The most common process for making paper is called the Kraft or Kraft Mill process. In this process, wood chips are cooked at 150 - 165 degrees Celsius under pressure in a liquid solution containing caustic (NaOH) and sodium sulfide (Na2S) to pulp the wood.
The solution of caustic and sodium sulfide is referred to as “white liquor.” These chemicals, along with heat and pressure, release the lignin from the fibers in the wood. The resulting pulp is washed, screened, and sent on to bleaching and becomes the feed stock for the paper-making process.
The waste from the pulping washing step (residual chemicals, lignin, organics, etc.) are removed and become what is known as “black liquor.” This black liquor is sent to multiple effect evaporators to be concentrated. From there it is burned in the recovery boiler.
The black liquor is burned in an oxygen-deficient atmosphere. This process forms a molten product consisting mostly of Na2S and sodium carbonate (Na2CO3).
This molten material is referred to as “smelt.” It is sent to a tank where water is added. The resulting liquid is known as “green liquor.” From the smelt tank the stream is sent to the green liquor stabilization tank.
The green liquor also contains small amounts of suspended solids, called “dregs.” The dregs are hazardous and must be removed. This is typically done in the green liquor clarifier.
The dregs from the green liquor clarifier are sent to a rotary vacuum filter called the “dregs filter.” Here the dregs are washed to remove residual chemicals and are dewatered prior to disposal. The liquids are recycled to the green liquor stabilization tank.
The clarified green liquor is fed to a slaker where NaOH is formed. The grit and unreacted lime settle to the bottom where they are removed by means of a screw conveyor.
The slurry flows through a series of agitated tanks in a process known as causticising. The main products of this process are calcium carbonate (CaCO3) and NaOH. The effluent of this process is now referred to as white liquor and is pumped to the white liquor clarifier.
The overflow from this clarifier is returned to the pulp digestion process. The settled CaCO3 precipitate is known as lime mud. This slurry is washed with a combination of fresh and recycled water.
Lime Mud Washer
This wash water is sent to the lime mud washer. The overflow from this unit is sent back to the smelt tank. The underflow is dewatered on a rotary vacuum filter known as the “lime mud filter.” The dewatered solids (CaCO3) are sent to the lime kiln to be converted to burnt lime (CaO). The filtrate from these filters is returned to the lime mud mixer tank.
In this way, a very high percentage of all the chemicals used are recycled and there is much less impact on the environment from waste disposal.
Surface Water Pretreatment
Surface water pretreatment prepares water for use in any type of treatment plant (i.e., power, chemical, petrochemical, etc.) and is needed when the source of water comes from a raw/contaminated source (usually river water) where the total suspended solids (TSS) can range from 50 mg/L – 200 mg/L. This treatment consists of four steps: chemical pretreatment, clarification, filtration, and sludge treatment.
Sodium hypochlorite (bleach) is added to kill any living organisms that may be in the raw water. Coagulant helps particles come together to improve clarity and settling. Polymer turns individual particles into larger clusters. The larger particles settle faster and form a more concentrated sludge.
RapiSand™ ballasted flocculation is a high-rate water clarifying system utilizing both chemical and physical treatments to remove suspended solids and unwanted particles. The process uses the proven flocculation technique of adding a dense ballast sand, allowing for much higher settling rates. Raw water is mixed with a coagulation agent to destabilize and neutralize particles in the water. During the next step of flocculation, sand and polymer are combined with the coagulated flow. The flocculation mixer provides the particles with enough energy to stay suspended and grow in size.
In the sedimentation area, developed floc particles (coagulant, polymer, sand, and solids) are allowed to settle to the bottom. Clarified water may pass through tube settlers or inclined plate settlers before it is drawn off the top of the sedimentation basin. The settled solids are gathered and pumped from the sedimentation area through a hydrocyclone, where the sand is separated for reuse and the solids are processed further.
If necessary, the overflow can be treated by filtration. The process starts in an equalization tank that allows for a constant flow into the dual media filters. As the water flows from the RapiSand to the dual media filter, polymer may be added to improve filtration. The resulting water is now less than 1 mg/L TSS. It may be directly sent to a cooling tower or it can be further filtered depending on the intended use.
If the water is being used for a boiler, then it may need to go through reverse osmosis. An ultrafiltration membrane system removes small suspended solids from the water to improve the efficiency of the reverse osmosis system.
The underflow of the RapiSand is pumped into a thickener. Polymer is added to improve settling and clarity. The thickened underflow is sent to a filter press or other type of vacuum or pressure dewatering filter. The slurry is pumped into the filter press under pressure to force the moisture out. The filter press may require additional polymer to function properly. The cake is then hauled off for disposal and the pressate and wash water is recirculated for further use. The pressate and cloth wash water get pumped to the reclaim water storage sump with the backwash from the filtration systems.