Starch is an essential ingredient used to make paper, and it is supplied to paper mills in packages or bulk containers. Various methods of transportation, including bags, super sacks, trailer trucks, and rail cars, are used to transport starch to the paper mills. Once delivered, starch is dispersed in water to create a uniform suspension. The process of dispersion can be achieved under atmospheric or elevated pressure. Additionally, chemical and enzymic conversion can be applied to modify starch and adjust its viscosity to meet the desired level. The content provides detailed information on each of these processes.
Delivery to the Paper Mill
Starch is supplied to the paper mill in packages or bulk containers, but shipping it as a suspension in water is costly due to the need to transport about 65% water.
Bags are shipped stacked in layers on pallets, while super sacks with lifting straps are used for handling with forklift trucks or overhead hoists. Bulk transport uses trailer trucks or rail cars.
At the paper mill, the starch is blown from the trailer into a silo, and various types of rail cars are also used for starch shipment. The starch’s dry bulk and suspension viscosity will be affected by the mode of drying.
Mechanical or pneumatic conveying systems are used to transport dry starch to and from the storage silo. Starch silos must be equipped with explosion relief decks, vacuum relief valves, and systems for monitoring starch content to prevent accidents.
Starch is discharged through a cone, and air pads or bin activators are used to prevent starch bridging.
Suspension in Water
Starch slurry is used in paper mills to make paper. There are batch and continuous systems used to make a uniform suspension of starch in water. The dry starch is added to water in a measured quantity, and adjustments need to be made for the water content of starch. The starch granules have a density of about 1.5 g/cm3 and settling needs to be prevented. The starch slurry has a high viscosity and settling rate, and preservatives need to be added to prevent spoilage.
Dispersion Under Atmospheric Pressure
When starch is heated in water, it swells and increases in volume. This creates a paste made up of swollen granules, fragments, and starch molecules. The paste is dispersed in large tanks with steam or heat, and the tanks are stirred to prevent vortex formation. The heating time required is at least 20 minutes at 95°C. Retrogradation can occur in some pastes, which requires holding the paste at a temperature above 91°C or quickly cooling it to 66°C. Dispersion of residues from atmospheric pressure cooking requires higher temperatures, jet cooking with a high rate of shear, autoclaving, or the use of alkaline solutions. Adding a strong base to the suspension can obtain complete colloidal dispersion of starch. Alkaline dispersions of starch are used to make adhesives for corrugated board.
Dispersion Under Elevated Pressure
Starch molecules need to be completely dispersed to be an efficient binder. This can be achieved by injecting steam into a starch slurry while it passes through a mixing device against back pressure. The high-velocity steam mixes with the slurry and raises its temperature rapidly. The equipment required for this process includes a positive displacement pump for the starch slurry, a back-pressure valve, and means to control temperature and pressure. Excess steam is used to further improve efficiency. Jet cooking of starch is a continuous process that can be activated or terminated by level controllers in the receiving tank for starch paste. The products of starch cooking with retention are similar to those obtained by dispersion with excess steam.
In most cases of cooking starch at high temperature, a slurry with higher solids content is processed than is needed for its application. Excess steam cooking is required to disperse bleached starches for use on the size press of the paper machine. Slurries of bleached starch are dispersed at high concentration and diluted to a discharge concentration in the range of 2 to 14% at a discharge temperature of 60-70°C. Jet cooking is also used to obtain complete dispersion of certain modified starches, especially starch ethers, and for enzyme inactivation after enzymic conversion of starch. Pigment and heat-stable coating adjuncts may be added to the starch slurry for processing through the cooker jet or added to the dilution water. The specific heat of the additives needs to be considered in the thermal balance for the system.
Chemical Conversion
Starch can be modified using a combination of dispersion, viscosity reduction, and chemical modification processes. In one common process, ammonium persulfate or other oxidants are used to reduce viscosity in a starch paste, which is created by injecting high-pressure steam into starch slurry. Protons released during this process further break down the starch, and a catalyst may be added to promote reactions. The resulting paste must be neutralized and may require the addition of a sequestrant to prevent colloidal destabilization. Starch slurries for this process are created by dispersing dry starch in cold water and maintaining suspension through agitation and pumping. The resulting paste is slightly alkaline and can be cooled by dilution water or heat exchange. Cationic starch can also be prepared by blending starch with an alkali and a cationic monomer, then controlling temperature and retention in a second reactor. The degree of substitution can be controlled in this process. Some paper mills have attempted to produce a cationic product on-site by complexing starch with a cationic polymer or cooking it with a quaternized fatty amine, but these methods may result in unreacted reagent or increased COD in the process water.
Enzymic Conversion
Starch viscosity can be adjusted to a desired level using amylases. Alpha-amylase breaks down starch molecules into smaller products, including maltose and limit dextrins. The enzyme is most active at pH 6.3-6.8 and is inactive at pH below 4 or above 9. Enzymatic conversion can be stopped by raising the temperature or by using enzyme poisons or oxidizing agents. Corn starch should not contain more than 0.4% protein for enzymic conversion. Enzymatic conversion can be conducted in manual batch, automated batch, or continuous processes. In the manual batch conversion process, starch is suspended in water, enzyme is added, and a time-temperature cycle is applied to produce the desired viscosity of the product. Automated batch process requires two tanks and proportional blending of dry starch and water. The slurried batch is pumped into the conversion tank and enzyme is added. Deactivation time depends on the concentration of the starch paste.