1 April 2002

Mining's New Age

Mining in the 21st century defies low-tech perceptions that summon Hollywood images of rural Kentucky, frigid British coal fields, or steaming South African diamond mines, where members of the desperate class toil in obscurity and dust.

Mechanically automated for years, robots, programmable logic controllers (PLCs), three-dimensional simulation and optimization software, and wireless Ethernet all have a place in the multibillion-dollar mining and metals industries.

—Nicholas Sheble

Mexican mining operation relies on redundancy.

As Mexico's role in the world economic community grows, its ability to make use of its natural wealth, including vast underground deposits of minerals and metals, is critical. Industrias Peñoles, S.A. de C.V. recently began mining lead, zinc, and other metals from the Francisco I. Madero Project (FIM) 600 kilometers northwest of Mexico City in the state of Zacatecas.

Peñoles plans to remove approximately 3 million metric tons of ore per year from FIM. Its ore processing system relies on redundant PLCs; software for human-machine interface (HMI) and supervisory control and data acquisition (SCADA); and a custom-designed, PC-based, advanced control system.

Peñoles expects the system, which went online in July 2001, to enable it to process 8,000 tons of ore per day.

Before lead and zinc concentrate can be loaded onto rail cars and shipped to a Peñoles refinery in the nearby city of Torreon, the mined ore must be put through a complex process of milling and flotation to separate the desired metals from unwanted waste material.

Rending Good from Garbage

Chunks of coarse ore arrive at ground level via a series of conveyor belts and feed into storage. From there, the ore goes to a semiautogenous grinding (SAG) mill, which performs an initial grinding. Five-inch steel grinding balls coming from an automated ball feeder, as well as water and the ore, feed into the mill.

A variable-speed drive controls the rotation of the SAG mill, and SAG ball loading regulates up and down based on the feed characteristics and throughput requirements.

The ore then passes through vibrating screens. Oversized material discharges through a chute to a return system and travels back to the SAG mill for further grinding. The rest of the material conveys to two ball mills for final grinding.

The resulting slurry then feeds into a bank of hydrocyclones, each of which is 20 inches in diameter. These devices separate the coarse and fine solids. The fines collect and undergo analysis and discharge to the facility's lead flotation system.

The coarse material that settles in the bottom of the hydrocyclones returns for further grinding in the ball mills.

The slurry undergoes mechanical separations and receives several more additions of depressants, collector reagents, thickeners, and cleaners that separate the desired metal from the ore. Further, a number of chemical analyses happen that eventually determine when the resulting zinc concentrate is ready for travel by railcar to a refinery.

Redundancy a Key

Peñoles needed an automated system to coordinate all the data and ensure maximum, consistent yield from the ore, with the flexibility to alter quickly and accurately the characteristics of the concentrates and the ability to maintain maximum uptime.

The company constructed an automated system based on a redundant PLC/SCADA package and a customized expert control system. The contractors and mine operators organized a metallurgical team to set the objectives for the system.

Software development occurred in several phases. First, engineers developed the logic programming for automatic operation of the process equipment, followed by PLC programming and the creation of operation interface screens. Consultants then programmed its expert system to optimize production and the recovery of silver also present in the ore.

Among the expert system functions are the following:

Controlling set points in SAG and ball milling
Checking emergency situations throughout the plant
Checking operating status of equipment evaluation of ore characteristics
Optimizing the overall operation through informed responses to changing ore properties

Control hardware consists of redundant CPUs on one backplane communicating with supervisory computers through the Ethernet protocol; five PCs, including three operator stations running HMI/SCADA software; one computer hosting the expert system; and one computer for program development.

In addition to Ethernet communications between the PLC and supervisory stations, the system uses the Profibus fieldbus protocol and fiber-optic cabling for optimal communications between the PLC and the plant's I/O points.

The system monitors and/or controls approximately 1,200 I/O points. Plant operations occur 24 hours per day. Two operators are on duty during each of the three daily shifts.

—Rolando Gonzalez, M3 Mexicana

Doing time on the rock: Making little ones outta big ones

Grinding ore and minerals is one of the most expensive tasks in the refining process. One milling method mixes steel balls with the ore in a large, round, rotating chamber not unlike a home clothes dryer. These mills can be 10 meters in diameter and 3 meters deep.

The walls of the chamber have lifters that carry the ore and balls to a certain height before falling and cascading against one another, breaking the ore into smaller pieces in the process. This is a SAG mill.

Today's high-capacity SAG mills expend large amounts of energy and consume tons of steel balls and shell liners while processing ore. Most energy is expended on unwanted ball strikes on the mill shell. The causes of these inefficiencies are unclear. The harsh environment of the interior of the mill prevents instrumentation from locating there, so investigation is difficult. At best, each mine site engineers a good operating philosophy based on past experience.

Yet one can listen to the operating SAG mill and intuitively discern that numerous balls strike the shell or lifters, in effect losing 30% of the energy in metal-to-metal collisions.

But operators keep the noise level of the operation mill high enough to produce enough grinding action in the belly of the mill. Thus, mill capacity maintains at the expense of increased metal wear, and electrical energy is expended on unwanted ball strikes on the mill shell.

Researchers at the Idaho National Engineering and Environmental Laboratory have developed a simulation code called Millsoft that shows exactly the motion of charge in SAG mills. They tested the code at an Argentine mine. The simulator suggested using a 48-lifter configuration instead of the usual 72-lifter configuration for a 36-foot-diameter mill.

The mine site increased production from 45,000 tons/day to 85,000 tons/day. The consumption of ball charge decreased by 25%, a savings of 10 tons of balls, or $5,000/day.

—U.S. Department of Energy

Ethernet accesses draglines in the tundra from desktop in the cube

Using a wireless Ethernet radio link, a techno nerd can access this shovel's electrical drive and control system from a remote computer through telephone lines or the Internet.

All operational and diagnostic machine functions are available remotely.

Regular operation monitoring and immediate expert troubleshooting maximize machine uptime and reduce service costs.

Currently operating on a shovel at a mine in Sweden, this development presents a benefit in preventive/predictive maintenance for instantaneous service support anywhere in the world. As mines are often in remote locations and require extensive traveling for service calls, this connection can effectively keep the manufacturer connected to the user's machine not only at commissioning but also over the whole machine life cycle.

Hardware consists of an upgraded, standard HMI computer onboard the shovel, an Ethernet radio link, and a standard PC with modem or Internet access at the mine. The wireless Ethernet system conforms to IEEE 802.11B standard.

Configuration of the equipment is very flexible and can easily be adapted to any mine layout. The use of standardized components keeps initial installation costs low.

The system software allows user technicians to run all functions available on the shovel's HMI computer remotely.

This includes monitoring shovel parameters such as voltage, current, speed, torque, master switch, and logic signals under normal operation and during fault conditions.

Temperatures of electrical and mechanical components translate to set trends and pinpoint maintenance needs. Factory software upgrades download and install on lunch breaks.

In addition to factory support, the mine maintenance personnel can also use the system from a comfortable office environment. When both the mine electrician and the factory expert are online, the system supports messaging and voice communication.