The ANDRITZ SulfoLoop™ solution is based on Wet-gas Sulfuric Acid (WSA) technology for converting wet, sulfur-rich gases and elemental sulfur into sulfuric acid.
In recent decades, pulp mills have become more profitable and environmentally friendly by reusing byproducts and finding new sources of income. However, cutting emissions and wastewater means more sulfur stays in the system, which raises the mill’s sulfidity. Traditional ways to reduce sulfidity often involve discarding valuable materials – like recovery boiler ash and salt cake from chlorine dioxide production – into the environment. This not only wastes resources but also removes sodium, forcing mills to buy sodium hydroxide to replace it, which increases costs.
The SulfoLoop system turns this challange into an opportunity. It recovers the sulfur coming from the pulp mill processes, thus eliminating the environmental and financial implications of purging sodium sulfate. The recovered sulfur streams are then transformed into concentrated, commercial-grade sulfuric acid for all the mill’s operational needs. The result is lower costs, reduced waste, and a sustainable solution that boosts mill profitability.
ANDRITZ SulfoLoop™ sulfuric acid plant (3D rendering)
ANDRITZ SulfoLoop™ sulfuric acid plant (3D rendering)
Softwood mills often face a sulfur surplus due to additional inputs, such as tall oil production, making sulfidity control in the chemical cycle particularly challenging. In these cases, sulfuric acid demand is higher, and achieving self-sufficiency becomes possible by supplementing with elemental sulfur – further enhancing the plant’s economic viability.
Hardwood mills, while typically receiving lower sulfur inputs, also gain significant benefits: complete self-sufficiency in sulfuric acid and full utilization of chlorine dioxide byproducts, driving efficiency and sustainability across operations.
The plant utilizes WSA (Wet Gas Sulfuric Acid) technology developed by Danish company Topsoe. While relatively new in the pulp industry, the technology is well-proven, having already been licensed to operate at nearly 200 locations worldwide, in many industrial sectors. ANDRITZ has already successfully introduced SulfoLoop systems into the pulp industry with installations at two mills in Brazil, Klabin Puma and Suzano Ribas do Rio Pardo with another plant under construction in Sweden Södra Cell Mönsterås.
The Topsoe WSA process is a wet gas catalytic process. The concentrated sulfuric acid is produced by condensation from a wet process gas. Drying of the process gas prior to treatment in a Topsoe WSA plant is completely unnecessary, which means generation of wastewater and loss of sulfur are avoided.
The purpose of the first step in the process is to produce an SO2-rich gas at the operating temperature of the oxidation catalyst in the SO2 converter. Sulfurous feeds are oxidized to SO2 by combustion and then cooled to the optimum catalytic conversion temperature in a waste heat boiler. The excess heat from this operation is recovered as steam.
The catalytic conversion of SO2 to SO3 takes place in one or more catalyst beds. The reaction heat is recovered between the catalyst beds by production of saturated or superheated steam. After the last conversion step, the gas is cooled and the SO3 reacts with water vapor to form gas-phase sulfuric acid. The process gas is finally cooled by a countercurrent flow of air in Topsoe’s proprietary WSA condenser.
Clean tail gas exits at the top of the WSA condenser and the sulfuric acid is collected in the bottom section from where it is cooled and sent to storage. Hot air generated in the WSA condenser may be used to preheat combustion air of the recovery boiler, to ensure optimal energy efficiency.
Producing high-quality lignin significantly increases sulfur input into the chemical recovery cycle and raises acid consumption, primarily due to the sulfuric acid required for the acid washing sub-process. To address this, integrating a Lignin Recovery system with the WSA process is a practical and cost-effective solution. The WSA process removes excess sulfur from the recovery cycle and converts sulfur compounds into recyclable concentrated sulfuric acid, which in turn is used in the process. This approach was successfully implemented at Södra Cell Mönsterås in Sweden, ensuring sulfidity remains under control – even at the world’s largest lignin plant.
SulfoLoop™ sulfuric acid plants are part of the ANDRITZ’s CircleToZero initiative, which includes new technologies aimed at optimizing pulp mill side streams. This global initiative for our pulp and paper customers brings together the continuous development and improvement of existing ANDRITZ technology solutions with an ambitious innovation goal for the future: To eliminate unused side streams, create new added-value products, and lay the foundation for zero emissions and zero waste production.
Closing loops equals huge savings on chemicals and enables self-sufficiency in sulfuric acid at pulp mills
Closing loops equals huge savings on chemicals and enables self-sufficiency in sulfuric acid at pulp mills
ANDRITZ chemical recovery solutions for kraft mills
One of ANDRITZ's latest innovations, LignaRec lignin recovery, shines a new light on the chemical cycle in a kraft mill. It offers tailored solutions to optimize the chemical balances in the mill and provides the ability to generate additional revenue and/or avoid the purchase of costly make-up chemicals.
Kraftanol™ biomethanol plants are part of ANDRITZ’s CircleToZero initiative, which aims at reduction, utilization and upgrade of pulp mill side streams.
Technologies for every major pulping, recovery, and power process.The major challenges facing pulp producers are to achieve steady-state production of uniform pulp quality with the lowest possible consumption of chemicals and water at the highest efficiency. And to do this within tight environmental regulations. ANDRITZ is one of the few technology suppliers to deliver a complete kraft pulp mill and to have it performing with world-renowned environmental excellence.
Brazilian pulp and paper giant Klabin is further closing the circular sustainability loop at its Puma I and Puma II projects in the south of the country. Soon it will be making its own sulfuric acid from captured non-condensable gases at its Ortigueira mill site. ANDRITZ expertise is paving the way.
