Successful modernization of two grate kiln pellet plants at Ferrexpo’s Poltava Mine

Metso Outotec analyzed the performance of the existing systems, developed an improved solution, and supported Ferrexpo’s implementation of this major modernization project without taking the equipment offline for more than a couple of months. Ferrexpo has been very pleased with the results and plans to proceed with similar upgrades to their other production lines.

Strong partnership based on collaboration and results

Ferrexpo and Metso Outotec have a long history of collaboration. Metso Outotec’s predecessor, Allis Chalmers, delivered the first two grate kiln (GK) pelletizing plants to the Poltava mine in 1976. The plants, with a rated capacity of 3 Mtpy, included a so-termed traveling grate for drying and preheating green ball iron ore pellets, a rotary kiln for high-temperature induration of the pellets, and an annular cooler to cool the indurated pellets ready for material handling and storage. Poltava commissioned two additional pelletizing plants of the same size and capacity from Allis Chalmers in 1980, four years later. These plants contained concentrator equipment such as grinding mills and filters, and agglomeration equipment such as balling drums and roller screens. 

Ferrexpo and Metso Outotec have formed a working partnership for maintaining and improving plant operations. Together, both companies are continually working to improve the processing capabilities, capacities, and efficiencies. In recent years, Metso has supplied flotation circuits with ten energy efficient Vertimills®, along with an upgraded grinding circuit.

Significant improvements by modifying the configuration

Ferrexpo Poltava still operates the four Metso Outotec GK processing lines to produce iron ore pellets. The plants processed ore from one mine site and produced one pellet type when they were first put into operation. Today, they process ores from several locations across the region and produce multiple high-quality pellet types. This means that the GK systems must be flexible to accommodate variations in ore quality. Facing the challenges of increasing demand for production and the need to increase process flexibility, Ferrexpo engaged Metso Outotec to compare the GK plants to modern ones and recommend areas for improvement.

Metso Outotec began by analyzing current design, operating parameters, and ore variations, determining that a process cycle modification would be the optimal approach to address Ferrexpo’s requirements. In a typical GK pellet plant, the traveling grate is comprised of multiple drying and preheating zones used to heat and partially indurate the pellets. The original traveling grates at Poltava were configured with an Updraft Drying (UDD) zone followed by a Downdraft Drying zone (DDD) and finally a Preheat zone (PH).  This configuration is identified as UDD/DDD/PH. The analysis performed by Metso Outotec engineers showed that modifying the grate configuration to DDD/DDD/PH would enable significant improvements in the process cycle with only minimal physical changes to the equipment. 

UDD/DDD/PH configuration results in gentle and conservative drying process

The existing UDD/DDD/PH configuration is a simple and conservative process cycle to operate. It results in the gentlest drying process for the green ball pellets entering the furnace, with low compressive forces on the pellets and minimal concern for the dewpoint of the exhaust gases. The pellets are less susceptible to spalling, cracking, and deforming or further-weakening due to dewpoint condensate. This configuration ultimately allows for a greater margin of error in controlling the UDD zone without adversely affecting the pellets.

The UDD zone has several advantages:

• Gases hitting the bottom of the bed are already slightly cooled by the grate plates that support the pellets, which results in less thermal shock and gentler pellet drying.

• The upward flow of gases allows the evaporated steam to readily escape the bed due to its lower density compared to air.

• The bottom pellets are dried first (dried pellets have increased strength compared to wet), resulting in improved resistance to cracking or deformation from compressive forces such as the weight of the pellet bed.

• The upward flow of gases greatly reduces compressive forces on the bottom pellets by counteracting the gravity of the pellet bed. This also minimizes cracking and deformation slightly expands the bed, improving bed permeability, which further improves gas flow.

• Any dewpoint condensate from the gases will occur on the pellets at the top of the bed where the compressive forces are weakest, and the effect on their strength is negligible. The slight heating of the grate plate in the UDD zone results in an increased off-gas temperature, as well as minimal concern for the dewpoint of the exhaust gases in the DDD zone that follows it.

