1. From Wood Yard
Wood handling is largely the same for all pulp processes. The wood arrives at the mill in the form of roundwood or, if it comes from a sawmill, in the form of chips. The roundwood is barked in large, slightly sloping and rotating drums, 4-6 metres in diameter and up to 60 metres in length. The bark loosens when the logs are rubbed against one another and against the walls of the barking drum. The bark is then used as fuel in the steam boilers at the mill. The barked wood is chopped into chips in a chipper. They are screened for size and any pieces that are too big are chopped up and returned upstream of the screen. The chips are stored in piles to the storage area.
Pulp Mill
The pulping process described here explains the sulphate or Kraft process, which is the most common. Another chemical process that is used is the sulphite method. Sulphate processing used in caustic based cooking liquor, while the sulphite uses an acid based chemical (red liquor). The packing recommendations shown will work equally as well in both systems. There are several types of ground wood pulping, but these require very little packing and will not be described in this report.
Wood chips are conveyed from the chip storage area to the prepared chip silo. From there they are taken to the digester area for processing. There are two types of digesters used for cooking the chips, batch and continuous. Both use a caustic based liquor to process the chips.
2. Batch Digester
In batch cooking the digester (cooker) is filled with chips, white cooking liquor and weak black liquor. The cooking liquor, which is drawn through screens, is circulated with a pump to a heat exchanger, where the liquor is heated with a steam before being returned to the digester.
The chips are cooked for about three hours in this caustic atmosphere. The impurities are absorbed by the cooking liquor and it becomes black liquor. This cooking action destroys the bond between the cellulose fibres and the glue-like material called lignin that cements them together.
3. Continuous Digester
This type of digester differs from the batch type in that the chips are processed in a downward flow through zones of steadily increasing temperatures and pressure until the cooking zone is reached. The chips remain in this area from two to three hours, and the mass is cooled, mixed with black liquor, and mechanically conveyed or “blown” to the blow tank.
Cooking liquor is continually circulated from the digester to heat exchangers where the liquor is reheated and reinjected into the digester. The ph of the cooking liquors is 13.5 to 14, and the operating temperatures are 240°F at 140 psi.
4. Blow Tank
The blow tank is a vertical vessel with the pulp-entering nozzle located on the tangent. As the pulp enters at high velocity, the shock breaks the final bond between cellulose fibres and the Lignin. The material in the tank is very thick and viscous and must be diluted before it can be pumped. Weak black liquor is returned from the washers to dilute the pulp.
5. Brown Stock washer
These washers are usually mounted in a series of four washers and use a counter-current flow to separate the black liquor and the impurities from the pulp. The pulp is washed with hot water, and care is taken to dilute the liquor as little as possible while removing the bulk of the chemicals. The chemicals and liquor, now called weak black liquor, is sent to the blow tanks for dilution.
After the stock has been washed, it leaves the brown stock washers and either goes to the prepared stock chest or to the bleach plant, depending on the final product to be made.
Fluids used in Chemical Pulping
White liquor Cooking liquor, pH 13.5 to 14, hot and abrasive.
Conc. black liquor Cooking liquor, pH 13 to 14, contains 50% solids.
Weak black liquor Black liquor and solids, pH 10 to 12, solids.
Water Both hot and cold.
Condensate from blow tank
Pulp and black liquor
Washed stock 3 to 4 % consistency.
Cleaned stock 3 to 4 % consistency.
6. Bleach Plant
Most pulping operations produce a material that is not suited for the production of white paper, so it is necessary to chemically treat this brown stock to the required whiteness. Lignin-removing bleaching is the most widely used method for chemical pulps. According to this method, chemicals are used which remove the lignin so that the natural white colour of the cellulose fibre emerges. To accomplish this, a series of treatments are used with a washing stage between each treatment stage. The common stages are Chlorination, Alkaline Extraction and Chlorine Dioxide.
Fluids used in Bleaching
Liquid chlorine pH 1 at 50°F.
Chlorine water pH 2, clean.
Bleached stock 3% to 5% consistency, abrasive.
Mill water 35 to 100 degrees clean.
Sodium hydroxide 50%, pH 14, 140°F.
Chlorine dioxide water pH 3 to 4.
Brine Salt and water, pH 11 to 13.
Spent acid 50% H2SO4, pH 1 to 2.
Paper Mill
Stock received in the paper mill area from the pulp mill can be either brown or bleached depending on the final product. The stock is held in an agitated tank to prevent settling out of the fibers. The stock in this chest is usually at a concentration of 5%. There is no such thing as a standard paper mill. However, the steps in the paper-making process can in principle be divided into four general areas of stock preparation; paper-making; coating and laminating; and finishing.
7. Stock Preparation
Stock from the storage area is moved into the beater room where the stock is refined and
treated before it goes to the paper machine. The first step is to place the stock in the beater tanks where the fibers are separated and toughened up. From the beaters, the stock goes to the refiners where the material is cut to uniform length. There are several different types of refiners but they all provide the same function. The most common are deflakers, which break up fiber bundles, jordans, conical refiners and disc refiners. The refined stock is cleaned by centrifugal cleaners, screened and pumped to the machine chest.
