PRELIMINARY STUDY ON ORE GRADE CONTROL TECHNIQUES FOR THE HUSAB MINE,

W. DONG, S. ZIAO, H. JINLONG, M. XIAOHU, Z. JIAN, C. XINGQI CGN Uranium Resources Company Ltd, Beijing, China

1. INTRODUCTION

The Husab mine is situated within the Desert in the Erongo region of western Namibia, 6 km south of the Rössing mine, which is majority owned by Rio Tinto, approximately 60 km east of the coastal town of and less than 100 km north-east of the Walvis Bay Port, the largest deepwater port in south- western Africa. It has good communications links and extremely favourable infrastructure conditions for development. As the most important uranium deposit to have been discovered since 2000, the Husab mine has a total uranium resource of nearly 300 000 t of U3O8 with ore reserves of 300 Mt containing 156 000 t of U3O8 at average grade of 518 ppm U3O8.

The Husab mine is the first ultra-large uranium mine to be constructed and operated by the China General Nuclear Power Corporation (CGN). Mine construction and pre-stripping commenced in 2013, ore mining commenced in May 2015 and the first barrel of uranium oxide was produced on 31 December 2016. The Husab mine has a designed annual mining capacity of over 100 Mt, an annual ore processing capacity of 15 Mt and an annual output of 6500 t of U3O8. It will be the largest open pit uranium mine with the largest mining capacity in the world.

With more than one year’s mining production and operation, the production management, equipment maintenance and technical management systems have been fully established at the Husab mine and its operational activities, such as the process plant and mining production, has gradually progressed. However, problems have arisen owing to the greater variation in the shape of orebodies than expected, inaccuracies in the measurement of uranium grade, large scale mine haulage fleets and precision loading errors, etc. Several outstanding issues have been identified in respect of consistency between the grade of resource model and that of mined ore, control of dilution, loss in the course of mining such as high dilution and loss ratios, and large variation in grade in the mining process. These issues will all have a negative effect and impede the stable production at the Husab mine. It is therefore urgent to carry out a study on the key techniques of uranium ore grade control in order to solve these mining production issues.

This study focuses on the entire process of mining production and has conducted the following optimization work including: establishment of a geological resource–grade control model system, optimization of the mining production procedures, application and optimization of controlled blasting technology to ensure a higher precision and application of rapid and accurate grade measurement by down-hole gamma logging. This optimization work will effectively improve the ore dilution and loss during mining and will further improve the production capacity and economics of the Husab mine.

2. ADVANCEMENT ON THE STUDY OF ORE GRADE CONTROL TECHNIQUES

2.1. Establishment and improvement of the three-stage resource–grade control model system

Currently, the ‘two stages’ resource–grade model system has been adopted at the Husab mine, which comprises a geological resource model based on the exploration database and grade control model based on down-hole gamma logging data from drill holes. The annual plan for mining production, blasting block

127 design, mining and stripping production plan and blast hole design are proposed on the basis of the geological resource model, and the mining block definition (composite model) and loading plan are proposed on the basis of the grade control model. However, owing to the fact that the drilling exploration grid is mostly 25 m × 25 m to 50 m × 50 m or larger and the fact that the geological resource model is not updated using the mining production data, the geological resource model has low accuracy and reliability with respect to mining production and thus it cannot be used to effectively guide the annual mining production plan, blasting block design, blast hole design or their optimization. At the same time, however, some model parameters reduce the accuracy of grade control model and thus it cannot accurately define the ore–waste boundary so as to meet the requirements of loading and ore blending.

On the basis of extensive discussion and site visits to other large scale open pit mines, combined with a review of actual mining at the Husab mine, this study proposes a hierarchical, segmental and stable ‘three stages’ geological resource model system consisting of a geological resource/reserve model, an interim model and a grade control model that can be updated dynamically. The grade control model is based on down-hole gamma logging data from blast holes by interpolating the ore grade for the block model of a specific blasting block and is used to guide the definition of mining blocks, wire connections for blasting, ore loading and blending. The geological resource model is created by using the geological database incorporating drill-hole data from resource drilling and blast holes from mining production. It interpolates the ore grade distribution for all the orebodies within the mining district and is used for optimization of the ultimate open pit boundary, for both mid-term and long-term mining, and for both stripping and annual mining production plans. The interim model proposed in this study is a transitional model between the geological resource and grade control models that establishes the interconnection between these models to form a comprehensive geological model system. This interim model is created by using down-hole gamma logging data from blast holes drilled for 2–3 benches above the current bench and resources drill-hole data below the current bench, it interpolates the grade distribution for the downward one or two benches from the current bench. It is used for the monthly and weekly mining and stripping plan, blasting block design and blast hole design and thus improves both blasting and mining efficiency.

