Jun 3, 2020 Mining blog

How grinding mill design changed over the last 30 years

Moris Fresko
Moris Fresko
Director, Global Engineering, Grinding Products
Grinding mills are some of the most important equipment in mines, processing over a few thousand tons of ore every hour. It is not uncommon to see these days that a single line of a grinding circuit consisting of a Semi Autogenous Grinding (SAG) mill followed typically by two ball mills generating over USD $1 million of revenues per day. As grinding mills operate around the clock, losses of revenue due to mechanical failures are unrecoverable.
Metso grinding mill at customer site.

The design of such grinding mills is extremely critical, requiring sophisticated software tools, proprietary calculation worksheets, and experienced technical engineers and drafters. This blog will review how the mill design process has changed over the last 30 years. While most of the changes have been positive, new issues and problems have risen in the process.

Today, we use computers in almost everything we do

As I started my career in this industry during the late 80’s, only a selected few had access to computers. Drafting (creation of detailed engineering drawings) was done manually on drafting boards; calculations were all manual; reports were typed on a typewriter; presentations were done with slide or transparency projectors; more sophisticated design using Finite Element Analysis (FEA) was carried on external computers via modem connections. While one may view those times as backwards or archaic, most of the know-how that we use today was developed back then. We see grinding mills designed during those times are operating successfully today with no failures. New grinding mills do not look much different than older mills either.

The design process has changed dramatically with computers though. A typical FEA of a grinding mill that would take about 3 hours on a minicomputer in the late 80’s takes about 3 minutes on a laptop computer today. This allows the analyst to add more detail to the models (increased accuracy) and check multiple options to optimize design and cost rather than being limited to 1 or 2 runs per day. Similarly, Computational Fluid Dynamics (CFD) calculations to model hydraulic flows have become practical only in recent years. With more computing power, computer graphics and 3-dimensional modelling have vastly improved. Cost of hardware has come down tremendously as well.

Figure showcasing Finite Element Analysis (FEA) software.
Tools such as Finite Element Analysis (FEA) software with 3-dimensional graphics have become readily available to design mills and view calculation results.

Most engineering calculations are done today using spreadsheets. This allows for fewer mistakes and clearer presentation and display of results. However, use of such proven and ready calculation sheets may discourage the designers to become experts as they can get to the results without necessarily understanding the formulas or technology behind these tools.

Drafting has probably seen the biggest changes over the years. It started with the introduction of Computer Aided Design (CAD) packages on personal computers such as AutoCAD. As time went by, 3-dimensional modelling and drafting become available. This allowed checking for clearances, interferences and fits between different mill components. Finally, CAD packages started offering dimension driven parametric capabilities, where a similar model could be generated within minutes by changing a few dimensions and the geometry would be rebuilt automatically, like using a spreadsheet. These models can then be passed on to FEA packages for structural assessment, without the need to create separate models within each respective software.

Three-dimensional mill model.
Three-dimensional mill models can be used not only in presentations, manuals, and training, but to check for clearances, interferences and fits between different components

The main benefits of these advances are that drawings are legible, easy to modify, easy to submit electronically, easily searchable, take no physical space to store, and so on. Lately we see more and more manufacturing facilities requesting native electronic files (not just electronic prints) so that they can program their cutting tools based on the models (thus speeding up the manufacturing process and reducing errors).

Lastly, in order to bring some order and control to all the electronic files, software Vaults are becoming common. This allows checking in and out drawings, mandatory approvals from drafters/checkers/engineers, automatic creation of bills of materials, purchasing requisitions when drawings are released, purchasing holds and alerts when drawings are revised, and meanwhile discouraging massive information theft among others.

Did grinding mills stop getting bigger?

Every time customers are interested in purchasing a grinding mill bigger than the previous size ever built, it brings excitement to the industry and to the engineering design teams. Extrapolating designs is a lot harder and challenging than using interpolation to make a mid-size equipment. When the first 42 foot SAG mill was designed and manufactured around 2010-11, there were a lot of issues with getting such big equipment pieces fabricated and machined. The costs go up exponentially as we try to push the envelope. That challenge falls on the engineers to optimize mill design using creative ways.

What we see from mining customers in the last 10 years is that there is no longer interest in purchasing bigger machines. This then translates to designing variations of existing equipment which requires less creativity or research.

Figure that shows that no major developments in terms of mill size increase took place in the last 10 years or so
The above figure shows that no major developments in terms of mill size increase took place in the last 10 years or so. Note that ball mills are heavier than SAG mills, therefore, it may be misleading to think that large SAG mill designs lead the way. Reference: Autogenous and Semiautogenous Mills, 2015 Update by F. Tozlu, M. Fresko, 2015 SAG Conference, Vancouver, British Columbia, September 2015; and other OEM installation lists.

Effect of OEM mergers on grinding mill design

A wave of grinding mill OEM mergers took place in the 70’s and 80’s. During those years, only a handful of OEM’s were competing with one another, and no more than two or three could bid on the large size SAG or ball mills. Engineering design offices started spending most of their efforts during those times to merge the best technology from each sister company they had acquired into their line of grinding mills. This started the wave of “standardization of mill components”. Research and technology development took a bit of a back step during these times.

Are grinding mills becoming a commodity?

A little bit, yes. Customers, and even sales and project management groups within mill suppliers want to minimize engineering costs and design delays. This discourages customization and finding creative new designs. During the last 30 years, mill manufacturers have not been able to convince customers or their engineering consultants to purchase completely standard or existing size mills. Instead, the industry is moving from completely customized equipment to nearly standard equipment with different options added on or taken away. Most of the engineering efforts these days are therefore going into creating tools to design grinding mills quickly with low overhead.

Training and IP Protection

As the design tools get more and more complicated, new employees need to be trained not only on the products and the underlying technology, but also on the existing proprietary design tools such as calculation sheets, drawings and models, storage and retrieval procedures. As employees retire, it is very easy to hand over all working files and tools digitally, but often a nightmare to figure out what is what, without proper training.

With all information being digital, Intellectual Property (IP) protection has become a serious concern in the last few years. Almost every company that is developing technology and producing detailed engineering drawings have experienced IT theft and pursued lawsuits against the offenders, mostly their former employees. In order to avoid IP theft, companies have implemented various gate-keeping tools (software Vaults) which in turn slow down the design process due to the added bureaucracy.

How did the workforce change over the last 30 years?

During the years when drafting was done manually, there were roughly 2 drafters for every engineer within the engineering departments. Today, we see this ratio to be about one-to-one. Engineers these days are also using CAD tools to create models and drawings, which they hardly did in the past. 

We also see engineering design shifting to lower cost countries as digital communication and global sharing of information is getting easier. Having technical people all around the world allows multiple designers to work during different time zones to accelerate the lead times of the projects. As manufacturing of components shift to cost competitive countries (CCC), having technical people near local shops also facilitate quality control and manufacturing problems resolution.

The percentage of female drafters and design engineers unfortunately has not improved over the years (there is much better representation in other departments). While mill suppliers encourage and make targeted efforts to improve hiring of female drafters and engineers, very few candidates are interested in these positions. Mining is still perceived as a male dominated industry, and these positions are occasionally required to travel to mine sites.

In brief

With the use of computers, the design of grinding mills has come a long way, mostly positive. As mill sizes get limited by manufacturing capabilities, the focus of the design process has shifted from pushing the envelope to creating tools to do it cheaper and faster. Design workforce is becoming more global. With the increased complexity of the tools used, we heavily rely on subject experts.