Weighing in the cost

Weighbridges play a huge role in controlling production and costs, writes Leon Louw, writer, editor and specialist in African affairs and mining.

Ensuring that every ton of extracted ore arrives safely at its destination has been a headache for as long as bulk mining operations exist. As theft and corruption become more prevalent, and as new ways of defrauding mines are devised every day, mining companies are focused on developing methods and systems to prevent excessive losses. But the fact remains: once the trucks load ore from the stockpiles, there are no solid guarantees that the same tonnages that leave the gate will arrive at the siding or port.

The most effective method is the tried and tested weighbridge; therefore, it is important to ensure that the best weighbridges are installed and maintained to manage and control stock and tonnages. When developing new mines, factors like mining methods, strip ratios, mine design, and all sorts of different ratios demand all the attention, with functions like stockpile, transport, and weight control being relegated to secondary functions that do not need immediate attention. Yet, it should be top of mind, as it is essential for a mine or quarry to know exactly what sort of tonnages enter and exit its gates and to monitor how much ore actually arrives at the destination.

According to independent mining consultant Nicolaas C Steenkamp, weighbridges are an essential part of most mine and quarry operations that rely on road transport of their product to their market. “The primary use of weighbridges is as a control measure — keeping track of the mass of product leaving the mine or quarry versus the mass of material delivered to the market or to the port. In Africa, this is essential, as there are cases of load skimming, substitution, and theft that occur between the mine or quarry gate and the port of market-delivery load-off points,” says Steenkamp.

At the stockpiles, the loading of the trucks can be done by front-end loaders or load-out stations. The benefit of load-out stations is that there is better control over the volume of material being loaded and the shape of the loaded material. A good loading shape results in more accurate weighbridge measurements.

Steenkamp says that weighbridges can also be used as a high-level indicator of the moisture content or quality control of the mined material by a skilled operator. Some products, such as soils, clays, and sand, absorb water, increasing the mass per volume. “It is common practice at most quarries to measure the moisture content of the material at the start of each morning shift. If the wetting is not due to rain, it can be an indication that the groundwater level or a perched groundwater level had been intersected or that dewatering of the excavation needs to be improved. It may also indicate a change in the physical properties of the material being recovered, either weathered material or waste material entering the stockpile if quality control measures fail,” Steenkamp explains.

Operations that mine different ore products from the same deposit usually have dedicated weighbridges for each product being produced. An example is the Postmasburg area, where operations have dedicated separate weighbridges for the manganese and iron lumpy ore being transported by road. Large mines can have up to six or more weighbridges, and there are a number of different weighbridges that can be installed, depending on the specific need and circumstances. It is fair to say that the better the weighbridge, the better control of the ore that leaves the stockyard. 

A weighbridge is a significant investment, and as much information as possible needs to be gathered before a final decision about which weighbridge to install is made. It is important to note, though, that weighbridges can also be hired. The final choice of which weighbridge to invest in depends on several factors, including cost, space, ground conditions, access, need for portability, frequency of use, and purpose (trade or non-trade). Furthermore, the legal requirements need to be considered when choosing a site.

Weighbridges do not always receive the attention they need, so Mining Mirror ventured out into the world of weighbridges and asked a few South African suppliers why weighbridges are important, what are the potential pitfalls, and what are the advantages of a good weighbridge. 

Danie van der Westhuizen, sales director at Masskot Scale

Why should a mine or quarry regard weighbridges as a critical component of its operations?

Measuring product by weight is the only real method to establish input and output and to measure loss control, as theft and corruption are the order of the day.

Is one weighbridge sufficient, and does it matter where it is located?

It will all depend on the size of the mine and logistics. A mine or quarry will need at least one weighbridge before a truck enters the yard, and another one to weigh trucks when they leave the gate. The volume of traffic will determine the number of weighbridges required to prevent unnecessary queuing of vehicles. Most large mines will have at least four weighbridges. Site logistics will obviously determine the positioning, but it is traditionally at the entrance of the mine or quarry.

What advice do you have for a mining company developing a new project in terms of the installation of a weighbridge, considering legal issues, the ideal site, what the best products are, and which type of weighbridge to install?  

They need to consider weighbridges that are manufactured and designed locally, and from companies that are reputable in the industry. Preference should be given to companies with ISO accreditation. Weighbridges used in the mining industry should be robust and reliable, with the necessary backup. Automation is becoming more popular, as it eliminates human error and theft.

