Paper360 - September/October 2015 - (Page 30)

techlink | SLUDGE DEWATERING Rotary Press Compacts Sludge to Cake The Fournier Rotary Press is a revolutionary product in the evolution of sludge management at pulp and paper mills. ROSS ELLIOTT Sludge dewatering is an important but often ignored process that impacts pulp and paper mill performance and sustainability (see Sidebar). Mechanical dewatering is used to concentrate sludge with technologies that include: belt press; belt press plus screw press; modern design screw press that accepts lower solids; and centrifuges. In all cases, the equipment works better with higher solids sludge. The lowest practical solids feed into belt presses and screw presses is 1 percent Total Suspended Solids (TSS) and 1.5 percent into a centrifuge. All equipment requires the use of polymers to coagulate/flocculate the solids in order to promote dewatering and retention. Belt presses have been the most common applied technology, but also the least efficient and most costly to operate. Centrifuges use a lot of energy and elevated levels of polymer. They also require constant attention because of sensitivity to feed changes. There are many different screw press designs and many different feed requirements and results from their operation. ROTARY PRESS REVOLUTION The Rotary Press is a game changer for making cake from mill sludge. It has found rapid acceptance with municipal sludge applications, and initial exposure to the pulp and paper industry has demonstrated tremendous results. There are five key factors that give the rotary press advantages over other technologies: * High solids cake * Low maintenance cost * Low operator attention * Low energy usage * Clean and quiet operation Sludge is pumped into a mixing chamber where it is combined with polymer to induce good flocculation (Figure 1). 30 Paper360º SEPTEMBER / OCTOBER 2015 The sludge then enters the rotary press channel (Figure 2) and goes through drainage, pressing and restriction zones to deliver the compacted sludge. The discharged cake either drops freely or is conveyed to a collection area. Sludge with TSS as low as 0.7 percent is fed into the rotary press at between 2 and 7 psi (10 to 50 kPa). It flows into the gap between two filtration plates. The drilled filter plates rotate at slow speed, between 0.2 and 2 rpm. As the sludge dewaters, the friction between the sludge and filter plates increases; friction drives the sludge through compaction and out the rotary press channel. A pneumatically controlled restrictor arm provides back pressure for constant cake solids. The outlet pressure is controlled to as high as 70 psi (500 kPa). At start-up, the restrictor arm forms a small gap which opens to full extension for cake production. Scrapers on each filter plate clean off any sticky material or carryover. The rotary press receives an automated five minute flush daily to clean the filter plate. No other clean-up or attendant cleaning action is required. The totally automated system monitors flows, alarm conditions and includes startup/shutdown sequences. Pulp and paper sludge develops a high degree of friction between the fiber-laden sludge and the filter plate, which equates to faster speed and higher throughput. Key factors that affect throughput and cake solids are the nature of the sludge, feed pressure, discharge pressure and the speed of rotation. A Rotary Press can have between one and six channels per drive (Figure 3). The feed to any channel can be independently shut off. Motor sizes

Table of Contents for the Digital Edition of Paper360 - September/October 2015

Over the Wire
Stora Enso Ostroleka PM5 Proves Less Is More
Valmet’s Strategy Combines Process Technology, Automation and Services
Doubletree Paper Upgrades Its Water Treatment System
WEIG-Karton Achieves Results With Web Inspection at Former and Press Section
Wants vs Cans
Rotary Press Compacts Sludge to Cake
TAPPI Journal Summaries
Foreign Exchange and the Pulp and Paper Industry
Consolidation Watch
Knowledge Builder
Association News
Online Exclusives
Index of Advertisers

Paper360 - September/October 2015