a positive coupling of particles through thin film connections, (ii) attraction forces between particles through hydrogen bonds, and (iii) creating of form-closed bonds through the sticking effect of added binders.
The basic compaction techniques used during the briquetting process are (i) pressing and (ii) extrusion. In both of these compaction techniques, solid particles are the starting materials. The individual particles are still identifiable to some extent in the final product. In both the techniques the pressing together of the particles takes place in a confined volume. If fine materials which deform under high pressure are pressed, no binders are required. The strength of such compacts is caused by van der Waals’ forces, valence forces, or interlocking. Natural components of the material can be activated by the prevailing high pressure forces to become binders.
The binding’s most common densification phenomena presents during the briquetting process are (i) the compaction under pressure of loose material to reduce its volume and to agglomerate the material so that the product remains in the compressed state, and (ii) an internal or external binding agent is necessary to prevent the compressed material from springing back and eventually returning to its original form.
The properties of the solids which are important to densification are (i) flowability and cohesiveness (lubricants and binders can impart these characteristics for compaction), (ii) particle size (too fine a particle means higher cohesion, causing poor flow), (iii) surface forces (important to agglomeration for strength), (iv) adhesiveness, (v) hardness (too hard a particle leads to difficulties in agglomeration), (vi) particle size distribution (sufficient fines needed to cement larger particles together for a stronger unit).
The material characteristics which include average particle size, particle size distribution, and binding properties influence the selection of binder for a particular application. A material with a wide particle size distribution tends to bind more easily than a material with a narrow particle size distribution since the smaller particles fills the gap between larger particles. For some materials pre-compaction of the material releases the trapped air and reduces the gaps between the particles.
Water can be integral part to the binding process either directly as a part of the binder composition or indirectly as a flow aid to help distribute the binder evenly throughout the material to be briquetted. In either case, the moisture level of the material both before and after the binder is added, affects the briquette’s ‘green’ (uncured) strength. The ideal moisture level depends on the briquetting process and the binder. As an example, the moisture content of the mixture for bituminous and sub-bituminous coal before briquetting in a double roll press can range from 8 % to 12 %.
In order to understand the suitability of the fine waste by-product material for briquetting, it is essential to know the physical and chemical properties of the material which also influence its behaviour after it is briquetted. Physical properties of interest include moisture content, bulk density, void volume and thermal properties. Chemical characteristics of importance include the chemical analysis. The physical properties are most important for the binding mechanism during the densification of the fine materials. Densification of fine materials under high pressure brings about mechanical interlocking and increased adhesion between the particles, forming intermolecular bonds in the contact area.
The binding mechanisms under high pressure can be divided into adhesion and cohesion forces, attractive forces between solid particles, and interlocking bonds. High viscous bonding media, such as tar and other molecular weight organic liquids can form bonds very similar to solid bridges. Adhesion forces at the solid-fluid interface and cohesion forces within the solid are used fully for binding. In case of materials with inherent binders (lignin) e.g. biomass, the lignin can also be assumed to help in binding in this way. Finely divided solids easily attract free atoms or molecules from the surrounding environment. The thin adsorption layers thus formed are not freely movable. However, they can contact or penetrate each other.
The softening lignin at high temperature and pressure conditions forms the adsorption layer with the solid portion. The application of external force such as pressure can increase the contact area causing the molecular forces to transmit high enough pressure which increases the strength of the bond between the adhering partners. Another important binding mechanism is van der Waals’ forces. They are prominent at extremely short distances between the adhesion partners. This type of adhesion possibility is much higher for powders. Fibres or bulky particles can interlock or fold about each other as a result forming interlocking or form-closed bonds. To obtain this type of bond, compression and shear forces must always act on the system. The strength of the resulting agglomerate depends only on the type of interaction and the material characteristics.
The physical properties are the most important in any description of the binding mechanisms of material densification. Densification of the material under high pressure brings about mechanical interlocking and increased adhesion between the particles, forming inter-molecular bonds in the contact area. In the case of biomass briquetting, the binding mechanisms under high pressure can be divided into adhesion and cohesion forces, attractive forces between solid particles, and interlocking bonds. Fig 2 shows different types of binding mechanisms.
Fig 2 Binding mechanisms
The speed of densification determines the relative importance of the various binding mechanisms. The aim of compaction is to bring the smaller particles closer so that the forces acting between them become stronger which subsequently provides more strength to the densified bulk material. The product is required to have sufficient strength to withstand rough handling. If uniform pressure is not applied throughout the entire volume of the material, it causes variations in compact density in the product.
