Do Nuclear Instruments Always Need
to Be Used?
Restricting the use of radioactive densitometers as a way to improve safety in mining operations
By Sidney A. A. Viana
Nuclear or radioactive densitometers (See Figure 1) are the most commonly used instruments to measure the density of ore slurries flowing in pipelines. Some advantages of nuclear densitometers are: non-intrusive and contactless measurement; easy external mounting on pipelines; no need for process operation stoppages during maintenance interventions; and robust construction. However, they also have drawbacks: the need for permanent safety care due to the risks of occupational accidents involving ionizing radiation and the need for field calibration using actual ore slurry from the production process, which rarely covers the full operating range, resulting in poorly representative calibrations.
Nuclear densitometers are a mature instrument technology with proven feasibility in several industrial applications. On the other hand, there are also several non-radioactive technologies for liquid density measurement. Certain industries such as chemical, petrochemical and pharmaceutical have been successful in using non-radioactive densitometers, which are not yet common in the mineral processing industry, due to a lack of knowledge about how to appropriately apply them, as well as due to the harsh industrial conditions of the mineral processing environment.
At Vale, a large global producer of iron ore, nickel and copper ore, radioactive densitometers have historically been used in the processing of ore slurries. However, an effort to replace such densitometers with non-radioactive alternatives was initiated in 2010 by the Southeastern Ferrous Department. Initially a survey on the existing radioactive densitometers was carried out, and feasibility studies to replace radioactive densitometers by non-radioactive alternatives were developed. The first replacements were done in 2011 using differential pressure densitometers . In the following years, other models of non-radioactive densitometers such as vibrating fork [13,14], microwave  and ultrasonic  were tested for feasibility to operate with iron ore slurries. Not all of them were proven suitable, and some suffered from fast wear caused by slurry abrasion. In 2014, the Executive Ferrous Operations Department requested the elimination of radioactive densitometers from all Vale’s iron ore operations in Brazil, based on the cases of success attained by the Southeastern Ferrous Department.
Restricting the Use of Nuclear Densitometers
Safety has been a subject of major concern for the mining industry. Removing radioactive instruments from Vale’s Brazilian iron ore operations would eliminate occupational and health safety risks related to the use of such equipment. This initiative was implemented by means of two work fronts.
Discarding of radioactive sources — The goal of this work front was the definitive elimination of radioactive sources that were out of operation and stored with no intended future use. In Brazil, the National Commission for Nuclear Energy (Comissão Nacional de Energia Nuclear, CNEN)  is the government institution responsible for regulating and establishing legal norms for the use of radioactive materials and equipment in the Brazilian territory. CNEN is affiliated with the International Atomic Energy Agency (IAEA). According to the CNEN norms, any radioactive source not being used in operation should be stored at a specific and safe place (a storage bunker), until it should be reused or discarded to a radioactive rejects receiver institution. However, no maximum time limit is set for the storage of radioactive sources by the users. This favors the occurrence of deactivated radioactive sources being indefinitely stored, with no prospect for reuse, constituting a safety liability. In a similar way, radioactive sources installed in the field but without effective use (for example, in a deactivated industrial plant) could also be discarded if they have no prospect of further use. In short, the discarding of radioactive sources intends to reduce or even eliminate the safety liability related to the storage of unnecessary sources.
Replacement engineering development — The goal of this work front was the development of basic engineering for the application of non-radioactive densitometers to replace radioactive units used in mineral processing facilities, taking into account the specific characteristics of each application and non-radioactive alternative (measuring principle, measuring range, installation aspects, etc.). The starting point for this work front was to understand the replacement initiatives done earlier by the Southeastern Ferrous Department, to identify the successful applications of non-radioactive densitometers and their potential for replication in similar mineral processing facilities. In short, the replacement engineering was intended to establish suitable technical standards for the use of non-radioactive densitometers in place of radioactive ones.
The Work Strategy
For all Vale’s iron ore operations in Brazil, an inventory of existing radioactive sources was created, as well as sources planned for acquisition in new projects. The inventory would help engineers understand how the equipment was distributed among the sites, allow them to plan the activities for discarding (work front 1) and to develop the replacement engineering (work front 2). In the inventory, the equipment was classified as: In Operation, Stored, Acquired in Project and Planned in Project (See Table 1). Figure 1 shows the numbers of radioactive sources ascertained by the inventory.
