Dilution behaviour at Codelco panel cave mines

Transcription

1 5th International Conference and Exhibition on Mass Mining, Luleå Sweden 9-11 June 2008 Dilution behaviour at Codelco panel cave mines A. Susaeta IAL Ltda, Chile E. Rubio Universidad de Chile, Chile J. Henríquez Codelco Central, Chile G. País Universidad de Chile, Chile Abstract Conventionally, there are three main parameters that define dilution behaviour in panel cave mining: draw point layout geometry (draw point spacing), rock mass fragmentation and draw pattern. Depending on the ore body, mining geometry, and operational behaviour the underlying gravity flow can be classified as isolated draw, interactive draw or isolated-interactive draw. Draw point extraction back analysis was performed at Codelco mines aiming to determine the underlying gravity flow from dilution observations performed from 7 mines of the three mining Divisions that operate panel caving. The draw point database consisted of 5000 draw points and represented 365 Million tons drawn between 1996 and The draw point database was subdivided by draw point spacing; fragmentation and drawing behaviour, defining 21 representative draw point clusters. These clusters were characterized by an empirical dilution curve and an underlying flow mechanism. For every curve, the dilution entry point and the percentage of full interaction was read. As a result of the study a design and operational guideline is proposed to attain interactive flow of panel caving in different rock masses subject to draw uniformity restrictions. 1 Introduction The dilution behaviour in a Panel/Block cave mine is relevant since it defines mining recovery and ultimate economic return of a given mine design and production plan. Traditionally, the dilution behaviour in a Block and Panel cave mine has been computed using template mixings or volumetric algorithms as described by Laubscher (1994) and computer based algorithms as described by Diering (2000). The following chart shows a diagrammatic representation of how the volumetric algorithm would simulate mixing in a single draw column. It is seeing that when extracting slice 4 of the draw column dilution starts to enter the draw point. This point in the life of a draw point is called point of dilution entry (PDE) which Laubscher describes as a function of fragmentation characteristics, draw point spacing and drawing performance. Clearly, the geometry of the dilution curve shown below would dictate the average grade to be extracted and the ultimate mining reserves to be mined. Figure 1 Volumetric mixing algorithm

2 The method shown above consists of the volumetric mixing algorithm introduced first by Heslop and Laubscher (1982). This method still widely used in the mining industry to simulate the process of mixing as a result of the underlying gravity flow process in Block and Panel caving mines. A different algorithm that includes the differential vertical flow of fines has been introduced by Diering (2000) that is part of the PC- BC Gemcom Block Cave suit. Also, some more sophisticated algorithms that integrate rock mass behaviour and the constitutive behaviour of particle interaction have been introduced by Pierce and Castro using particle flow codes and cellular automaton respectively. Nevertheless, it is rear to find calibration of these sophisticated algorithms and the actual dilution observations performed at the mine sites. Thus, mixing templates based on simple rules still valid as they can reconcile with the dilution curves presented in this paper. The following chart shows a simulation conducted in a single draw column in which three dilution models have been simulated in order to state the mining reserves for a given cut off grade. In these models the main parameter to define dilution behaviour is the percentage of dilution entry (PDE). This would characterize the point at which the first dilution shall be observed at the draw points. Figure 2 Diluted Columns for PDE=35%, 65% and 85% Based on a draw column of 100m of ore of 1% copper and an overlying column of 100m of waste of 0% copper the expected mixed grades at the draw point for the three mixing models presented above are shown below. Figure 3 Grade inside diluted Columns. 168

