Creating Pulp
Posted Oct 24, 2023 06:55 PM
The Pulp Making ProcessThe process of turning wood into pulp isn't very complex, but requires many different operators working together in order to produce. Pulp is used to create one of the worlds most used commodities, paper. This blog will be a brief run-down on the cycle used to create pulp and the various byproducts needed in order to constantly recycle cooking chemicals.
Some things that will be required in order for this operation to work, are different departments which specialize in each part of the cooking process. With that, we need to start with a wood-yard. The wood-yards main focus is to maintain wood inventory and to move and break down logs as needed for operations.
Logs will be stripped of its bark and cut down into wood-chips to be sent to two different departments for use. After grinding the bark, it is sent by a conveyor belt system from the wood-yard to an industrial bark boiler located at a power & recovery department. The power & recover department is responsible for providing the mill with water and steam which to power the mill, making the process possible. It also is responsible for handling waste & water treatment to ensure any water sent back to the river systems is the same or better than when it was pumped into the mill. The bark will then be fed through chutes onto a slowly rotating grated bed to burn and produce steam which will be used to power the mill. This allows the pulp mill to use wood chips in a piece of equipment called a Digester, which will separate the lignen molecules in the wood allowing the creation of pulp.
Lignen is what we call the fibers in wood that hold it together.
Before we can do this though, we would need the chemical to cook the pulpwood in for the pulp. In this case we will be needing white liquor. To make this, we will need an industrial recovery boiler, evaporator system and lime kiln. After pulp is made, the lignen residue and uncooked white liquor is returned to the power & recovery department into a storage referred to as Weak black liquor, or also know as weak black soda. The white liquor, originally an amber color similar to a Coca-Cola will contain that lignen residue and would turn black after being removed from the digester. The weak black liquor has a high water content, so the amount of 'solids' which are used during the recover boiler cooking process are determined by how weak the liquor is. We can increase the solids of the liquor by running the liquor through evaporators.
By running weak black liquor through evaporators, it will do as it says and evaporate water from the liquor, leaving behind a higher solid content in the liquor. The liquor is measured with a hydrometer and whenever it meets standards it is stored in a Heavy black liquor storage vessel for later use. The heavy black liquor is pumped to the recover boiler area and fed through another set of evaporators called Cascade Evaporators. There is a large sort of turbine inside constantly and slowly turning and churning the liquor within. However, instead of blades, it contains pipes that are constantly turning and within the pipes is hot condensate, which heat and evaporate any remaining water. Once you reach around 68% solids, you can use it in the cooking process. The liquor will be sprayed into the boiler using liquor guns, at the bed the liquor will cook and begin to form smelt. Smelt has an appearance similar to lava. The black liquor (smelt) pours out of the boiler through a series of spouts and falls into a Dissolving Tank. Within the dissolving tank, there is weak wash sent from lime kiln, which is where we'll make our white liquor for the digester. But first, whenever the black liquor falls into the tank and combines with the weak wash, it turns into Green Liquor.
After we have our green liquor we are ready for the lime kiln to do its job to complete the cooking process. The lime kiln operator will pull the green liquor from the dissolving tank and move it to a clarifier for storage and testing. The lime kiln operator must monitor different aspects of the green liquor for environmental and safety reasons and procedures. The operator will manage the alkalinity and sulfidity of the of the green liquor, and one the required values are met, it will be sent to a slaker to combine with lime produced from the kiln. This combination forms a chemical reaction that results in the green liquor becoming White liquor. The white liquor and pulled from what is now lime mud and sent to the pulp mill digester.
After going through clarifiers to remove as much white liquor from the lime mud as possible, the lime mud is sent to a mud storage and agitated to avoid hardening. The mud is pumped to a lime mud filter with a large rotating drum. This drum has an internal vacuum that will suck any remaining moisture, liquor or water, from the lime mud, drying it out before screw feeding it into the lime kiln to be cooked back into pure hot lime.
Now that we have our white liquor and pulpwood, they can be fed to the digester and the resulting byproduct is pulp.