It is advisable that weighbridges are installed pitless above ground with enough clearance below to make it easy to clean, which is a problem at most mining operations. The onus to keep the weighbridge clean is on the user, as regulated by the National Regulator for Compulsory Specifications (NRCS). It is very important to make a decision based on quality and not opt for the most cost-effective solution. Many owners make the mistake of purchasing weighbridges on price rather than quality. Many weighbridge companies do not verify weighbridges in accordance with the SANS specifications, which could affect the client’s profitability.

Which weighbridges are best suited for the mining industry?

Mines should use an extra-heavy-duty weighbridge with the least defection under a full load. Weighbridges used in the mining industry are the same for all the different mines and minerals. Weighbridges on coal mines seem to corrode easier, and collieries should consider hot-dip galvanised weighbridges to ensure longevity. Heavy-duty guide rails are recommended for safety.

What does the site preparation for weighbridges entail?

There are various options, which will be determined by the installation. For a permanent position, one would build proper civil foundations. On the other hand, temporary installations only have to be placed on a flat concrete slab. A critical aspect, however, is the sub-base, which has to be designed and built to carry the vertical loads on the load cells.

What is the typical maintenance regime of a weighbridge?

The maintenance regime will also be determined by site conditions, and maintenance can be carried out monthly, three-monthly, biannually, or annually. Personally, I recommend a good service every three month.

What does a mining company need to do to ensure that weighbridges are kept in an optimum condition and to maximise its operational life?

Regular maintenance is a must and keeping the weighbridge and foundation clean goes a long way to prolong the life of the weighbridge.

Which external factors damage the efficiency of weighbridges?

Accumulated dirt, no regular maintenance, damaged cables, poor or bad power supply, and calibrations and verifications not done to the South African National Standards (SANS) specifications.

What are the biggest challenges faced by weighbridge suppliers in Africa?

The biggest challenge is to compete with imports from China and unscrupulous operators cutting corners. All South Africans should support “Made in South Africa” products, thereby also creating jobs.

Danie van Vuuren, mechanical engineer at Loadcell Manufacturing and Services t/a Route Calibration

Why are weighbridges important in the mining industry?

All products sold by weight need to pass over a verified type-approved weighbridge. Weight is directly linked to sales, general business, public safety, and wear and tear on infrastructure — roads especially. In mining, weighbridges enable large-scale weighing with accurate results and minimal interference with the daily production of the mine, as trucks can pass over weighbridges quickly, allowing accurate tracing of product or ore movement within plants or sites. 

How many weighbridges should a mine install on its property and where do these weighbridges need to be constructed?

Depending on the operations and the size of the site, weighbridges can be installed at the main entrance and exit gates of the site, with others requiring more frequent weigh-ins between plants for inter-company transit or product management. 

Some Mining Mirror readers are junior miners currently developing new mines. What advice would you give them when it comes to installing weighbridges?

For companies with an output of more than 500t per month over the weighbridge, a multi-deck weighbridge with individual weighing needs to be installed. When buying a new weighbridge, it is imperative to ensure that the weighbridge you are getting is definitely approved for trade by the NRCS with a SA number, and that all the parts are approved.

 What do site preparation and construction of weighbridges entail?

It entails mostly civil excavation. Weighbridges should preferably be placed on high-lying ground to prevent the collection of water.

There are multiple phases in the construction of a weighbridge. These include the civils for the structure/housing/location of the operator, as well as instrumentation with software, load receptor, loadcells, junction box, and instrumentation. 

How often does a mine need to do maintenance work on its weighbridges?

Depending on the operation and the environment where the weighbridge is located, a quarterly checking schedule is recommended. Proactive maintenance is always better than reactive maintenance. This can be done informally with a known mass, or with a comparison mass that has been weighed on another weighbridge. In this way, any deviations from expected weight can be logged and traced, and problems identified before large changes or issues with the weighbridge occur. Apart from this, calibration and verification of the weighbridge at intervals recommended in their type-approval documents are necessary. This exercise should take place annually or biennially. The information can be accessed by contacting the NRCS to request details on your weighbridge, with the SA number as the reference.

 What should a mining company do to ensure that weighbridges are kept in an optimum condition and to maximise its operational life?

Proactive maintenance is the best course of action. Furthermore, operators need to ensure that the weighbridge is clean and free of waste and damage. It is important that all moveable parts of the bridge are still floating (plates) and that there are no obstructions or exposed cables that could accidentally be pulled out or damaged. 

Name a few factors that could damage or seriously affect the efficiency of weighbridges.

Overloaded trucks using the weighbridge cause severe damage. The continuous use of air brakes on the weighbridge could cause the plates to shift from the loadcells, which results in overloading the loadcells or breaking their mountings. Other factors that limit the life of a weighbridge are inadequate lightning protection and inadequate protection against rust or other corrosive materials or mediums.