3 As a result of the above for a cut off grade of 0.7% copper the best height of draw (Hrecov), the mining recovery (% Rec), the average grade in the mineable column (Lm) and the copper content (Copper) are presented in the following table. Clearly it is shown that the impact of the dilution entry point has a tremendous implications on defining the mining reserves and the copper content which are the main parameters delineating the mining business. However it should be noted that these results are highly dependant on the cut off grade and also the ore body under analysis. The results would be quite different if the ore body had a constant grade without waste on top of the ore body, which is often found in porphyry copper deposits. Based on these results it is worthwhile analyse the dilution behaviour of Codelco mines to put all the elements regarding dilution behaviour under perspective, aiming to use these results to further calibrate mixing templates or particle flow codes in the process of stating Panel and Block cave mining reserves. Table 1 Column report for different dilution entry points at 0.7% Cu cut off grade 2 Background and Hrecov Hnom % Rec Lm Copper (m) (m) (%) (%) (t) PED= PED= PED= PED= Traditionally it is understood that dilution behaviour is the result of the underlying gravity flow process that takes place in the ore body that has been previously caved. The gravity flow is induced by the propagation of void through the broken ore that lies on the production level crown pillar. The fundamentals of gravity flow for panel caving currently used began with sand experiments performed by Marano (1980), Heslop and Laubscher (1981) performed at Shabaine mine at the beginning of 70 s. These experiments were performed in a three-dimensional model of 2400mm of height and a base of 760mm x 760mm. The firs experiment consists of several draw points spaced at 206 mm apart in which drawing is performed simultaneously. This experiment show that there are drawing zones formed of about 108 mm wide in which there is no interaction between draw zones. This mode of flow is often called isolated flow. The second experiment consists of several draw points spaced at 108 mm apart in which the drawing is performed simultaneously. This experiment shows that there is a uniform draw down of the draw column. This mode of flow is often called interactive flow. The third experiment consists of setting up the draw point spaced at 152 mm apart in which the drawing is performed simultaneously. This experiment shows that there is interactive draw with a large amount of mixing along the draw columns, i.e. fines segregation is observed along the draw column. This mode of flow is often called interactive flow. Finally the last experiment consists of performing uneven draw as draw points were drawn sequentially in a draw point layout spaced at 108 mm. The results of this experiment show that there is chimney and there was no interaction across the major apex. This mode of flow is often called isolated flow. The following diagram shows the fourth experiments performed by Marano (1980) 169

4 Figure 4 Gravity flow experiments (Marano, 1980) Susaeta (2004) reports results from a research project in which gravity flow experiments were performed in a 1m x 1m x 2,4m sandbox physical model. The results of this research very much confirmed the results obtained by previous authors. Nevertheless, the author provides insights regarding the third case described by Heslop and Laubscher as interactive draw introducing the concept of Isolated-Interactive draw. In this new mode of flow Susaeta introduces the rate of drawing down differentiating the rate of flow of the draw point itself (vta) compared with the rate of the material overlying the major apex pillar (vti). The following diagram illustrates the concept. 170

5 Figure 5 Isolated Interactive flow model (Susaeta, 2004) The author describes flow modes as a function of the degree of interaction (Gi=vti/vta) which is the ratio between the interactive and isolated draw rate. Thus, if vta>vti would induce Isolated-Interactive flow or vta=vti would induce Interactive flow or vta>0 and vti=0 would induce Isolated flow. The following diagram shows the expected flow modes as a function of the degree of interaction. The author also states that the main underlying parameter to make the transition between interactive flow and Isolated Interactive flow is the draw point drawing performance. The authors validates his hypothesis showing a relationship between the uniformity index behaviour over the life of a cluster of draw points and the degree of interaction measured from remaining reserves obtained from major apex core drilling. Based on these flow modes the author proposes three main models of dilution as shown below. Figure 6 Dilution models derived from the Isolated-Interactive flow mode The percentage of extraction where dilution appears, or in practical terms starts to grow, is called Isolated Dilution Entry Point (PEDZA). At the same time, the percentage of extraction where the dilution tendency changes its slope starting to increase after the PEDZA, is called Interactive Dilution Entry Point (PEDZI). As it has been shown by many Block and Panel cave researchers the underlying gravity flow that dictates the dilution behaviour is highly dependant on the draw performance at production level in a short time interval. Thus, in order to characterize the expected dilution behaviour one needs to account on how even or uneven draw points are going to be mined over time. Laubscher(2004) proposed the draw control factor, an index varying between 0 and 1 which is a linear function of the coefficient of variance of tonnages mined between 171

