One of these is lime. As noted in T 617 - Analysis of lime, “two forms of lime are used in the pulp and paper industry: quicklime or unslaked lime and hydrated or slaked lime.” We have discussed these in past posts, but to clarify: quicklime (calcium oxide, CaO) is a white crystalline mineral that is derived from the rapid thermal decomposition of limestone and generally obtained in a kiln; slaked lime (calcium hydroxide Ca(OH)2) is a somewhat-similar-in-appearance white powder that derives from the mixing, or “slaking”, of quicklime with water.
Each of these forms of lime is incorporated into a different pulping procedure. Ultimately, the primary goal of pulping is to dissolve away the lignin that binds together the cellulose fibers of wood. Each process makes use of lime in different ways to fulfill this task.
Lime in Kraft Pulping
Kraft pulping (also known as the sulfate process) is the most widely used pulp process. During this procedure, wood chips are digested at elevated temperature and pressure in “white liquor”, essentially a water solution containing different chemicals that dissolve the lignin. Since kraft pulping is often done in batch digesters, after going through a process of cooking, washing, pressing, and drying, the cooking chemicals and heat are captured and used again in the same process to dissolve more lignin.
Slaked lime is essential to this process, since it is one of the main chemicals present throughout its different steps. The kraft process begins by mixing caustic soda (NaOH) with the initial wood chips under high heat and pressure. After attacking the wood’s lignin, the caustic soda is converted to sodium carbonate (Na2CO3), which is then pumped into a vessel containing slaked lime. With the introduction of slaked lime, the two react to form calcium carbonate and caustic soda, the latter of which is once again mixed with wood chips to continue the process all over again.
However, the cyclical nature of the process doesn’t end with this stage. The calcium carbonate is heated to form quicklime, which is then slaked so that it can be mixed with sodium carbonate once again.
This procedure clearly benefits greatly from the versatility of lime in changing its chemical formula when exposed to heat and other factors. This grants the capability to prepare the cellulose for paper-based products while successfully recycling the chemicals involved.
Lime in Sulfite Pulping
Sulfite pulping proceeds quite similarly to kraft pulping, except it uses different materials in its lignin dissolving chemical mixture. Unlike kraft pulping, since there is no innate chemical and heat recovery procedure, in sulfite pulping the mixture used (called “red liquor”) drains through the bottom of the tank before being treated and discarded, incinerated, or even sent to a plant for recovery of heat and chemicals.
In the past, this process incorporated quicklime when it was added during the preparation of calcium bisulfite as a cooking liquor. However, this method is slowly being phased out of sulfite pulping due to waste disposal problems. Instead, quicklime is now used more often to neutralize the discharged acidic water at the end of the pulping process, and it is sometimes relied upon for other waste purposes.
For the resulting pulp product obtained at the end of either one of these processes, lime can also be used to prepare calcium hypochlorite bleach liquor. This bleach can provide the pulp with its desired hue of whiteness that we see in many types of paper.
T 617 - Analysis of lime was published by the Technical Association for the Pulp, Paper and Converting Industry (TAPPI), a standards-developing organization that is devoted to standards relating to the pulp, paper, packaging and converting industries.
If you’re interested in learning more about the different types of lime and their applications (which includes their different uses throughout history), please refer to our past posts:
Calcium Oxide: From Ancient Warfare to Modern Industry