Overcoming the limitations with Metso Outotec’s support

The UDD/DDD/PH configuration has a few key disadvantages that ultimately limit the GK system’s capacity and overall fuel efficiency. As the bed of pellets in the UDD zone is dried from the bottom up, the wettest pellets are at the top of the bed as they leave this first drying zone. If the gas temperature or flow rate is too high in the following DDD zone, the pellets at the top of the bed will dry too quickly, spall, and/or micro-crack, thus reducing pellet quality. Consequently, the drying rate in the DDD zone must be lower than the rate the equipment can achieve. If the UDD/DDD zones combined don’t remove sufficient moisture from the middle of the bed, the gas temperature and flow rate in the following PH zone must similarly be limited to avoid spalling and/or micro-cracking, which compounds the situation. Limiting the gas temperatures and flow rates in the DDD and PH zones ultimately reduces the heat transfer potential of the system.. While this provides the gentlest drying for the pellets when they are at their weakest, i.e., when entering the system, the UDD/DDD/PH configuration is therefore limited in terms of heat transfer, which also affects production capacity and fuel efficiency. Overcoming these limitations was the focus of the modification performed by Metso Outotec.

Proper design and control to reduce the risks

The DDD/DDD/PH configuration can highly stress the pellets entering the furnace if not designed and controlled properly and can quickly result in damaged pellets, reduced capacity, and poor pellet quality. The hottest gases see the top of the pellet bed first and then pass downward through the pellet bed and grate plates. Accordingly, the pellets at the bottom of the bed see the least amount of drying and are at further risk of weakening due to dewpoint condensate from the gases. The pellets at the bottom of the bed also undergo the greatest compressive forces due to the weight of the pellet bed and the pressure of the gases pushing the pellets against the grate plates. The compressive forces will compound if the bed begins to deform, as the voidage in the bed will decrease and the gas pressures will increase. A deformed bed will carry forward to the subsequent DDD and PH zones, impacting the gas flow and efficiency through the entire traveling grate. The risks associated with improper design and control of this configuration include spalling and/or micro-cracking of pellets at the top of the bed; deformation, cracking or crushing of pellets at the bottom of the bed; reduced capacity; and reduced fuel and energy efficiency.     

Conversion to a DDD/DDD/PH configuration

The aim when converting a traveling grate to a DDD/DDD/PH configuration is to preserve the gentle drying inherent in the UDD design while eliminating the limitations of the UDD/DDD/PH configuration. This requires an in-depth review and detailed knowledge of heat transfer, pellet behavior, gas composition, and fluid flow. Simply rerouting the gas flow in the first zone from UDD to DDD would result in degraded, if not unsustainable performance. Metso Outotec carried out a detailed review and assessment of process parameters to determine the optimal thermal and flow cycles, thoroughly tested the cycles with varying ores and pellet types and modified the machinery design to achieve the control and flexibility required for the various cycles.

Optimal performance achieved by process control

Taking advantage of the DDD/DDD/PH configuration requires strict control of the process in order to achieve optimal performance. Unlike the UDD system, which can be simply set up for gentle processing, the DDD system must be set up precisely. This is because drying the pellets either too conservatively or too aggressively results in suboptimal operation. Condensing water on the bottom layer must be avoided when drying too conservatively. If not, pellet moisture increases, which results in weakened and collapsed pellets. The residual moisture will continue to be a problem in the subsequent high-temperature DDD and PH zones even if the bottom pellets survive the first DDD zone. This ultimately limits production and fuel efficiency in much the same way as the UDD/DDD/PH configuration is limited. Drying too aggressively and creating spalling and excessive compressive forces is also problematic, meaning gas temperatures and flow rates must also be designed and controlled to avoid both these and thermal shock. Ultimately, optimal process cycles with a careful balance between temperature, mass rates, and composition of drying gases were established for the different ores and pellet types.