6 a draw point and its neighbours in a period of time of a week. A modification of this index was proposed by Susaeta (2004), who considers not only the relative tonnage drawn by the neighbouring draw points, but also the inactive draw points. The system allows evaluating each shift drawn tonnage per draw point associating it to a uniformity index. The uniformity index is computed as follows: : Inactive number of draw point neighbours. : Correction factor, 99/89. te po : Extracted tonnage of the studied draw point. te pi : Extracted tonnage of neighbour i. n (tep0 t min) VUI (t max te pi ) (3) t n 2 max tmax: Maximum extracted tonnage in the period taking into account all neighbours. tmin: Minimum extracted tonnage in the period taking into account all neighbours. n: Number of draw point neighbours, 7. As a result of the uniformity index calculation one could classify a given period of time, for example 3 shifts, as Uniform, Semi Uniform or Non Uniform as shown below. i 1 Table 2 Uniformity index table [0-0.2) [ ) [ ) [ ) [0.8-1) 0 Uni Uni Uni Semi Semi 1 Uni Uni Semi Semi Semi 2 Uni Semi Semi Semi Non 3 Semi Semi Semi Non Non 4 Semi Semi Non Non Non 5 Non Non Non Non Non 6 Non Non Non Non Non Then, the life cycle of a draw point could be characterized as a function of the percentage of the time that a draw point has been drawn non uniform or the amount of tonnage draw from a draw point that has been drawn uniform. All these indicators could assist mining engineers to characterize the drawing behaviour of a cluster of draw points and correlating this behaviour with measured mining recovery and dilution. This exercise was performed at Codelco mines aiming to find at different sites the dilution models presented before and perhaps infer on the flow modes present as a function of different rock masses as means of fragmentation, draw point spacing and drawing performance. 3 Codelco Mines Back-Analysis In April 2006 Codelco Chile decided to prepare a guide to standardize the methodology to determine reserves for panel caving across the organization. In order to determine the mining reserves flow modes had to be stated for different rock mass fragmentation, draw point spacing and drawing performance. Since there is no a constitutive law that defines the Panel Block cave underlying gravity flow behaviour, the flow modes were inferred from dilution models fitted from empirical dilution observations collected over the years at El Salvador, Andina and El Teniente mines. The database used in the study is described below. 172

7 Table 3 Codelco Production Databases Mine Sector Years Extracted Draw Tonnage Points [Mt] [#] Andina Parrillas, III P Andina LHD, III P Salvador ICW Salvador IN Teniente Teniente Teniente Qda. T Teniente Esmeralda Teniente Regimiento Total Codelco defines primary rock as a rock mass that lacks of discontinuities and shows a Laubscher Rock Mass Raiting (1989) greater than 70. Then, the rock mass fragmentation of a draw point was classified as Secondary, Mixed or Primary depending on the amount of primary rock present in the column. The definition of this fragmentation tags are defined as follows Secondary rock column: 0-15% of the draw point column in situ model with Primary Rock. Mixed rock column: 15-50% of the draw point column in situ model with Primary Rock. Primary rock column: % of the draw point column in situ model with Primary Rock. Then, every draw point of the database is assigned a fragmentation tag based on the amount of primary rock present in the column. Also, every draw point has associated a mine layout that characterizes its draw point spacing. Finally, the drawing performance was characterized using the uniformity index proposed by Susaeta (2004) in which the time periods were three shifts and six neighbours. Thus, for every draw point and shift the uniformity index is computed using a computer application showing a uniformity index time series of a draw point or a cluster of draw points. Thus, for every shift a draw point is classified as uniform, semi uniform or non uniform according to the classification showed on table 1. Then an indicator of draw performance called CUI is computed as the percentage of tonnage drawn as uniform or semi uniform over the 100% tonnage extraction. The 100% tonnage extraction of a draw point is computed as the tonnage to reach the interface between insitu economic column and broken rock. For example the following selected draw points show their uniformity index in a period of time and the evolution of dilution over their life time. Based on the geometry of the curve PEDZA and PEDZI are assigned to the cluster and added to the analysis.. Figure 7 Uniformity index and dilution visualizer. 173

8 All the analysis were performed for individual draw points identifying clusters of draw point showing CUI in a given range. Also, the clusters of draw points were selected in such way that rock type and draw point spacing was the same for all draw points in the cluster. The following charts show some of the clusters analysed as part of the study. 3.1 Empirical Dilution Models A cluster of draw points from the mine Esmeralda of El Teniente that is located in primary rock and a draw point layout of 15x17.2 shows the following dilution behaviour for different ranges of CUI. Figure 8 Esmeralda primary rock dilution behaviour, 15x17.2 layout. Figure 8 shows the observed dilution for draw points showing a CUI in the range of 40-80% and %, the number of draw points in the cluster are 43 and 208 respectively. It must be noted that the behaviour of this draw points are shown until 150% extraction (%E), nevertheless the draw points where selected by their CUI measured upto 100% of drawn. It is seeing in the graph that for primary ore there is a interactive draw behaviour for a LHD 15 x 17,2 m layout, where PEDZA is highly sensitive to the CUI range. Draw Columns of Andina-LHD sector are composed only of mixed rock. The diluted material is called Rhyolite which is a geologic marker, included as a fraction of the overburden material. The curve behaviour considering draw points with two CUI ranges are shown below. Figure 9 Mixed rock column dilution behaviour, 13x13 LHD Layout. 174