Fluids used in Bleaching
Clean water
Stock chest pulp 2% to 5% consistency.
Beater room, pulp 1/2% to 1% consistency.
White water 1/2% consistency or less, abrasive.
Aluminum Various concentration, abrasive.
Paper Making
8. Paper Making
The machines used for the manufacture of paper are technically, highly sophisticated. The biggest of these machines (called "Fourdriniers") have a width of 6 to 10 meters and a length of up to 200 meters. Another type of machine that is commonly used is the cylinder machine where a sheet is formed, and this is picked up by a felt, which carries it along the machine. There are many different chemicals, coatings, and pigments that are added to the paper as it moves along the machine. The most common are clay, latex, titanium dioxide, size and dye.Despite variations in their construction, all of these paper-making machines contain the same basic elements: headbox; wire section; press section; dryer section; and reel. The actual design of these elements depends on the type of paper being made.
Fluids used in Paper Making
Clean water
White water 1/2% consistency, abrasive.
Head box pulp 1/2% consistency.
Couch pit pulp 1/2% to 5% consistency.
Condensate from dryer rolls
Clay slurry
Alum
Latex
Rosin size
Starch
Chemical Recovery
The sulphate process is based on a cycle principle ensuring that the raw materials are used efficiently and that emissions and energy losses are minimized. During the cook the cooking chemicals have been transformed. Recovery of these chemicals is absolutely essential for both environmental and economic reasons. This is accomplished in the chemical recovery department at the mill. The recovery system at the sulphate pulp mill consist of evaporation, recovery and causticization. Furthermore, the black liquor coming from the pulp wash also contains lignin from wood and other dissolved organic matter. This, too, is taken care of in the mill's chemical recovery system (power house) and is used as fuel in the recovery boilers.
9. Evaporators
Weak black liquor, 15% to 18% solids, is recovered from the brown stock washers and pumped to multi-effect evaporators. This black liquor contains most of the sulphate chemicals and the organics from the wood. Because the chemicals can be saved and the organics burned for fuel, most of the water must be evaporated. Each of the effects is a vertical heat exchanger and some designs operate under a vacuum.
Weak black liquor is pumped into the system at 15-18% and leaves at 50-55% solids. As there is still too much water for combustion, the liquor is put through a finisher which increases the solids content to 63-65%. This liquor is known as heavy black liquor or concentrated black liquor and is pumped to the recovery boiler for burning. Each of the effects has a calculating pump that takes feed from the bottom and pumps it to the next unit.
Fluids used in Evaporators
Clean condensate
Dirty condensate
Weak black liquor 15% to 18% solids, pH 10 to 12.
Black liquor 35% to 50% solids, pH 12 to 13.
Heavy Black liquor 63% to 65% solids, pH 12 to 14.
Tall oil 10% solids, pH 9 to 11.
Recovery
10.Recovery
Heavy black liquor is pumped to a black liquor storage tank at the recovery boiler where it is mixed with salt cake, sodium sulphate, as a make–up chemical to replace the chemicals lost during washing and evaporation. The black liquor is then pumped by the nozzle pump into the boiler where the black liquor is vaporized and burned. The organics in the liquor burn as fuel, while the chemicals fall to the bottom of the boiler and flow out as smelt. The smelt flows into a dissolving tank filled with weak wash liquor from the causticizing area. The smelt is agitated and recycled to break up the molten smelt and prevent an explosion. The liquor in the dissolving tank is called green liquor.
Causticizing
11. Causticizing
Green liquor from the dissolving tank is pumped to the causticizing area where it is treated with milk of lime, calcium hydroxide, to form white liquor. As green liquor contains impurities called dregs, it first must be filtered in a clarifier. The clarified green liquor is pumped to the slaker where it is mixed with burnt lime, calcium oxide. The lime-green liquor mixture flows to two or three causticizers in a series to complete the reaction. The liquor is separated from the lime mud and becomes white liquor. The calcium carbonate precipitate is burned in the lime kiln to form calcium oxide for use in the causticizing area
Fluids used in Causticizing
Green liquor Abrasive.
Green liquor dregs Abrasive and hot, 250°F.
Water
Weak liquor Abrasive.
Lime slurry 10% to 20% solids.
White liquor mud Abrasive, pH 13.5 to 14.
White liquor pH 13.5 to 14.
Boiler
12. Boiler
Almost every paper mill has a power boiler in addition to a recovery boiler. These boilers are usually multi-fuel and burn anything from coal to wood bark. Most mills have on-site generating plant and produce all of the electricity they use. Make-up water for the boiler is treated and pH adjusted to help prevent scale build-up in the boiler tubes. All condensate is collected, deaerated, and returned to the boiler water make-up tank. Water is pumped into the boiler by a multi-stage, high pressure boiler feed pump. The water is heated in the generated tubes and is flashed to steam in the steam drum. The steam is guided over blades in the turbine generator and electricity is produced. The spent steam flows across a water cooled condenser and falls as condensate into a hot well. The condensate is pumped through a preheater where the temperature is raised to level required by the boiler feed pump. The water is then pumped back into the boiler and the process starts over. The same water flow as described above occurs in a recovery boiler but a recovery boiler is normally a low pressure boiler.
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