An assessment of the radioactive sources was made as far as the feasibility of elimination. Once all radioactive sources had been identified and recorded in the inventory, they were classified by their operational use and status, shown in Table 1. Those classified as In Operation were further confirmed as Necessary (without feasible replacement by a non-radioactive alternative) or Not Necessary (with feasible replacement by a non-radioactive alternative). Those classified as Not Necessary were then engineered for replacement. An additional classification regarding the elimination status of the radioactive sources is shown in Table 2, for the stored sources. Figure 2 shows the numbers of stored sources following this classification, for all operations departments. For example, 25 stored sources (23% of the total stored sources) were classified as needing to be eliminated.
In Brazil, activities for discarding radioactive sources must be done in compliance with the CNEN norms, specifically NN-5.01 and NN-3.01 . Although the discarding activities can be carried out by the owner of the radioactive sources, it is strongly preferable to contract a disposal services company specialized in those activities. By this way, four service companies specializing in handling, packaging, transporting and delivering deactivated radioactive sources were consulted and evaluated, and one of them was selected for contraction.
To allow proper replacement of radioactive densitometers, specific technical documentation (e.g., specification sheets, datasheets, mechanical drawings and mounting arrangement drawings) was developed for the application of alternative non-radioactive densitometers, according to application criteria defined by VALE. An engineering services company was contracted to provide such documentation.
Work Front 1: Discarding of Radioactive Sources
Radioactive sources that have been laid up and with no prospect of reuse are always a safety liability. In Brazil, the discarding of that equipment is subjected to specific legal regulations, and must be performed as a planned process of handling, packing, transporting, and delivering to a specific receiver branch of CNEN. Among the CNEN branches qualified to receive deactivated radioactive sources, three were considered: CDTN (Centro de Desenvolvimento de Tecnologia Nuclear, in Belo Horizonte, MG), IPEN (Instituto de Pesquisas Energéticas e Nucleares, in São Paulo, SP), and IEN (Instituto de Engenharia Nuclear, in Rio de Janeiro, RJ).
Work Front 2: Replacement Engineering
The object of this work front was to develop the basic engineering for replacement of radioactive densitometers by non-radioactive alternatives. Some of those alternatives were already in use at Vale and other Brazilian mining companies, but not all of them were operating efficiently. The objective was to characterize more accurately the cases of feasible application of non-radioactive densitometers to minimize the occurrence of failures due to the inadequate implementation of potentially feasible applications or the implementation of actual infeasible applications.
Several non-radioactive densitometers
were evaluated for mineral processing:
• Coriolis : Measures the density by contact with a flowing liquid, using the Coriolis principle, and are exclusively applied to pipelines. They are currently available for small- and medium-diameter pipelines. For large-diameter pipelines, they must be installed on a bypass pipeline that samples the liquid from the main pipeline. They have higher accuracy and precision, but are not suitable to operate with liquids containing solids in suspension, due to clogging and/or wear by abrasion. They are ineffective for applications with ore slurries.
• Differential Pressure : Measures the density by contact with the liquid using the hydrostatic pressure, which relates the pressure of a liquid column to its density. They have very good accuracy and precision for applications in tanks and reservoirs, if both pressure seals remain submerged. They cannot be directly applied to pipelines, since the liquid flow invalidates the hydrostatic principle. For such cases, the manufacturers usually offer a “sampling vessel” for insertion of the densitometer, to be connected to a pipeline with pumped flow. However, practical experiences have shown that this solution is ineffective as the density measurements are corrupted by pressure variations in a pumped pipeline, and the “sampling vessel” is often clogged by settled solids from the slurry.
• Vibrating Fork [13,14]: Measures the density of a liquid by sensing the resonance frequency of a pair of vibrating blades inserted into the liquid. They have very high accuracy and precision, and can be applied with success to tanks and reservoirs, preferably with suitable mixing, for representative measurements. They cannot be applied to pipelines, due to quick wear by abrasion.
• Microwaves : Measures the density of a liquid from the time propagation delay of microwaves transmitted through a fixed path within the liquid. The higher the fluid density, the shorter the propagation delay. Those densitometers usually have a tubular shape, for pipeline installation. They provide good accuracy and precision, but are unsuitable for measuring abrasive liquids due to the quick wear of their internal coating in contact with the liquid. In a test conducted by Vale in 2014 with a polyurethane- coated microwave densitometer, the equipment lasted only 20 days, due to internal wear.
• Ultrasound : Measures the density of a fluid from the time propagation delay of ultrasound waves transmitted through a fixed path within the liquid. The higher the liquid density, the shorter the propagation delay. Like microwave densitometers, they are usually constructed in a tubular shape for pipeline applications, and suffer heavily from abrasion.