It has been found that the room temperature properties of briquettes depends upon on various processing parameters such as solid / water ratio, briquetting force, compression time, and the particle size distribution of the feed stock. The optimum values of these are inter-related and depend on the composition of the briquettes.
Based on the compacting pressure used, the briquetting processes can be classified as (i) high pressure, (ii) medium pressure, and (iii) low pressure. In the high pressure briquetting, the pressure applied is above 100 MPa. In the medium pressure briquetting, the pressure applied is in the range of 5 MPa to 100 MPa while in the low pressure briquetting, the pressure applied is upto 5 MPa.
High pressure briquetting process uses a heating device while the other uses a binder. This process is normally carried out for materials having inherent binders (lignin) and hence no external binder is used. However, some of the materials need binders even under high pressure conditions. Medium pressure briquetting processes may or may not require binders, depending upon the raw material whilst low-pressure machines invariably require binders. All briquettes using inherent binders (lignin) or external hydrophilic binders (starch, molasses, gum, clay) are not waterproof and disintegrate when they come into contact with water or stored under humid conditions.
The briquetting of organic materials requires higher pressure as additional force is needed to overcome the natural springiness of these materials. Essentially, this involves the destruction of the cell walls through some combination of pressure and heat.
Types of briquetting machines
There are several kinds of briquetting machines available for the production of the briquettes. Their mode of operation varies from one principle to another. The briquetting machines can be basically categorized as (i) screw press, (ii) piston press, (iii) roller press, and (iv) manual press. Fig 3 shows these types of presses.
Fig 3 Types of presses
Screw press – A screw press consists of screw extruder and a die. There are three types of screw presses. These are (i) conical screw press, (ii) cylindrical screw press with heated dies, and (iii) cylindrical screw press without externally heated dies. In the screw press, the material (with inherent binders, lignin) to be briquetted is continuously fed into a screw with heated dies, which forces the material into a cylindrical die to the point where lignin flow occurs. The briquetting technology is based on the pressure of a special screw which pushes raw material within a chamber which becomes progressively narrower. The pressure is built up along the screw rather than in a single zone as in the piston machines. Binder is needed only in small amount in a screw press densification; however, it can be necessary if the required temperature (200 deg C to 250 deg C) to dissolve lignin is not achieved, or in the materials which do not have the lignin content.
The screw press has low capital cost, but high maintenance cost than the piston press because of the substantial wear on the screws, which is to be reconstructed regularly. Its specific energy demand is also higher. Screw press was initially built and used for briquetting sawdust, but it can be used for briquetting of other materials. The briquettes produce in a screw press have a concentric hole, which gives them a larger specific area. They are also homogeneous and do not disintegrate easily. Briquette density from the screw press is normally in the range of 1,000 kilograms per cubic meters (kg/cum) to 1,400 kg/cum.
Piston press – It is a mechanical press consisting of a ram (piston) and a die and it is driven by either by an electric or a hydraulic drive. The feedstock is punched into a die by a reciprocating ram with a very high compaction pressure to obtain a briquette.
In case of the piston press with a mechanical drive, the machine develops a compressive force of around 200 MPa and is typically used for large-scale production, ranging 200 kilograms per hour (kg/h) to 2,500 kg/h. The achieved briquette densities are normally in the range between 1,000 kg/cum to 1,200 kg/cum. The capacity of a mechanical piston press is defined by the volume of material which can be fed in front of the piston before each stroke and the number of strokes per unit of time. Capacity by weight is then dependent on the density of the material before compression. The moisture limit of feedstock in most cases is 15 %. However, the ideal operating region is 8 % to 12 %. A lower limit of 5 % is acceptable as anything less causes friction and hence increases energy demand. In comparison to the screw press, it has long life of wearing parts and a low power consumption rate. It also needs a higher level of maintenance and the briquettes produced are of low quality. Additionally, it generally gives a better return on investment (ROI) than the hydraulic piston press.
The hydraulic piston press operates like the mechanical piston press. However, the energy to the piston is exerted by a cylinder operated by a hydraulic system. The briquetting pressure with hydraulic press is considerably lower and this is because of limitations in pressure in the hydraulic system, which is normally limited to 30 MPa. The piston head can exert a higher pressure when it is of a smaller diameter than the hydraulic cylinder, but the gearing up of pressure in commercial applications is modest. The typical production capacities of the hydraulic machine are in the range of 50 kg/h to 400 kg/h and can tolerate higher moisture contents than the normally accepted 15 % for the mechanical piston presses. It normally produces briquettes with a bulk density lower than 1,000 kg/cum since the pressure is limited. In general, the briquettes produced have a uniform shape and size, typically 40 mm diameter cylinders, and the quality of the product is much higher compared to the mechanical piston presses.