Tests to confirm the new grate cycle

Metso Outotec performed extensive testing at its lab facilities in Pennsylvania following development of the concepts and theoretical models. Each iron ore source has its own unique properties that must be considered. Determining drying limits and pellet compressive strength through the entire drying cycle is of utmost importance for a  successful conversion. An additional challenge in this case was to ensure a stable process for two different pellet blends along with blending ore with unique characteristics from various mines. Pot grate and batch kiln tests were used to confirm the new grate cycle. The satisfactory results were shared with Ferrexpo once they were established so that they could consider the potential value of increased capacity and/or improved fuel efficiency.

Modifications with minimum downtime

Several physical modifications to the traveling grate were required to ensure a successful modification in addition to adjusting the process cycles. The DDD/DDD/PH configuration is much more sensitive to variations in ore quality, so the design included several features to provide flexibility. These included redesigned dead plates, used to isolate process gases between zones, adding bed bypass ducts, the ability to operate at a reduced pellet bed height, new process control dampers, and modified process controls. These features provided Ferrexpo with the operational flexibility they needed to meet demanding processing and pellet-quality targets.

The new configuration has a vastly different pressure balance within the system as well. Sections that were previously under positive pressure are now operated under vacuum. This influences system leakages, temperature loss and most importantly, the existing process fan operating point. All existing equipment had to be evaluated to ensure it could perform within the new configuration’s parameters.

Ferrexpo also required all modifications to be planned so that the process line would only need to be shut down for a relatively brief period. With this in mind, Metso Outotec engineers designed the new system components in such a way that most of them could be manufactured offsite. This allowed for quick installation and minimized the need to move or completely remove existing components and structures.

Increased production capacity with new configuration

Ferrexpo ultimately decided to move forward with the suggested modifications and contracted Metso Outotec to manufacture some of the key material components. The changeover was strictly managed and accomplished on-schedule, and the new configuration provides Ferrexpo with the operational flexibility needed to meet their demanding processing and pellet quality targets.

The new configuration involved modifying the grate, reviewing the operating characteristics of the existing process fan, rerouting the recirculation gas flow, upgrading the scrubbers, modifying the process control strategy, and installing new exhaust gas ducting and a new exhaust gas fan. Ferrexpo and Metso Outotec were both able to successfully overcome the challenges encountered during the project and improve the process with the new configuration. The modifications were implemented in Q4 of 2019 and the equipment restarted before the end of the year with support from Metso Outotec field engineers. Pellet quality and increased production capacity were the most notable improvements with the new configuration.

We have a long established working relationship with Metso Outotec, who have helped us deliver increasing pellet volumes and pellet quality for many years now, and we are excited about the next phase in developing this relationship. The approach adopted by Metso Outotec for this latest phase of work has been one of collaboration with our operational teams, working together to establish a cost effective, energy efficient pathway for us to increase production whilst also preserving the high quality of our pellets. We look forward to working with Metso Outotec to deliver a further three million tonnes of pellet capacity through this phase of work.” Jim North, CEO (Ferrexpo)

Ferrexpo plans to proceed with similar modifications to the other processing lines at the Poltava mine in the future, and Metso Outotec will be supporting these. 

More about Ferrexpo

Ferrexpo, a Swiss-based commodity trading and mining company and the world’s third largest exporter of iron ore pellets, generated total sales of USD 1.7 billion in 2020. The history of the company dates back to 1960 when the government of the former Soviet Union (U.S.S.R.) commenced mining operations in the Poltava Region, now part of Ukraine, to exploit the huge reserves of magnetitic iron ore. The concentration plant started operation in 1970 and produced its first pellets seven years later.

The Poltava GOK steel plant was privatized a year after Ukraine achieved independence in 1991 and became part of Ferrexpo, its holding company. Ferrexpo was subsequently listed on the London Stock Exchange in 2007 and has since opened a second mine.

Read more on their website.