[Abstract] This paper, based on practical experience, introduces the design and development of an automatic control system for cyanide processes based on DCS. It details the hardware and software design of the automatic control system for a gold mine cyanide plant, as well as the connection of the communication network. Through the design of four main components—field data acquisition, upper-level monitoring in the control room, LED large-screen display via DDE, and remote access—centralized control of field equipment and information resource sharing are achieved, improving the overall automation and management level of the gold mine and compensating for the generally low level of automation in gold mines.
Keywords : Cyanide distributed control system PLC, gold mine
1 Introduction
Cyanide smelting, a key step in gold ore refining, plays a crucial role in the entire metallurgical process. However, my country's cyanide technology is still immature and lacks standardized procedures. Most processes are completed independently by enterprise technicians through trial and error, with manual operation and monitoring of most equipment. Due to the large number of equipment in workshops, harsh conditions, and significant external interference, coupled with stringent process requirements, achieving optimal control is difficult. Furthermore, the continuous expansion and modification of the process flow further increases the complexity of on-site operations.
With the widespread use of computers, computer-based management systems are gradually gaining popularity in modern enterprises. Computers handle the rapid processing of large amounts of data and repetitive tasks, while managers perform the creative work of analyzing, judging, and making decisions based on the results. This human-machine system, which makes the best use of human and resources, is the current main paradigm for computer-based enterprise management. Therefore, to ensure more stable technical indicators and smoother processes in the cyanidation process, while simultaneously increasing the extraction rate, reducing worker workload, and improving the working environment, full automation of the cyanidation process is imperative.
2. Overall Design of Automatic Control System for Cyanide Plant
2.1 Cyanide process
The main processes include feeding, grinding, alkaline leaching and filtration, leaching, displacement, pre-flotation filtration, comprehensive recovery, and flotation tailings filtration, ultimately separating gold, silver, copper, and lead; the tailings are recovered and used to extract other non-metals (such as sulfur). The process flow diagram is shown in Figure 1.
Figure 1 Schematic diagram of the cyanidation process for gold and silver ore
2.2 Main Control Requirements of the System
The system has 108 analog input points, 101 analog output points, 698 digital input points, and 73 digital output points. Its main control functions include:
(1) The status (start, stop, fault) of all motors is displayed on the computer in the main control room. Ordinary motors can only be controlled on-site, while motors with frequency converters can be controlled on-site, remotely, and partially automatically. The frequency and current of the motor can be displayed on-site. In manual mode, the motor can be started and stopped on-site, and the frequency converter frequency can be set. In remote control mode, the operator can start and stop the motor through the computer in the main control room, and set the frequency converter frequency. In automatic mode, the PLC can compare the data collected on-site with the process requirements, and control the motor output frequency through calculation to meet industrial requirements and realize closed-loop control of the motor.
(2) Collect and summarize important data (liquid level, pressure, density, flow rate, pH, etc.) in real time at the production site, and automatically control the relevant instruments based on this data to improve the level of process indicators. At the same time, display this data on the LED screen in the workshop so that workshop workers can make targeted decisions about their operations. Finally, archive this data to form trends and reports, and print them out regularly.
(3) Accumulate important workload indicators such as the amount of ore processed on site and the number of filter press outlets in each section.
2.3 Control System Composition
2.3.1. Introduction to SIMATIC PCS7 Series
This system is primarily built using field control equipment from Siemens AG, mainly utilizing the SIMATIC PCS7 series. The SIMATIC PCS7 is an advanced process control system developed by Siemens based on the TELEPERM series distributed control system and the S5 and S7 series programmable controllers, combining advanced electronic manufacturing technology, network communication technology, graphics and image processing technology, fieldbus technology, computer technology, and advanced automation control theory. It uses the excellent WinCC host computer software as the human-machine interface for operation and monitoring, utilizes open fieldbus and industrial Ethernet for field information acquisition and system communication, and employs the S7 automation system as the field control unit to achieve process control, receiving field sensor signals through flexible and diverse distributed I/O. The SIEMENS PCS7 process control system possesses the following characteristics:
1. High reliability and stability;
2. High-speed, high-capacity controller;
3. Client/server architecture;
4. Centralized, top-down configuration method;
5. It can be flexibly and reliably integrated into older systems;
6. A centralized, user-friendly human-computer interface;
7. Batch processing packages with recipe functionality;
8. An open structure that allows communication with the management level;
9. It is integrated with fieldbus technology.
SIMATIC PCS7 employs programming software and a field device library compliant with the IEC 61131-3 international standard, providing continuous control, sequential control, and high-level programming languages. The field device library offers a large amount of information on commonly used field devices and function blocks, greatly simplifying configuration and shortening project cycles. SIMATIC PCS7 features standard interfaces such as ODBC and OLE, and utilizes open networks such as Ethernet and PROFIBUS fieldbus, thus possessing strong openness and easily connecting to host computer management systems and control systems from other manufacturers. Figure 2 shows a system diagram built using SIMATIC PCS7.
