Menardi manufactures filtration products utilized in Kraft pulp mills. Kraft mills make up over 70% of the US mills with Sulfite and Ground-wood mills having the remaining share. Developed in Germany during the 1870’s the first domestic Kraft Process plant opened in Roanoke Rapids, NC in 1930. The Kraft method is the process of cooking wood chips under pressure in an alkaline (caustic) chemical mixture, known as white liquor. The chemicals used are too expensive and too toxic for disposal so they are recycled.
Regeneration of the sodium based chemical compounds from the spent white liquor (black liquor) occurs by means of separation of the inorganic compounds as well as the reduction of the compounds and the addition of lime. The caustic recovery process is as follows:
- Obtain used cooking chemicals (Black Liquor) from the pulp mill digesters and washers.
- Concentration of the chemicals in the Black Liquor, through evaporation.
- Incinerating the concentrated Black Liquor in a recovery boiler to capture the sodium carbonate (Na2CO₃) and the sodium sulfide (Na₂S).
- Green Liquor produced by mixing the smelt of the recovery boiler with weak wash (dilute White Liquor).
- Recausticization of the sodium compounds (converting Green Liquor to White Liquor) by slaking with lime, known as White Liquor processing.
- Recovery of the lime from the calcium carbonate (CaCO₃) for reuse in recausticization known as Lime Mud processing.
Recausticization, simply stated, means the converting of Green Liquor to White Liquor. Menardi products utilized during the re-causticization process is as follows:
- Pump green liquor from the smelt-dissolving tank to the Green Liquor Clarifier.
- Removal of inert solids (called dregs) from the Green Liquor Clarifier.
- After dewatering with a Rotary Vacuum Drum Filter also known as the Dregs Filter
- Dregs are then ready for disposal.
- The Green Liquor then goes to the slaker for an addition of lime. This addition forms calcium hydroxide Ca (OH)2.
- Recausticization occurs when calcium hydroxide Ca (OH)₂ reacts with sodium carbonate (Na2CO₃) in the Green Liquor and forms two bi-products. Sodium Hydroxide (NaOH), the active ingredient in white liquor and calcium carbonate (CaCO₃), also known as Lime Mud. Recausticized liquor then goes to a White Liquor Clarifier (old technology) or to a White Liquor Pressure Filter.
- Lime mud settles in the clarifier or filtered out in the pressure filter.
- White Liquor recovery is now complete and transfers to the digesters as sodium hydroxide (NaOH) and sodium sulfide (Na₂S).
- The Lime Mud is sent to the mud mix tank or Lime Mud Pressure Filter and washed with filtrate to recover any of the sodium chemicals left in the Lime Mud. The lime mud moves to the Pre-coat Filter and is washed again, then dried to 70% solids. Dried mud goes to the kiln (one of the most energy intensive processes in the recovery cycle) to be calcined.
Menardi’s monofilament polypropylenes are fabricated into filter blankets (or ‘face wires’) and are used on the Dregs Filters and Lime Mud Pre-coat Filter. The White Liquor Pressure Filter and the Lime Mud Pressure Filter use candle filters made from polypropylene felt, usually laminated with ePTFE.