9 It is seeing again that a better uniformity of draw performance leads to a lower dilution entry, both in percentage of extraction and total dilution up to 100%E. The Pedza & Pedzi show interactive-isolated flow behaviour. Draw columns of Andina-Parrillas (grizzly) draw points are composed only of secondary rock according with the rock definition mentioned above. The diluted material is Rhyolite and the sector is characterized by four grizzly layouts: 9x9, 9.4x9, 9x11 and 9x11.3. Analysis was performed grouping data as it is shown in the following graphs considering uniformity analysis until the 100%E. Figure 10 Secondary rock dilution, 9x9 & 9.4x9 Grizzly Layout Figure 11 Secondary rock dilution, 9x11 & 9x11.3 Grizzly Layout 3.3 Dilution Results Summary The summary of all the analyzed data is presented in Table 3, where the three draw function variables: geometry is defined by the Layout/Method, the fragmentation by the Rock column description (secondary, mixed and primary), and the draw uniformity by the Uniformity index (% uniform + semi uniform tonnage drawn of the column up to 100% extraction). The flow behaviour of each of the different cases is defined as Isolated Flow (Is), Interactive Isolated Flow (I-I) and Interactive Flow (n), considering the shape of the dilution curve. The flow mode is also characterized by its dilution entry point for the isolated and interactive flow (Pedzi & Pedza). The number of draw points that belong to each of the different data clusters is also presented in the table. 175

10 Table 4 Summary of PEDZA and PEDZI for Selected Clusters Mine Sector Layout/Method Draw Points Rock column CUI (mxm) # %U+S %E %E Pedza Pedzi Flow Mode Andina Parrillas IIIP 9x9/grizzly 51 Sec I-I Andina Parrillas 9x9/grizzly 52 Sec I-I Andina Parrillas 9x9/grizzly 32 Sec I-I Andina Parrillas 9x9/grizzly 8 Sec. > I-I Andina Parrillas 9x9/grizzly 51 Mix I-I Andina Parrillas 9x9/grizzly 18 Mix I-I Andina Parrillas 9x9/grizzly 9 Mix I-I Andina Parrillas 9x9/grizzly 2 Mix > I-I Andina Parrillas 9x11/grizzly 14 Sec I-I Andina Parrillas 9x11/grizzly 19 Sec I-I Andina Parrillas 9x11/grizzly 14 Sec I-I Andina Parrillas 9x11/grizzly 38 Sec. > I-I Andina LHD IIIP 13x13/LHD 35 Mix I-I Andina LHD IIIP 13x13/LHD 65 Mix I-I Andina LHD IIIP 13x13/LHD 75 Mix > I-I Salvador IN 13x13/LHD 17 Sec. > Is Salvador IN 13x13/LHD 121 Mix > I-I Salvador IN 13x13/LHD 14 Prim >75 75 >100 In Salvador ICW 15x15/LHD 8 Mix > Is Mine Sector Layout/Method Draw Points Rock column CUI (mxm) # %U+S %E %E Pedza Pedzi Flow Mode Salvador ICW 15x15/LHD 20 Prim > I-I Teniente Queb.T. 7.5x7.2/grizzly 667 Sec Is Teniente Queb. T. 7.5x7.2/grizzly 72 Sec. > I-I Teniente Queb. T. 7.5x7.2/grizzly 47 Mix >100 In Teniente Esmeralda 15x17.2/LHD 43 Prim I-I Teniente Esmeralda 15x17.2/LHD 208 Prim > I-I Teniente Teniente 4 15x17.2/LHD 62 Mix <40%* 18 - Is Teniente Teniente 4 15x17.2/LHD 137 Prim <40%* 28 >100 I-I In : Interactive Flow, I-I : Interactive Isolated Flow, Is: Isolated Flow The chart below shows PEDZA versus CUI for Andina Grizzly sector shows the effect of draw point spacing and CUI in regards to the percentage dilution entry point defined in this study as PEDZA. This chart outlines and confirms the Isolated-Interactive draw theory in which for a fully interactive draw point spacing layout the influence of draw performance decreases as shown for 9x9 draw point layout. Different behaviour is observed for the Isolated-Interactive flow induced by a draw point layout of 9x11 in which the CUI is highly significant the moment of analysing dilution behaviour. 176