2.2. Optimization and improvement of mining production procedures

Since pre-stripping, which commenced in 2013, and ore mining, which commenced in 2015, a set of mining production procedures were established and the mining production rate gradually increased. However, in the absence of a more accurate geological resource–grade control model, there is still much uncertainty to the entire mine production. It is therefore difficult to meet requirements with respect to the tonnage and grade of ore to be mined and thereby ensure the smoothness of mine production. The geological resource models are re-created and updated for the first stage of mining areas in Pit #1 and #2 based on an understanding of the mine geology and combined with grade data. This further defines the spatial distribution of the orebodies and the grade distribution, which will provide guidelines for preparing the mid- term, long term mining and annual mining plans. The interim model proposed in this study provides more accurate ore–waste boundaries within the benches to be mined and thus it lays a better foundation for preparing the monthly and weekly mining plans, blasting block design and blast hole design. These more scientific and reasonable grade control models and accurate ore–waste boundaries will greatly enhance the blasting efficiency and the output of mined ore, and reduce the ore dilution and loss and at the same time. It is also favourable to blend ore directly within the pit to reduce ore transport.

2.3. Application of advanced controlled blasting technology and its optimization

Up to now, mining production has focused on increasing the stripping capacity at the Husab mine and is not completely transitioned to focus on mining capacity to provide the ore in the tonnages and grades required by the process plant. Therefore, relatively simple blasting technology and blasting scheme are adopted currently at the mine. Different blast schemes have not been adopted owing to differences in the mechanical properties of rocks within different blasting blocks. In addition, the advanced blasting technology has not

128 been adopted to effectively separate the ore and waste. It is therefore necessary to improve blasting efficiency. In this study, the quality of rock mass was assessed on the basis of studying the physical and mechanical properties of the main ore and rocks within the mining district. The rock masses are divided into two categories according to their blastability. The first is rock that is easy to blast and comprises near-surface loose and poorly cemented sandstone and calcrete. The second category is rock mass that is difficult to blast and mainly consists of hard units of granite, marble, gneiss, schist and quartzite.

On the basis of analysis of the blasting parameters currently used at the mine, and considering the requirements of ore loading, hauling and primary crushing of ore, it is believed that the current blasting parameters are effective for blasting within the ore-bearing areas. However, it is not reasonable that these parameters be used for blasting within the areas of waste without any modification and these could be improved greatly. Consequently, significant research work has been conducted to improve blasting parameters for use within the areas of waste, and desktop analysis and digital simulation are also undertaken. The next step is to carry out the field trials on-site after which they can be applied at the mine.

In addition, it is difficult to control the ore dilution and loss owing to the fact that there is no interim model or accurate grade control model to guide mine production and the so-called special technologies, such as pure blasting, separating blasting and slag-remaining blasting. These are not applied at the Husab mine and, therefore, the field trial of blast induced movement monitoring does not provide a satisfactory result. It is proposed in this study that different blasting technology be adopted, including schemes to control the blasting and successfully separate the ore and waste according to their distribution within the mining area, based on the more accurate interim and grade control models. For the large-scale blocks with extensive areas of ore and waste, it is proposed that individually designed blast holes on the block of ore or waste be used to successfully separate the ore and waste when the ore area or waste area meets the requirements of an individual block. When the area or waste area is not large enough to be designed an individual block, modification of the blasting parameters such as the blast hole design, charge structure, blast delay time and initiation mode is proposed in order to generate physical zones that are easy to identify. Such zones could show visible difference in sizes within the ore or waste areas and flutes at the boundaries of ore and waste and by this means the ore and waste can be finally distinguished and guide the ore loading, thereby reducing ore dilution and loss.

2.4. Application of uranium grade measurement technology by down-hole gamma logging

Currently, the uranium content of ore is determined primarily by chemical analysis and assisted by radiometric measurement at the majority of uranium mines in the world except those in China and elsewhere in central Asia. It not only takes a considerable time but it is also expensive to determine uranium by chemical methods of analysis and thus it is difficult by this means alone to ensure the smooth operation of the Husab mine. A radiometric measurement system has been established, including the technology, equipment, operating manuals and technical standards, etc., over a period of several decades, drawing on the practices employed in the exploration and production of uranium mines in China. Since the start of the Husab mine, the radiometric measurement system and related operating manuals and standards were introduced gradually. This included key equipment facilities such as the down-hole gamma logging system, truck scanner system, belt scanner system at the primary crushing station and corresponding calibration of technical parameters, and the construction of standard calibration models on the site. This ensures that the grade of ore will be determined accurately and on time, fully meeting the production requirements at the Husab mine.

3. CONCLUSIONS

A set of comprehensive set of mining production procedures and technical standards were initially established when the Husab mine was put into production. The continuous research achievements in this study show that effective results have been achieved in the fields of the resource–grade control models,

129 optimization of the mining production procedures, controlled blasting technology and radiometric measurement technology. Thus, the mining capacity and efficiency has been improved effectively and the ore dilution and losses have been reduced, laying a strong foundation for the ramp-up in production at the Husab mine.

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