Roller press – Roller press is considered the global standard technology to produce pillow-shaped briquettes using diverse types of feed stocks. The roller press works on the principle of pressure and agglomeration. It consists of dual cylindrical rollers of the same diameter, rotating horizontally in opposite directions on parallel axes. The two rollers are arranged in such a way that a small gap exists between them and the distance from each other depends on factors such as the feed stock type, the particle size, the moisture content, and the addition of binders. During operation, the raw material is fed into the press and forced through the gap between the rollers on one side. It is then pressed into a die forming the densified product, which comes out on the opposite side. The smooth production of briquettes using this technology needs high quality rollers with dies on which the briquettes are shaped. The type of roller or die used determines the shape of the briquettes and typical bulk densities range from 450 kg /cum to 550 kg/cum.
Manual press – Different types of manual presses exist for the densification of the materials. Some come in the form of piston or screw presses but are operated with bare hands and hardly uses electricity. Manual press is designed for the purpose of briquette making or adapted from existing implements used for other purposes. Manual clay brick making press is a good example with which briquettes can be made from the feedstock. The manual press is made from both metal and wood with the latter being the most common. These machines operate with very minimal pressure. Binder addition to the feedstock is needed. Manual presses are characterized by low capital costs, low operating costs, and low levels of skill needed to operate the machine. However, it has a low production capacity of around 5 kg/h or 50 kg/h.
Binders for briquetting
Binders have been used for briquetting coal and charcoal for more than 100 years. Starch came into prominence as a binder for coal and charcoal in the 19th century with the large scale development of briquetting. Use of cement for binding iron and steel plant waste materials is relatively a more recent application.
Effective briquetting of iron and steel plant wastes needs a binder to hold the briquette together and improve its durability. Several types of binders have been used for different raw materials, and each binder has different effects on the quality of the cold bonded briquettes. The binders can be organic or inorganic agents. Some of the identified binders of organic nature are heavy crude oil, tar and pitch, starch, and molasses. The inorganic binders include clay, bentonite, sodium silicate and cement.
As an example, pitch can be used as a binder for the mixture of mill scale and blast furnace flue dust. Another example is the use of 2 % bentonite as a binder for the mixture of blast furnace dust, converter dust, electric arc furnace dust, and sludge from blast furnace and basic oxygen furnace. Studies have also been carried out for using molasses, starch, and sodium silicate as binders for the mixture of several types of dust and mill scale. The binder types, amount of binder, and water addition, have significant effects on the thermal behaviour of the briquettes.
The most important binder for briquetting the metallurgical waste of iron and steel plant is inorganic bentonite. Bentonite is a silicate with a low melting temperature. The silicate components melt, pull particles together and promote briquetting of the iron oxide grains. Upon cooling additional solid bonds are added (recrystallzation processes). Thus, a high thermal stability of the briquettes is achieved. However, the disadvantage is, that bentonite increases the silica content in the briquettes. Further, the briquettes, whether produced with organic or inorganic binders, can be assumed to have a poor reducibility than iron oxide pellets due to their larger size and lower porosity.
The important criterion for a suitable binder for a certain application is its cost effectiveness. Starch, cement, molasses, and lime are widely used since they are inexpensive and readily available. Other binders such as alkali cellulose or phenol aldehyde resins can offer improved performance or additional benefits but they frequently cost more and can be less readily available because of location and supply constraints.
Molasses has been used since long either on its own, or with lime, particularly slaked lime [Ca(OH2)] for briquetting coal and similar materials as well as waste materials of iron and steel plant. The composition of molasses can vary widely depending on its source. Molasses is highly viscous but becomes more fluid when heated, allowing it to be more easily mixed with slaked lime prior to the briquetting coal and char fines. Typically, the molasses-slaked lime binder has ratio of molasses to lime in the range of 2:1 to 4:1.
