Posted by un_homme_de_nuit
Sure I wouldn't mind a full explanation!
Still shocked that the company left all of their machines inside without any effort to sell them. (Maybe paper making equipment isn't worth the hassle to sell, or there's a significant decline in the pervasiveness of the paper mill)
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the paper industry in north America is a declining industry.
I worked at what is called a Kraft Process Integrated Paper Mill. Here's how it works. Logs and woodchips come in by truck and are sorted hardwood or softwood. The logs get sorted on one side of the flume, chips at a chip station on the hill. The logs get thrown/loaded into a flume full of water and floated up to the debarker where the bark (duh) is pulled off the log. That bark goes to another pile and then sent via pipeline to the powerhouse to be burned as fuel. so now the log is debarked it meets its doom at a huge chipper. Let me put it this way, if you went through this thing, there would be a puff of pink and you'd be no more. ;)
the chips and chipped wood are carried up a conveyor to the top of the chip silos. At our facility there are two hardwood and one soft wood silo. The chips come down the silo into a cone and then into a shaker table that drops them on to another conveyor, what we call the 'short belt'. At this point we can run both hardwood silos (which we usually do) or shut them down and run the softwood silo (depending on where we are in the digester). So the chips go down the short belt, meet at an exchange and are dropped on to the long belt that takes the chips to the chip cyclone. From the cyclone they make their way into what we call a steaming vessel. This gets the chips 'gooey' and ready to be introduced to the top separator of the digester.
The digester at the plant I worked at is best thought of as a 187ft tall stomach. Steamed chips go in the top along with what we call 'white liquor'. White liquor is a powerful sodium hydroxide caustic we make on site (more on tha later). This chews up the lignin that holds the wood fibers together in the chip. From there, the chips make their way down the digester alternating their way through more screens, extraction zones, steam and more liquor. By the time the chips reach the bottom and head to the blow tanks, the liquor is no what we call 'black liquor' as it is full of fats, lignin and other debris from the chips. The 'black liquor' gets pumped off to the powerhouse to be burned in what is called a recovery boiler. This is where things get interesting.
Black liquor goes into different tanks and the recovery boiler operator sends it to different evaporators where it is then introduced to the recovery boiler. It gets burned inside of it turning to what is called smelt. That smelt makes its way out of the bottom of the boiler and into a 'receiver tank'. Here the smelt meets water and a solution we call weak wash and that solid gets beaten back into a liquid called 'green liquor'. That green liquor gets pump to the recausticizing plant next door. It ends up in a huge tank called a clarifier where the carbon and other impurities are settled out of the green liquor. Those impurities are pumped are called 'dregs' and get pumped to a dregs filter where all the liquid is pulled out, sent to the weak wash tank and blade scrapes whats left into a screw that carries the solids out to a pad where they are loaded into trucks for delivery to the landfill. Ok, back to the liquidy stuff... the green liquor from the clarifier is pumped into storage tanks and then later gets sent to the slaker.
Now think of a slaker as a big caustic Kool Aid pitcher. It's about 15,000 gallons and sits just beside the kiln. Now the liquor gets pumped to this where it meets lime (Calcium carbonate- my chemistry is a little rusty as I haven't done the job in almost two years). The hot lime (500+F) meets the liquor (heated via steam at what we call a pick heater) meet in the slaker and mix forming a caustic slurry. This slurry overflows into #1 causticizer then to #2 and then finally #3. The whole time it is in these tanks, it's getting agitated and gaining strength. At the end of that, it reaches what we call the white liquor receiver tank. From there, pumps pump it across to the white liquor clarifier. This is where, though a chemical additive, the mud settles out of the slurry (underflow) and the white liquor (overflow) gets pumped to storage tanks and from there, sent back to the digester. It's a closed loop.
Now, about the mud (underflow)? that gets pumped all the way back to what we call a mud washer tank (yes, mud washer) What happens there is the mud is mixed with water and weak wash (a weak caustic solution). Again, you have an underflow and overflow situation. The overflow is called weak wash and that goes to a storage tank. We use that for pH control, emissions maintenance and make up on different systems. It's important because our weak wash is pumped to not only the pulp mill, but the powerhouse as well. Now the underflow, mud, gets pumped back to a mud storage tank. From there, it's still a slurry, and is pumped to the mud filter receiver tank and then to the mud filter itself. Think of this as a drum covered in screen that uses a vacuum pump to pull the liquid out of the mud. That liquid goes back to mud washer for use in that process. The mud itself comes over the drum as a semi-dry cake, is scraped off by what we call a doctor blade, meets the upper screw that feeds the lower screw that feeds the kiln.
