Binder for the Cold Consolidated Pellets
Many materials may require binder in the production of cold - consolidated pellets. The choice of binder is varies from different and processes. According to our practice, we are going to make a general description of the choice of binder.
1. Principles of selection
In general, the selection of binders should follow these principles:
• Available in the local market.
• If it is suitable for the material.
• Lower prices.
2. Binder types
| Inorganic Binder | 1. Sodium silicate (R2O·nSiO2) 2. Bentonite 3. Refractory cement 4. Standard cement 5. Quicklime 6. Slaked lime 7. Magnesium chloride | 1. Lower price 2. Better strength under high temperature 3. Increase ash 4. Adding amount is relatively high (Generally 3%-10% for different materials) |
| Organic Binder | 1. Syrup, pulp waste liquid 2. Sodium carboxymethyl cellulose (CMC) 3. Starch glue 4. PVA(Polyvinyl alcohol) 5. Asphalt 6. Petroluem coke 7. Peridur (peridur XC-3) 8. Sodium humate | 1. Less ash involved, achieve the most evaporation under high temperature 2. low strength under high temperature 3. Adding amount is relatively low (0.3%-10%) 4. Relatively high price |
| Composite Binder | 1. Syrup+slaked lime 2. 0.4% CMC + 8% Bentonite 3. Other kinds | 1. It has a good comprehensive performance 2. Relatively high price |
3. Description of inorganic binder performance
3.1 Sodium silicate
The chemical formula is R2O·nSiO2, where R2O is alkali metal oxide, and N is the ratio of the mole number of silica and alkali metal oxide, which is called the friction number of sodium silicate.
Technical index：
| Silicon Dioxide % | Sodium Oxide % | Water insoluble substance % | Iron % | Baume Degree | Modulus | Soluble solids |
| ≥24.6 | ≥7.0 | ≤0.2 | ≤0.02 | 0.35-0.37 | 3.5-3.7 | ≥99 |
| ≥26.0 | ≥8.2 | ≤0.38 | ≤0.09 | 0.39-0.41 | 3.1-3.4 | ≥99 |
| ≥29.2 | ≥12.8 | ≤0.36 | ≤0.08 | 0.5-0.52 | 2.6-2.9 | ≥99 |
| ≥25.7 | ≥10.2 | ≤0.38 | ≤0.09 | 0.44-0.46 | 2.2-2.5 | ≥99 |
| Notes: 1. The module 3.3 and module 2.4 used more in the market. 2. The price is about（800-1400）Yuan/t, it depends on its modulus and concentration. 3. Supply method: Drums with usually 300kg and 1500kg. Tank trucks are also used. | ||||||
Silicon dioxide is more commonly used in the cold consolidated pellets as its relatively low price.
It is effective for most metallic and nonmetallic powders and is generally added in the range of 3% to 10%. Cold consolidated pellets have a good high temperature strength, but some materials are restricted because they bring in more harmful elements.
Water is normally added to dilute the silicon dioxide，The mass ratio of water to silicon dioxide is 4∶ 6, or 3：7. Some will modify the sodium silicate treatment,by adding appropriate additives to improve its bonding performance, thereby reducing the use of sodium silicate.
At present, we use the following materials: manganese metal, manganese ore powder, inconel iron, red nickel ore, iron-containing dust dust, iron oxide, etc., all of which use silicon dioxide as binder to prepare cold consolidated pellets.
3.2 Bentonite clay
Bentonite is a nonmetallic mineral with montmorillonite as the main mineral composition, and it has strong hygroscopic and expansive property：
① Strong water absorption is to control the free water and water regulation to increase capillary force.
② Lubrication increases the molecular force, and this lubrication functioning as：One is to reduce particle friction, increase pellet plasticity, reduce porosity, increase pellet density and increase pellet strength. Another function is, Montmorillonite can be coated on the surface of iron-containing powder particles and nianping particles to reduce the surface roughness, reduce the contact distance between particles and increase the attraction between particles. The attraction between particles comes from van der Waals forces (intermolecular attraction), electrostatic forces, and magnetic forces.These forces are small in most cases, but increase significantly as the distance between particle size decreases.
③ The surface electronegativity increases the electrical adhesion. Bentonite binds the iron powder together through colloidal electrical properties and electrical molecular forces.
Bentonite is widely used in the preparation of oxidized pellets and has been used in cold consolidated pellets, but it is not widely used. We believe that the moisture content of more cold consolidated pellets is actually lower than that of oxidized pellets, and bentonite is only effective when it meets water.
The current market price of bentonite is about (500-800) Yuan/t. Pellets are commonly used to have two kinds of sodium base and calcium base. Because Ca2+ has twice as much positive charge as Na+, Na+ number is twice as much as Ca2+ number under the condition of similar surface charge of clay minerals. Therefore, the water absorption capacity of sodium bentonite is higher than that of calcium bentonite (the former water absorption rate is 600%~700%, the latter water absorption rate is 200%~300%). The expansion ratio of sodium bentonite is 15~20 times higher than that of calcium bentonite.
