30 QUARTER 4 2017 | TAPPSA Journal

SLUDGE MANAGEMENT

The management of sludge from paper mills has become an important intrinsic cost due to regulations in many countries restricting its disposal. At the same time papermakers want to increase their productivity by improving usage of the raw material and reducing losses and wastes. Therefore a signification challenge is the sustainable valorisation of by-products.

Kadant has developed some technologies that can help to reduce the total rejects. One such technology is a new cell specially designed to replace existing secondary flotation cells, when a mill is producing deinked pulp (DIP).

Secondary flotation cells necessary on deinking lines

There is a need to develop a dedicated secondary cell for replacement of existing cells in order to boost yield and reduce fibre and fines losses. Kadant has observed a general double trend in the supply of raw material for deinking lines.

The first trend or effect is that with newsprint production shrinking, so to does the collection. This in turn reduces the deinking mix. Paper mills have had to take in more magazine grades as raw material to compensate and with this the level of fillers is increased.

The second effect is that over time the fillers added in the production of magazines and other graphic papers has increased in order to reduce the fibre quantity while retaining bulk, and reducing the production cost.

Consequences of both effects are that the raw material used for DIP lines have more and more fillers and that even if the papermakers have found strategies to produce paper with more mineral content, the difference between filler content at the inlet and in the final paper results in direct losses. This gap has the tendency to increase, reducing the overall yield of a DIP and generating even more sludge.

Most of the large DIP lines – over 400 bone dry metric tonnes (BDMTD) – are operating with primary and secondary cell arrangements from a well-known supplier. The flotation modules are either placed in pre-and post-flotation, based of five to seven primary cells and two to four secondary units. Secondary cells are designed to treat the foam rejected from primary cells.

With an increased level of fillers in the raw material, the secondary cells are no longer capable of handling the

By Kadant’s FRÉDÉRIC VAULOT, R&D director; JALILA BACHIRI, Technology Centre manager and STÉPHAN POTTIER, R&D project manager.

Technology developed for advanced flotation to improve sludge management

MAK-CS 120

31 TAPPSA Journal | QUARTER 4 2017

SLUDGE MANAGEMENT |

primary rejects, due to the high flow of foam and the high level of fillers. It is thus difficult to control the level of losses while keeping, at least the same level of ink elimination. This leads to a degradation of the brightness as well as an increase of the Effective Residual Ink Concentration (ERIC) values for the accept.

Reducing losses with a specially designed cell

Based on Kadant’s MAK-C successes, the company has designed a proper secondary cell especially designed for yield recovery.

Special design for primary cell rejectsKadant’s brief was to design a cell capable of processing sludge from primary cells on large DIP lines. Key features included:

Special derivation of the well-known MAK-C cells with specific ability to process primary sludge.

No need to dilute between primary cells and MAK-CS but reduce the need of recirculation increase capacity of the line.

Operating capacity for filler content of over 50-65%

Quality of accepts needs to be better than primary feed.

Material losses reduced by 30-50 %

Yield increase by 1 to 2%

To ensure compatibility with the high level of aerated foam (comprising high level of fillers), Kadant has done a series of improvement tests in mill conditions. The optimisation of the design went through several stages including the foam retention time, which leads to water drainage and final foam consistency. Selectivity studies show that there is an optimum value, which can be reached thanks to a reject control valve offering much more flexibility and control than a simple overflow – as is the case with the original replaced cells.

Focus on secondary cellKadant carried out different tests with pilot MAK-CS (16m3/h & 200m3/h) as a secondary with different RRv. The results are easy to remember:

• 2 to 3 points brightness gain.• Fibres and total losses divided by 2.• Feed flow of secondary could divided by 2 to 3.

Tertiary stage feasibility

MAK-CS is possible as a tertiary stage. Tests have shown that MAK-CS tertiary is able to recover around 30% of cellulosic material from final rejects. There is possible yield improvement of 1%. The quality of the accepts (bright) from MAK-CS as tertiary cell is comparable to the primary inlet cells.

MAK-CS cells are available for renting through an R&D partnership. Kadant has two sizes available - 16 and 120 m3/h.

Conclusion Kadant will help the papermakers with their innovative solutions through the Technology Centre capabilities and its unique mill trial cell units.

Primary Cells (5 cells)

Secondary Cells (2-3 cells)

DIP system: 600 tonnes per day, Europe

Through an R&D partnership, Kadant and a customer initiated trials in the mill using a dedicated MAK-CS cell (120 m3/h). Over time, the mill has suffered with erosion of yield mainly due to an excessive reject rate from the secondary cells. The initial plan was to reduce the mass reject rate by over 25%.

It is also important to note that the MAK-CS cell does not need any dilution prior to the inlet and accepts are not recirculated. This means that the flow entering in the cell is two to three times lower than when using original cells. Pumping power savings is an additional advantage of the cell.

In fact, the results were much better than anticipated and the Kadant team was able to reduce the mass reject rateby 50% compared to the reference situation (generating an annual savings of around 1.5 million Euros). As a result, the mill immediately ordered two cells for the pre-flotation. Another option is to implement a MAK-CS as a tertiary cell. The advantages will be reduced, and so will the size of the cell. The investment cost would be similarly reduced but it is an intermediate solution.

C A S E S T U D Y