Figure 2 Schematic diagram of SIMATIC PCS7 system
2.3.2 System Introduction
Based on the superior performance of PCS7 and in accordance with the tender requirements, we designed and selected the equipment for this system, which can meet the process requirements with optimal configuration. This system has two central control rooms. Central control room #1 is equipped with a CPU414-2DP master station, which controls, monitors, and manages the main production processes including the feeding section, grinding section, alkaline leaching and filtration section, leaching section, and displacement section. The central control room monitors, manages, and operates the current status and trends of the production process, thereby ensuring the stable and reliable operation of the production equipment. Based on the process and the distribution of field equipment, six field control stations (#1, #2, #3, #4, #5, and #6) are configured, each using ET200 distributed I/O. The central control room (No. 2) is equipped with a CPU412-2DP master station, which controls, monitors, and manages the main production processes, including the pre-flotation filter press section, the integrated recovery section, and the flotation tailings filter press section. Based on process requirements, two field control stations (No. 1 and No. 2) are configured, each using ET200 distributed I/O. The field control stations are connected to the master station via a PROFIBUS-DP bus, and the master station transmits process data and information to the central control room operator station via a high-speed data channel. The operator station also has online/offline configuration capabilities and self-diagnostic functions. Each field control station in the distributed control system can independently complete its own control tasks.
The data acquisition process is roughly as follows: The output signals of the field sensors are collected by the signal templates of each station, converted into corresponding digital signals, and then sent to the 400PLC master station via the communication module. The 400PLC master station performs various calculations and processes on the data sent from each station as required, and then transmits it to the server via the MPI network. Data is transferred between the client and the server via OPC.
The entire system uses Siemens' WINCC V6.0 (host programming) and STEP7 V5.4 (slave programming) as its development platforms. Real-time signals from the field are collected via the ET200M distributed system distributed throughout the plant. The S7-400 master station processes the data transmitted from each slave station, and through calculations, achieves automatic control of key control points. Siemens' STEP7 is primarily used for PLC programming. The host server configuration uses SIMATIC WINCC to achieve real-time data display, data query, and report printing. Simultaneously, the DDE connection in WINCC is used to import key data into Excel, which is then connected to the workshop's LED screen via a local area network. The displayed data is updated every minute, allowing field operators to monitor the work progress in real time. The 400 master station connects to the various slave stations via PROFIBUS-DP to complete data transmission; this connection method is suitable for distributed processing scenarios where data is relatively dispersed. To ensure the real-time performance and accuracy of data acquisition, the 400 master station connects to the host computer's data acquisition card via the MPI protocol. The signals collected by each substation are processed by the communication module and then transmitted to the 400 master station via the PROFIBUS bus. The 400 master station processes the data and transmits it to the host computer (server) via MPI. The server is then integrated into the enterprise network, making client expansion extremely simple. Simply connect the computer to the local area network and share data using the OPC read/write protocol built into WinCC. The overall system configuration is shown in Figure 3.
Figure 3 System Overall Configuration Diagram
3 System Hardware Design
3.11# Main Site Module Selection and Introduction
There are 42 two-wire 4-20mA analog input signals in the No. 1 field control main station. We selected 6 8-point AI modules SM331-7KF to form the No. 1 field control substation.
The specific distribution of substation #1 is as follows:
There are 55 four-wire 4-20mA analog input signals in the No. 1 field control main station. We selected eight SM331-7NF 8-point AI modules to form the No. 2 field control substation.
The specific distribution of substation #2 is as follows:
There are 439 digital input signals in the No. 1 field control main station. We selected 16 32-point DI modules SM321-1BL, 8 of which form the No. 3 field control substation, 5 of which form the No. 4 field control substation, and 3 of which are placed in the No. 6 field control substation.
The specific distribution of substation #3 is as follows:
The specific distribution of substation #4 is as follows:
There are 75 analog output signals in the No. 1 field control station. We selected 11 8-point AO modules SM332-5HF. Eight of them formed the No. 5 field control substation, and the other three were in the No. 6 field control substation.
The specific distribution of substation #5 is as follows:
There are 56 digital output signals in the No. 1 field control master station. We selected two 32-point DO modules SM322-1BL, which, together with three analog output modules and three digital input modules, formed the No. 6 field control substation.
The specific distribution of substation #6 is as follows:
3.2 Connection at the Signal Acquisition Site
The wiring methods for field sensors in this system are generally of two types: four-wire and two-wire. For voltage, current, and power signals, the instruments use a four-wire connection: the field instruments have a dedicated power supply and output 4-20mA current. For pressure and flow signals, a two-wire connection is used. This connection has two methods: one is that the instrument and signal acquisition module are connected in series and then powered by a 24V power supply on the terminal block; the other is that the signal acquisition module's internal power supply directly powers the field instruments. The latter method is used for most two-wire instruments. The acquired signals use a unified numbering system: station number—signal input template number—signal input template channel number, for easy future reference.
