The ERCO R-Series are proprietary chemical processes developed by ERCO that are used by bleached pulp producers every day to generate chlorine dioxide (ClO2) at their sites. We provide select, customized services for every R-Series process installation, from design through commissioning, along with full lifecycle support including modifications and enhancements as your mill’s needs expand or evolve.

With a track record of almost five decades of successful operation and continuous innovation, ERCO’s R-series ClO2 generators are in service in over 100 pulp mills around the world. Every R-series generator consists of customized process equipment of the highest quality, advanced supervisory controls and ancillary systems, and materials suited to our customer’s mill standards wherever possible.

ERCO stands behind what it designs and supplies and has routinely provided each process plant with guarantees of production capacity, product quality and chemical efficiency.

In many cases, through relatively minor changes to equipment, piping, process, control software and inputs, ERCO is able to convert or modify an existing process to one having a higher production capacity, more beneficial by-products, and where possible avoid the need to remove and replace major process equipment.

The benefits of choosing from ERCO’s R-Series processes are clear: industry-leading chemical product quality, established environmental acceptance, high process efficiency, cost savings, flexibility and, above all, safety.

Process Flexibility

The same basic process equipment can be utilized by most of the ERCO R-Series processes to produce ClO2. Only the feed chemicals and process operating conditions need to be changed in order to achieve the desired process configuration. In some cases, ancillary or “add-on” equipment is installed and seamlessly integrated into the existing process plant, along with any additional piping and controls that are needed.

All necessary modifications are custom-designed and overseen by ERCO’s experts. ERCO’s process flexibility allows you, the mill operator, to always choose the process chemistry that is most beneficial to your mill operation.


The R3 was developed to meet the need for increased process safety, higher chemical efficiencies and reduced sulphur by-products than earlier ClO2 processes. The process utilizes sodium chlorate, sulphuric acid and sodium chloride (as the reducing agent) to produce ClO2 solution, sodium sulphate (crystal by-product), and a chlorine by-product as chlorine water or hypochlorite. Although the R3 enjoyed great success for many years following its introduction in the 1970s, new environmental rules eliminated elemental chlorine from bleaching processes and the R3 largely fell into disuse. Nonetheless, it provided the foundation for the development of ERCO’s highly successful R8 process, described further on.


The R3H process is based on R3 chemistry, however, replaces part of the sulphuric acid and all or part of the sodium chloride with hydrochloric acid (HCl). This was done to achieve a balance in the production and recovery of sodium sulphate (sodium and sulphur) required by the mill’s chemical recovery system.


The R5 was developed for mills that do not require any sodium sulphate by-product. The process utilizes only HCl and sodium chlorate to produce ClO2. Because no sulphuric acid is used, crystalline (solid) sodium chloride is co-produced that can be disposed of or used to make brine for the manufacture of sodium chlorate. The by-product chlorine can be absorbed in cold water or reacted with sodium hydroxide to produce hypochlorite bleach for use in the bleach plant. When integrated with sodium chlorate manufacturing, the technology is known as the ERCO R6 Process, described below.


The R6 ClO2 plant is an R5 system integrated with a sodium chlorate and HCl plant, so that the only raw materials required by the mill are electricity and elemental chlorine make-up. This technology is beneficial to bleached pulp producers that are situated in remote areas which, as a result, may experience supply chain challenges in receiving chemicals needed by the other process configurations. The by-product gaseous chlorine from the R5 process and the hydrogen gas from the chlorate plant are combined to manufacture the HCl on-site that is then recycled to the R5 process.


The R7 uses the same chemistry as the R3H process. The gaseous chlorine by-product, however, is reacted with sulphur dioxide to form a mixture of sulphuric and hydrochloric acids. These are then recycled to the ClO2 generator, effectively eliminating the chlorine by-product and reducing the amount of sodium sulphate produced. As with the R3H, the R7 process can be used to achieve a balance in the production and recovery of sodium sulphate (sodium and sulphur) required by the mill’s chemical recovery system.


In response to stringent environmental protection standards first introduced in the 1970s, ERCO introduced the R8 process that quickly became and remains today, the most widely used ClO2 generating technology amongst elemental chlorine-free (ECF) bleached pulp producers. By utilizing methanol as the reducing agent to produce ClO2 from sodium chlorate and sulphuric acid, essentially no by-product chlorine is formed and there is also less sulphate by-product created than with any other ClO2 process that produces chlorine as a by-product.


In combination with the ERCO R10 or uncatalyzed ERCO R11, the R9 design features electrolysis of all or part of the sulphate by-product from the associated ClO2 process. The acidic solution produced in the electrolysis cells can be recycled back to the ClO2 process or used elsewhere in the mill, thus reducing the mill’s acid requirements. The electrolysis cells also produce valuable sodium hydroxide that can be used in the mill.


The acidic by-product saltcake (sodium sesquisulphate) from the R8 process is converted to “neutral” sodium sulphate in the ERCO R10 process. The acid recovered from the acidic saltcake in the R10 process is returned to the R8 process to reduce the acid requirement and the amount of saltcake co-product by approximately 24 percent. Usually, the reduction in sulphur that is otherwise returned to the mill’s chemical recovery cycle can generate significant savings by reducing the amount of sodium makeup required in the form of caustic.


The R11 ClO2 process utilizes hydrogen peroxide as the reducing agent to produce ClO2 from sodium chlorate and sulphuric acid. Essentially no by-product chlorine is formed, and, in addition, the mill’s effluent system performance may also benefit from its use. When a catalyst is employed, sodium sulphate is produced under low acidity. If a catalyst is not used, the process is operated at high acidity, and sodium sesquisulphate is produced. When integrated with the R10 process, either methanol (R8 process) or hydrogen peroxide can be used in the same plant, thus providing significant operating flexibility.

ERCO R101®

The ERCO R101® is a small-scale, single-chemical, electro-chemical generator that produces a ClO2 solution by passing ClO2 gas through a gas transfer membrane into a receiving water stream, usually requiring disinfection. ERCO R101® is best suited for disinfection of drinking water for municipalities.


Electrolysis is used to convert part of the sodium chlorate feed stream of the R8 or the R11 process to chloric acid. The addition of the R12 system will reduce the amount of sulphuric acid normally fed to either process and depending on the amount of chlorate converted to chloric acid, will reduce the amount of acid consumed and by-product sulphate produced proportionately. This allows the mill to “balance” its sodium and sulphur make-up requirements. Sodium hydroxide is produced as a valuable by-product that can be used in the mill.


Crystal sodium chlorate from the generator and make-up sodium chlorate are passed through electrolysis cells to produce chloric acid, entirely replacing the sulphuric acid feed to the R8 and R11 processes. As a result, no by-product sulphate is produced. Only sodium hydroxide is produced as a by-product. The R13 benefits mills that do not require sodium or sulphur make-up for their chemical recovery cycle and cannot dispose of the saltcake co-produced by other ClO2 processes that use sulphuric acid.


The ERCO Chloride Removal Process (CRP) is designed to improve the operation of the mill’s recovery boiler by decreasing fouling. The CRP system selectively removes chloride and potassium from recovery boiler precipitator ash and returns purified sodium sulphate crystals to the mill’s recovery cycle. With lower levels of chloride and potassium in the ash, the boiler can operate at higher temperatures, significantly decreasing fouling. Commercial installations of the CRP have consistently shown increased recovery boiler uptime thanks to a reduction in shutdowns caused by “chill-and-blow” and water wash operations.

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