Briquette binder plays a key role in the process of briquette production. The quality and performance of briquette also depend on the quality of briquette binder. Different types of briquette need different briquette binder. Binder used in briquetting process can be divided into inorganic binder, organic binder and compound binder. The inorganic binders have many excellent advantages, such as abundant resource, low cost, excellent thermo-ability and good hydrophilicity. However, a major problem arising from the usage of inorganic binder is related to the ash increased in significant amount. The organic binders have many excellent advantages, such as good bonding, good combustion performance, and low ash. But organic binder is easy to decompose and burn when it was heated, so the mechanical strength and thermal stability of organic binder briquette are poor, and its price is high. The composite binders are composed of two binders at least with the different binder playing the different role. The compound binder can make full use of the advantages of all kinds of binders, such as it can reduce the supplying amount of inorganic binder, reduce the cost of organic binder, improve the quality of briquettes, and get better performance of briquettes.
Binders have been classified in several ways. This includes (i) binding mechanism such as molecular forces, electrostatic forces or free chemical bonds, (ii) chemical type such as organic, inorganic, or compound, and (iii) it behaviour during briquetting. Carl A Holley in 1983 has outlined a comprehensive five group classification system for the binders. These five groups are described below.
Inactive film binders – An inactive film binder is typically a liquid solution which uses surface tension to pull the material’s particle together. It can also be a dry solid which is mixed thoroughly with the material to be briquetted before a solvent such as water or alcohol is added. In this case, the solid acts as both lubricant and glue, forming a solid bridge between particles when the solvent dries. Examples of inactive binders are water, alcohol, oils, wheat flour, molasses, starches, casein, glucose, dextrin, alginates, and gum arabic.
Chemical film binders – A chemical film binder coats the particles of a material with a thin film and causes a brief chemical reaction which bonds the particles together. The resulting briquette is frequently water-proof when a chemical film binder is used. Examples of chemical film binders are sodium silicate and dilute acid or sodium silicate and lime.
Inactive matrix binders – An inactive matrix binder embeds the material to be briquetted in a matrix (or framework) of the binder. Some inactive matrix binders such as coal tar pitch need to be heated to reduce their viscosity during the briquetting process but it sets hard when allowed to cool. Examples of inactive matrix binders are petroleum asphalt, carnauba wax, paraffin, wood tar, colloidal alumina, and metal stearate.
Chemical matrix binders – A chemical matrix binder uses a chemical reaction between two binder components for binding the material particles together. Examples of chemical matrix binders are quick lime (CaO) and water, molasses and slaked lime, and water and plaster of Paris.
Chemical reaction binders – A chemical reaction binder uses a chemical reaction to form a strong bond between the material particles to be briquetted and the binder. Examples of the chemical reaction are electric arc furnace dust mix with a water and quick lime binder, and power plant fly ash (which contains lime) mixed with water. A reagent such as dilute sulphuric acid or phosphoric acid is sometimes added to cure the strength of the briquette.
A distinction between organic and inorganic binder materials is possible. Carbonaceous organic materials can be included in the briquettes to improve the reduction kinetics (reduction rate and reducibility) owing to the presence of a larger number of reaction sites simultaneously and due to the shorter diffusion paths compared to inorganic binders. However, the addition of carbon normally lowers the strength of briquettes. Hence, briquettes normally contain less than 10 % carbon. Another disadvantage of the organic binders is the volatility of the organic substance during thermal treatment (begins at around 300 deg C).
For an application, typically, the most cost effective binder is used in the smallest concentration which produces briquettes with acceptable durability and weathering characteristics. Durability is especially important for the waste materials of iron and steel plant since the briquettes need to hold together under high temperature and reducing conditions.
Optimal binder concentration depends on the binder type, the material’s characteristics, and the characteristics of the added components. For example, with the use of pre-gelatinized starch to bind bituminous coal fines, 2 % of the binder can be used. Another example is starch in amounts ranging from 2 % to 7 % can be used for briquetting coal chars with strong briquettes produced at 4 %.
Binder is playing a vital role in the process of briquetting. It has a great influence on the decrepitation temperature of the briquette. Binder is required to have several requirements which include (i) mechanical properties which provide the briquettes protection from deformation under load, resistance to disintegration / fracture by impact and compression, resistance to abrasion, and compressive strength etc., (ii) chemical composition which does not increase silica, phosphorus, sulphur, and arsenic etc in the briquettes, metallurgical performance which maintains briquette’s metallurgical properties, such as high reducibility, little swelling, and little pressure drop during reduction, (iv) processing behaviour which means that the adding, mixing, dispersion of binder is not be complicated or essentially change conventional briquette production cycle, (v) good thermal stability, (vi) cost factor, and (vii) easy and local availability.