Bentonite should be crushed or rolled, dried and finely ground before use. The average natural moisture content is from 30%(25%~50%) to 7%~8%. To keep the bentonite active, the drying temperature should not exceed 150℃. The ground bentonite is less than 0.044mm grain size accounting for at least 90%, equivalent to a specific surface area of 8000cm2/g. Bentonite should be transported in sealed containers, such as pneumatic, tank truck or bagged.
In the preparation of cold consolidated pellets, single bentonite is used as binder. For example, Longmen Iron and Steel corporation in Shanxi, China, mixes solid waste such as blast furnace dust, steel slag powder, converter dust and iron scale with bentonite to make cold consolidated pellets. Also there is use of bentonite and other additives together as a binder.
3.3 Slaked limeCa (OH)
Calcination is the formation of calcium hydroxide (Ca(OH) particles after digestion with water or steam. It is a kind of adhesive with small particle size, large specific surface area, good hydrophilicity and cementation ability. The addition of lime had a good effect on the compressive strength, falling strength and dry ball strength of raw ball. However, the good effect of compressive strength was only found on concentrate with large specific surface area, but had little effect on the compressive strength of raw ore. The binding force of slaked lime is worse than that of bentonite, and the binding force of slaked lime added with 2% is equal to that of bentonite added with 0.5%. In addition, the density of slat is small and the adding quantity should not be too much. Otherwise, according to the volume, the proportion of slat in the material is too large, which will reduce the migration speed of capillary water in pellets and affect the pellet forming speed.
Lime is usually used as a binder in conjunction with organic binders such as molasses.
3.4 Quick lime（CaO）
Quicklime is the product of calcined limestone at 900℃. The bulk quick lime (CaO) needs to be broken and finely ground before it can be used as pellet binder. The quick lime needs to be fully digested after adding the mixture before it is suitable for pelletizing. The former Soviet Union studied the use of active quicklime instead of bentonite to reduce impurities brought into pellets. The experimental results showed that the iron content of pellets could be increased by 0.4%~0.8% by replacing bentonite (0.7%~0.8%) with active lime (1.2%~1.5%), but the strength of pellets was lower than that of bentonite pellets, and the ultimate drying temperature of pellets was also lower than that of bentonite pellets.
With quick lime as binder cold consolidated pellets is needed to carefully use for materials such as furnace dust sludge with high water content, the quick lime can rise to reduce the effect of moisture, but it needs enough digestion time and the process is complicated. If lime is not good at digestion, so there are risks of cracking cold consolidated pellets. In practice, it is possible to use quicklime as binder when treating materials with large moisture content, but usually it cannot be used alone.
3.5 Magnesium chloride
Magnesium chloride is not recommended as binder , although it has some rapid hardening, spontaneous heat and other unique properties, but it contains salt halide and other harmful substances which should be used with caution. Some enterprises use magnesium chloride as binder in metallurgical dust ash cold fixing pellets, but the current users are very few.
3.6 Refractory cement, standard cement
Cement is not recommended as cold consolidated pellets binder, because it may increase the ash content and bad elements, but it can be used for some materials, we've done a silica pressure experiment for one of our Canadian client, the suitable raw material moisture on (3-5) % of cement has a good effect of ball formation, it comes with the extension of curing time and high strength of pellets.
4. Description for the organic binder performance
4.1 Industrial syrup, pulp waste liquid
The use of industrial syrup as binder has a significant relationship with sugar content. The syrup has a rich diversity, and the bonding ability is directly affected by the sugar content, which is C12H22O11. here. There are usually optical and chemical methods to measure sucrose content, but they are complex and difficult to be used in practice. Therefore, the method of arranging industrial samples is generally adopted, and the strength of the final ball is used to judge the advantages and disadvantages of the syrup. For the same quality of syrup, the amount of addition is usually determined by the pressure ball experiment.
Pulp waste liquid is a dark brown thin paste liquid, soluble in water, specific gravity 1.2-1.5. The technical index of different manufacturers will be different. And a technical index of a manufacturer is provided here: water content 68.9%, dry matter 31.1%, in which water insoluble matter < 0.652%, mineral substance 9.96%, PH >7.
4.2 Peridur
Peridur is an organic binder derived from a natural cellulose. Its appearance is a loose white powder. It is mainly composed of long chain molecules containing a large number of hydroxyl groups. Cellulose is a polysaccharide in which the repeated anhydrous glucose units are linked by glycoside bonds (-1,4 sugars). Peridur is prepared by replacing the hydrogen atoms of the three hydroxyl groups of anhydrous glucose in an appropriate manner which is soluble in water. By physical adsorption, water molecules are bound around their carbon chains and thus have strong properties of fixing water. 1g Peridur can bind 4.95g water, 5~10 times as much as bentonite (0.66~0.91g water). So it helps to improve the strength of the ball without slowing it down. Peridur is made from cellulose, so it is non-toxic and contains no elements harmful to the environment or smelting, such as phosphorus and sulfur. Peridur does not contain silica, so the iron content of pellets will not be reduced by the addition of Peridur as a binder. To produce pellets with similar quality, the addition amount of Peridur is only 10%~20% of that of high-sodium Wyoming bentonite. Peridur is burned when pellets are fired, so the porosity of pellets increases and the reduction of pellets is expected to increase by 20%~30%. Peridur XC-3, produced by the Dutch company ENKA, has been used in industrial production.