The kiln is almost 300ft long, 9 feet in diameter and rotates... the mud is 'cooked' on its trip down the kiln and changed from one form of calcium carbonate to another. That lime drops out the front into a grizzly (breaks big chunks into little chunks) and then into a lump breaker (again, makes the product smaller) and that break dumps to a shaker into the bucket elevator that dumps the product into the hot lime silo to be reused.
Now, going back across the road to the pulp mill, the chips are now what we call pulp as they are pumped into the knotters. These removed uncooked wood and other debris, separating it where it then flows into a dumpster in the basement to be dumped in the landfill. At this point the pulp is pumped to either #1 or #2 blow tank. From there, the pulp makes its way into the two pulp washers. Again, the pulp is pumped into these devices were huge vacuum drum covered in mesh pick up the pulp and suck the remaining liquor out of the pulp. The #1 washer feeds the pulp to another screw and that pulp gets fed to #2 washer. The whole time, the pulp is having the remaining liquor washed out of it. From the washers it goes to the screen room which seperates more large debris and from there (after a series of tanks and screens), the pulp meets the deckers. The basically work just like #1 and #2 brown stock washers, again, washing more liquor out of the pulp. At this stage the brown pulp goes to the Brown stock Hi-D silos, one for hardwood one for softwood. This is their last stop before the pulp hits the bleaching plant.
Now no two mills are alike. That is something to keep in mind when I describe the process here. The process we follow uses Chlorine dioxide as the primary bleaching agent, but the bleaching process is multistage. There are 5 'washers' on the operating floor- 15, 25, 35, 45 and 55. Now I am going to give you the speed of light version of how this works... 15, 35 and 55 are ClO2 stages, 25 and 45 are caustic. The ClO2 stages bleach the pulp, the caustic stages act as a wash to get any of the impurities out. And each washer feeds a corresponding retention tower. These are huge 100ft towers that allow the bleaching chemicals to work on the pulp. In paper manufacture, especially the paper we make, brightness is king. You want your paper as bright as possible. At the 55 stage, we add a couple of different chemicals to make the pulp as bright as possible. From that final washer, the pulp is pumped to one of four storage silos. 7,8,9 and 10. Numbers 7 and 10 are for hardwood pulp, 8 is a swing silo (from when swing the process from hardwood to softwood or vice versa) and 9, the smallest silo is for softwood. After this stage, the pulp goes to paper machines.
The pulp passes to the additive room where different chemicals are added to make whatever grade of paper. That pulp now goes out on to a fourdenier, or a wire, from a headbox. The wire is vibrating so it can align the wood fiber before it hits the first set of dryer cans. The dryer cans are just what they sound like, rollers heated by steam from the power house. Now it's starting to look like paper. Between #1 dryer section and #2 is the starch press. This where a form of corn starch is added to the paper to help durability and formation. From there, on to another dryer section. Now when it comes off the last dryer section, it meets the calendar stack. This is a press that sets the thickness or 'weight' of the paper. Now keep in mind that this whole sheet, all 350ft of it is being pulled by the winder at the front end. The sheet comes out of the stack and wound on to a reel which can way 60 tons+. The winder kicks the done reel off and auto loads another reel. The crane operator hooks the crane to the finished reel, spins it 180 degrees and loads into on to the cradle for the rewinder. This where the reel gets cut into rolls for shipping or processing downstairs. This is one of the most dangerous departments in a paper mill to work at. Once the reel is sliced into rolls, the rolls are put on a belt, go through a series of interchange belts and scales and are lowered to the finishing room. This is where the will either be made into free sheet paper or wrapped to be shipped to a customer or other conversion site. That was my last job there, roll ramp operator. I wrapped the rolls to be shipped. From there, the rolls go down my belt into the warehouse and on their way.
I worked in that place for almost 11 years and saw it all. ;) Sorry for the overlong post. Believe it or not, I actually skipped some stuff?
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