11 Figure 12 Secondary rock dilution, 9x11 & 9x11.3 Grizzly Layout 4 Proposed design table as per flow behaviour Considering the results showed before and selecting those clusters of draw points showing a CUI greater than 80%, rock mass and draw point layout are correlated with the inferred flow mode in order to establish a design guideline for the future Block/Panel cave mines of Codelco Chile. The following table summarizes these results. Table 5 Design table as per flow mode for uniform draw (CUI>80%) Layout Rock Column (In Situ) Secondary Mixed Primary 15x17.2/LHD Is Is Is Is I-I I-I 15x15/LHD Is Is Is Is I-I I-I 13x13/LHD Is Is I-I I-I In In 9x9/Grizzly I-I I-I I-I I-I 7.5x7.2/Grizzly I-I I-I In In In : Interactive Flow, I-I : Interactive Isolated Flow, Is: Isolated Flow It is interesting to note based on the results shown above that the overall trend for a dilution perspective is to reinforce the use of close spaced layout draw points. There is a no an easy answer to whether or not a Block and Panel cave operation should minimize the amount of dilution. It would depend on the ore body and grade distribution across the ore body. There are some other considerations to include in the analysis as layout productivity, fragmentation, development cost, ore body characteristics. The optimal draw point layout should obey to a comprehensive analysis that includes all these aspects of the mine design. It is aimed that the table presented above could support the dilution analysis related to the decision of draw point spacing. 5 Conclusions The dilution curves constructed for Codelco mines presented in this paper follow the dilution models proposed by the Isolated Interactive flow theory. It is inferred that all the three modes of flow are present at Codelco mines the different modes unfold for different draw point layout and fragmentation. It was shown that the draw performance has a tremendous effect on the dilution behaviour of a draw point. In particular when draw point spacing has been designed in such a way that interaction is minimal the relevance of even draw is crucial to achieve Isolated Interactive flow. For a mixed rock mass the recommended draw point spacing is 13m to achieve Isolated Interactive draw with performing even draw. For primary rock the draw point spacing should be at the most 15m to achieve Isolated Interactive flow.. Several operations around the world will be looking at Block or Panel cave 177

12 designs for their ore bodies that are in the range of mixed and primary rock. The tendency to use wider draw point spacing could eventually affect the dilution behaviour and the overall mining reserves. It is interesting to note based on the results shown above that the overall trend for a dilution perspective is to reinforce the use of close spaced layout draw points. This result goes against the industry trend of using widely spaced layout in order to achieve higher productivity and more reliable rock mechanic design. Nevertheless, it is highly important to review the basics of mine design that must be founded in the ore body characteristics and geological setting rather than quick and incomplete economic return. The sustainability of block and panel cave operations would force the industry to look at methods that could enhance the way how dilution behaves within the mining and metallurgical processes in order to optimize energy consumption. Yet, it is believe that in years to come must attention shall be addressed over dilution behaviour disregarding too much attention over economic return could not only jeopardize the life of a mine but also create a non recoverable sank of energy. Acknowledgements The authors of this paper would like to thank CODELCO for the permission to publish these results and especially to all the planning engineers of the three Divisions that participated in the development of the standardization guideline. Acknowledgements should also be given to the University of Chile for supporting and holding the development of the project that sustains the resulting standards of summarized in this paper. References Diaz, H., Susaeta,A, (2000), Modelamiento del Flujo Gravitacional, Revista Minerales, in Spanish. Susaeta, A. (2004) Theory of gravity flow (Part 2), MassMin Proceedings 2004, A.Karzulovic &M.Alfaro, Minería Chilena, Santiago, Susaeta, A. (2004) Theory of gravity flow (Part 1), MassMin Proceedings 2004, A.Karzulovic &M.Alfaro, Minería Chilena, Santiago, Susaeta.A., Rubio.E, Pais.G., Troncoso.S, Barrera.S, (2006), Guía Estandarización Metodología de Determinación Recursos Extraíbles en Hundimiento por Paneles Codelco Chile, IAL Ltda.. Internal Report. Marano, G., "The interaction between adjoining draw points in free flowing materials and its application to mining", Chamber of Mines Journal, Zimbabwe, pp Laubscher, D.H., "Block cave manual, design topic: drawpoint spacing and draw control". For the International Caving Study , The University of Queensland, Brisbane, Australia. Laubscher, D.H., "Cave mining - the state of the art", The Journal of the South African Institute of Mining and Metallurgy, vol 94 no 10, pp Heslop, T.G., and Laubscher, D.H., "Draw control in caving operations on Southern African Chrysotile Asbestos mines", in Design and Operation of Caving and Sublevel Stoping Mines, pp Ed. D.R. Stewart. SME- AIME, New York. 178