What are the biggest challenges faced by weighbridge suppliers in Africa?

The distance to clients and the cost of transporting the traceable reference mass pieces to their sites for testing and maintenance. 

Donovan Pye, director at Richter Scale

How many weighbridges should a mine install to prevent unnecessary bottlenecks?

It depends on volume. Once a mine or quarry needs more than around 75 trucks weighed each day, a second weighbridge should be considered due to the time constraints involved in weighing each truck in and out every day. Avoiding bottlenecks is essential to move product in or out, so if volumes are higher, then the mine would need to consider a dedicated inbound and outbound weighbridge. In extreme cases, the volume might justify multiple incoming and outgoing weighbridges on one property. These are normally always installed at the entrance/exit of the property for control purposes.

What should mining companies consider when installing a weighbridge at a new mine? 

Firstly, the strength and reliability of material and equipment are very important. Many of the imported weighbridges are simply not built to withstand the robust South African mining environment. Many are poorly manufactured, and this will become an issue when the volumes increase and the unit needs to perform. We have replaced many collapsed or bent imported decks.

Secondly, make sure that the company supplying it is SANAS accredited and has the equipment, experience, and NRCS authority to certify and repair the equipment they are offering. Many companies either use third parties or do not have their own weighbridges NRCS type approved and thus mix and match decks, loadcells, and instruments from various suppliers and pass these off as their own products. Many decks have never even been tested by the NRCS type-approval department.

Lastly, if they are based in South Africa and are moving more than 500t per month from their mine, they need to comply with the National Road Traffic Act, which puts the onus on the company loading the trucks on their premises to check the load distribution and total GVM of each outgoing truck. They then need to provide the driver with the evidence (printout) that the truck is loaded correctly. For this they need to install a multi-deck weighbridge. Which means the 22 or 24m-long weighbridge is broken up into four separate platforms to enable the truck axle groups to be checked instantly. Standard weighbridge tickets/slips will also be produced by such a weighbridge.

Do different mines require different weighbridges?

Some mines need weighbridges for checking loads closer to the supply/pit before they pass over the legal-for-trade and SANAS-verified weighbridges. Cleaning can be a big issue, so pit-mount weighbridges are in the minority due to extra maintenance needed.

What does the site preparation for weighbridges entail?

This is soil and condition dependent. Remember, close to 60t (80t for non-South African operations) will be going over the scale every hour of every day, so preparation is critical. Any bad soil or turf needs to be removed, backfilled, and compacted.

 What does the construction of weighbridges entail?

The civil work is crucial to get an accurate weighing at the end of the day. One cannot expect basic slapdash civils to be good enough for a high-volume site. Ramps on and off the weighbridge are also critical. They need to be built well to avoid the hammering they take every day from heavy trucks destroying the ramp in a short period of time. They also need to be wide enough for drivers to comfortably drive on and off and to avoid accidents by trucks driving off the sides of the ramps.

Side barriers are also essential on a weighbridge to prevent accidents caused by trucks driving over the side of it — the barriers are there to prevent this. One recovered truck will more than pay for the additional costs of the barriers. Concrete needs to cure sufficiently before the unit is tested or used. Once civil works are complete, the deck installation takes no longer than one day. Calibration is usually about half a day.

How often does a mine need to do maintenance work on its weighbridges?

Debris and dirt are the enemy. Built-up debris under the scale will cause weighing errors.

Daily visual inspections, minimum weekly cleaning, and monthly maintenance on any bolts or nuts or visible items. We recommend at least a yearly service. Calibration and SANAS verification are biannual requirements by law — many operations do this voluntarily every 12 months.

What does a mining company need to do to ensure that weighbridges are kept in an optimum condition and to maximise its operational life?

Self-maintenance, servicing by an accredited and experienced company, and keeping calibrations and verifications up.

Name a few factors that can damage or seriously affect the efficiency of weighbridges.

Poor design, weak material used on the deck, poor or insufficient civil works, bad installation, and no maintenance.

What are the biggest challenges faced by weighbridge suppliers in Africa?

Many of the imported weighbridges have never been tested by the NRCS. A lot of them bend or collapse once used in an industrial environment. Non-accredited companies sell these products to clients who do not know any better.

It is important as a mining company to do your homework before investing in a weighbridge. Management and procurement need to go and see the company; ask to see some of their installations — don’t just look at the brochure. Or they can contact the NRCS if they need more information. It is an expensive exercise to replace a weighbridge after the fact.

By Leon Louw, writer, editor and specialist in African affairs and mining.