3.3 Communication between stations
Each substation is connected to the 400 PLC master station via PROFIBUS-DP. For distributed processing scenarios where data is relatively dispersed, this connection method is undoubtedly economical and reasonable. Its more prominent advantage is that if one or more substations experience a power outage or other malfunction, it will not affect the normal operation of other substations. For substations that are far apart, a repeater is added to each substation to ensure the accuracy and real-time performance of data transmission. Field instrument signals acquired by the substations are processed by the communication module and then sent to the 400 PLC master station via PROFIBUS-DP. After processing by the 400 master station, the signals are transmitted to the host computer via MPI. The 400 master station uses the MPI protocol to connect to the host computer's data acquisition card, the SIEMENS CP5611, which is compatible with the MPI protocol. Using MPI can very economically solve the problem of high-speed signal transmission. However, the transmission distance using MPI is not very long; therefore, repeaters are installed in the data acquisition boxes of the more distant substations to extend the transmission distance and ensure reliable signal transmission.
3.4 Data Network Communication
The general communication structure of this system is as follows: the master station connects to each substation via PROFIBUS_DP to complete data transmission. The 400 master station connects to the data acquisition card of the host computer via the MPI protocol, and the server is integrated into the enterprise network. This makes client expansion extremely simple; simply connect the computer to the local area network and share data using the OPC read/write protocol built into WinCC. To help operators monitor production in real time, the system displays the data collected by the PLC on the workshop's LED screen. This is mainly achieved by connecting the computer connected to the large screen to the system's monitoring computer network via a hub, forming a small local area network. Data is retrieved from the WinCC runtime system's database and transferred to the external application Excel using a DDE connection, and the imported data is periodically saved. This allows for dynamic tracking of all process data that needs to be displayed on the large screen.
4 System Software Design
The entire system's software uses Siemens' WINCC V6.0 (upper-level programming) and STEP7 V5.4 (lower-level programming) as development platforms.
The 400 master station processes the data transmitted from each substation and realizes automatic control of each important control point through calculation. It mainly uses Siemens' STEP7 to program the PLC.
The host computer, configured via WinCC, primarily enables real-time data display, archiving, querying, and report printing. Simultaneously, it utilizes WinCC's DDE connection to import key data into Excel, which is then connected to the workshop's LED screen via the local area network for display, with the displayed data updating every minute.
4.1 PLC Program
4.1.1 PLC Network Configuration Diagram
Figure 4 PLC network configuration
4.1.2 Main Loop Program Block OB1 Flow
Figure 5 OB1 execution sequence
4.1.3 Main numerical conversion program
Numerical conversion program
Figure 6 PLC numerical conversion program segment
4.3 Application of DDE in this system
This system requires displaying data collected by the PLC on a large LED screen in the workshop in real time, facilitating on-site operation for workers. The main method is as follows: First, the computer connected to the large screen is connected to the system's monitoring computer network via a hub, forming a small local area network. The IP addresses and other relevant parameters of each computer are configured. Then, on the monitoring computer, data is transferred from the database of the WinCC running system to the external application Excel using a DDE connection. In Excel, the data format and content to be displayed on the large screen are designed, allowing for real-time dynamic display of various data. Since the large screen system cannot dynamically read data from Excel, various methods were explored, ultimately using Excel's macro processing commands to periodically save the imported data. This allows for dynamic tracking of all process data to be displayed on the large screen.
Figure 7 below illustrates the interaction between the Excel WinCC Explorer and the WinCC runtime system.
Figure 7 Data interaction between WinCC and Excel methods
5 Key Subsystems – Deoxygenation Tower Pressure Control System
Vacuum deoxidation is crucial in the replacement section, directly affecting the gold replacement ratio. The vacuum inside the deoxidation tower is achieved by a hydraulic jet pump, while the inlet and outlet of the tower are controlled by electric regulating valves to regulate the slurry flow. Due to wear and tear from prolonged use, the flow rate changes non-linearly, and the pressure within the pipeline fluctuates. Therefore, it is difficult to control the deoxidation tower pressure using ordinary PID algorithms. Thus, we employ fuzzy control in the deoxidation tower pressure control system, transforming the control strategy based on expert experience into an automatic control strategy to achieve the desired control effect. The pressure transmitter on the deoxidation tower converts the negative pressure inside the tower into a 4-20mA current signal, along with the valve position feedback signals from the two electric regulating valves. These signals are converted into digital signals by the PLC's analog input module, enter the fuzzy controller, and then output two 4-20mA current signals to control the two electric regulating valves respectively, ultimately achieving stable negative pressure. The software structure flowchart is shown in Figure 8.
Figure 8. Flowchart of deoxygenation tower pressure control software
6. Conclusion
This system is a comprehensive and wide-ranging monitoring system that integrates modern information technologies such as communication, networking, PLC, computers, adaptive control, fuzzy control, intelligent detection, and automation. It performs real-time data acquisition and analysis to achieve overall monitoring of the entire cyanidation process, solving the long-standing problem of mining enterprises being unable to implement effective automatic control. The system is currently in operation and is functioning well.
About the author:
Shu Jianwei, male, is currently a postgraduate student. His main research area is industrial control.