• Tomographic : Measures the density of a liquid by tomographic principle, by sensing the liquid resistivity of the passage of an electric current through the liquid. The higher the liquid density, the greater the resistivity. Those densitometers consist of a set of electrodes arranged on a tubular or rod-shaped body. For applications with abrasive slurries flowing in pipelines, there are tubular models with ceramic inner coating, which is more robust to wear than other coating types like polyurethane. The rodshaped models are suitable for density measurement in tanks and reservoirs. It is a promising new technology, but still with few user references about its practical performance.
• Gravimetric : Measures the density of a fluid by the gravimetric principle. A straight tube with known length and diameter is supported by one or more load cells, which sense the mass of the liquid inside the tube. The liquid density is determined by dividing the sensed mass to the known internal volume of the tube. It has reasonable accuracy and precision, if the pipeline operates fully filled by the liquid. It cannot be applied to vertical pipelines. Excessive mechanical vibrations on the measuring tube will affect the load cells and may corrupt the density measurement. In mineral processing plants, ore slurries pass through pumping pipelines and storage tanks, in addition to the mineral processing equipment. Tanks and pipelines have more appropriate conditions for the installation of density measuring instruments, depending on the measurement technology used. Two key aspects to consider in the application are:
• Abrasion — Flowing ore slurries are strongly abrasive because of the movement of ore particles relative to object parts in contact with the slurry. Nonradioactive densitometers usually perform measurements by contact with the liquid, and are therefore subjected to some degree of abrasion, which may significantly reduce their lifespan. Any application of non-radioactive densitometers to ore slurries must be designed to minimize the effects of abrasion on the instrument.
• Typical Installation Design — Each type of non-radioactive densitometer has a specific physical construction, which determines the way it can be installed. For example, tubular-shaped microwave densitometers can only be installed in pipelines. On the other hand, rod-shaped differential pressure densitometers are more suitable for installation in tanks. Therefore, the development of “typical installation standards” for each application of non-radioactive densitometer, considering the specific application needs regarding the instrument and the process, allows the standardization of the applications and increases the chances of success.
The assessments for elimination of stored radioactive sources allowed the discarding of 86 sources in 2014. According to Figure 1, the original total number of radioactive sources (In Operation + Stored + Acquired in Project) was 509 sources. The 86 sources discarded represents a reduction of 17% in the total number of existing sources. For radioactive sources classified as Planned in Project, recommendations were made to change the engineering designs to use non-radioactive alternatives whenever possible.
The replacement engineering work front identified 71 feasible applications of radioactive densitometers that could be changed to use non-radioactive alternatives. The engineering project documentation for those applications were developed and delivered to the representatives of each department. However, the actual replacement of those densitometers depends on the implementation of the respective projects, through the acquisition and installation of the non-radioactive alternatives.
The results of discarded radioactive sources were considered an important milestone for the reduction of safety liabilities in the iron ore operations. Given the geographic dispersion of the site operations, the involvement of management levels of each site was fundamental to the success of this project. Systematic planning and follow-up was crucial to keep everyone informed of advances, problems and difficulties throughout the project.
The discarding of more radioactive sources in the following years depends mainly on the implementation of the replacement engineering projects, along with the maturation of the knowledge about the application of non-radioactive densitometers in mineral processing facilities. For applications with impracticable replacements, the use of radioactive densitometers should be kept.
An important benefit of this project was the definition of objective criteria for the use of non-radioactive densitometers as an alternative to radioactive ones. The lack of such criteria favors the indiscriminate use of non-radioactive alternatives, resulting in incorrect applications.
Although the nuclear instrument technology has its advantages, non-nuclear alternatives with similar measurement performance and robustness should be preferred. The goal is to attain a better balance between instrument accuracy and safety liability regarding the use of nuclear instruments.
Sidney A.A. Viana is a specialist automation engineer at Vale’s Ferrous Automation Engineering Department. He can be reached at firstname.lastname@example.org.
The success of this project was possible due to the effective engagement and teamwork of many people. The author acknowledges the partnership of his local department colleagues, Herbert Mascarenhas, Gustavo Martins, Diogo Pires, and Gilberto Resende, for their contributions to the planning and management of this project. Thiago Rezende (Southeastern Ferrous Department), Julio Faria and Solange Nunes (Pelletizing Department), and Michel Martins (Southern Ferrous Department) are specially acknowledged for sharing information about the equipment from their departments, and for supervising the execution of project activities on their site operations. Finally, the author thanks Luciano de Biasi, Kleber Saldanha, and Kênio Figueiredo (former engineering managers) for their managerial engagement and administrative support.
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