Chemical Recovery in the Kraft Process
The schematic on the right represents the standard chemical recovery process in a kraft mill. Presumably, it describes the situation at PTPC fairly well.
The molten smelt which is continuously withdrawn from the recovery furnace consists mostly of sodium sulfide (Na2S) and sodium carbonate (Na2CO3). These molten salts are mixed with weak wash water in a dissolving tank where it forms what is known as “green liquor”. The green liquor is then sent to a clarifying tank where insoluble compounds settle out.
The clarified green liquor moves on to a “slaker” where it mixes with lime (CaO) and is held long enough for the lime to become calcium hydroxide (Ca(OH)2) in solution. It then enters the causticizers where the sodium carbonate is transformed into sodium hydroxide according to the reaction:
Na2S + Na2CO3 + Ca(OH)2 → Na2S + 2 NaOH + CaCO3
The sodium sulfide (Na2S) in the green liquor remains unchanged in this process, and the calcium carbonate (CaCO3), also referred to as “lime mud”, precipitates out of solution. That leaves a solution of sodium sulfide (Na2S) and sodium hydroxide (NaOH), also known as “white liquor”, which is sent to the kraft pulp mill for use in the digesters. Typically 95-98% of the cooking chemicals in the kraft pulping process are recovered in this cycle.
To put it less technically: green liquor + lime ==> white liquor + lime mud
The lime mud which settles out of the white liquor in the clarifying tank is washed before being sent to the lime kiln to be turned back into lime. The wash water, referred to now as “weak wash” or “weak white liquor”, is used to dissolve the molten salts in the smelt dissolving tank.
The washed lime mud (essentially limestone and water) is transformed back into quicklime (CaO) according to the chemical reaction:
CaCO3 + heat → CaO + CO2
This reaction takes place in the lime kiln at 1650oF or higher. At PTPC, the fuel used to produce this heat is recycled fuel oil. The lime exiting the kiln then cycles back to the slaker to be mixed with the green liquor prior to entering the causticizers.
Emissions and Control
While not in the same league as the combustion units in the power plant, the smelt dissolving tank and the lime kiln are both significant point sources of air emissions at the mill. Both emit primarily particulates and ammonia. The lime kiln also emits nitrous oxides. The dissolving tank is equipped with a wet scrubber (Ducon UW4, installed in the 1970s) to help control those emissions. The lime kiln uses a wet scrubber as well, in this case a Venturi scrubber, last modified in 2003.
Solid Waste
The insoluble compounds settling out of the green liquor clarifier are withdrawn as a highly alkaline dark green-black slurry known as "green liquor dregs". It is comprised mostly of unburned carbon (char) and lime mud, as well as compounds of sodium, magnesium and sulfur and other elements. These dregs are generally washed and/or filtered to retrieve process chemicals and reduce the pH. In most plants the dregs would then be destined for the landfill, but apparently PTPC has a buyer for theirs.
"Lime grits" are the undissolved solids which settle out of the slaker, a highly alkaline material comprised mostly of lime and lime mud. These grits are removed, washed and disposed of in the landfill. The wash water is added to the dissolving tank, helping to preserve process chemicals -- notably Na2O with a pH of over 13. This helps lower the pH of the disposed grits to something under the 12.5 threshold above which the EPA considers the waste to be hazardous.
Lime mud itself is sometimes part of the waste stream, mostly when the lime kiln is down for repairs or maintenance and is not available to convert the mud back to lime. Nationally, excess lime mud accounts for over half of the waste stream of kraft pulp mill causticizing plants.
References...
Tappi/Kraft chemical recovery process: http://www.tappi.org/Downloads/unsorted/UNTITLED---05EPE189pdf.aspx
Trinity BART report addendum: http://www.ecy.wa.gov/programs/air/globalwarm_RegHaze/BART/PTPC_BART_Addendum.pdf
WA industrial carbon footprint reports: http://www.sustainable-economy.org/art?cid=13
