NV100 Acoustic Mill Monitor

• It’s about process optimisation – better crushing rates and yield
• It’s about reduced maintenance costs – longer liner life
• It’s about increased production run time – less blockages in the mill output circuit

The NV100 solution provides both a visual indication of mill noise as well as offering a feedback mechanism to control the feed rate of ore blend into the mill. Thus optimizing the system easily, maximising throughout, stabilising the mill quickly after restarts and extending liner and lifter life.

Overview

The NVMS NV100 is essentially an electronic version of the human ear, but without the perceived variability between operators and the need to go home. The electronics are housed in a sealed IP66 rated stainless steel enclosure and the microphone is protected by a robust hostile environment housing. Power is drawn from a standard 110/240V supply and the measured sound pressure level in decibels is filtered around the mills particular harmonics and converted to a 4-20mA signal which is then supplied to your DCS system.

At the heart of the system is a powerful sound level analyser and the entire solution is developed and manufactured by NVMS. The NV100MT Hostile Microphone contains and protects a high quality Brüel & Kjær microphone, the same transducer used extensively in automotive R&D, aerospace and other uncompromising and highly demanding applications.

Though capable of impressively reducing operating costs, the system is simple, elegant, well tested and easy to understand.

Background

Typically a mill operator has a limited amount of empirical data available for assessing how the mill is behaving, as it is impossible to physically see inside a rapidly rotating mill. Therefore assumptions have to be made in order to efficiently setup a mill for optimal performance. This usually involves making small adjustments to one input factor (perhaps rotation speed or blend of ore entering the mill) and waiting to see if stable and consistent output quality can be achieved.

• Feed rate in tonnes per hour or feed conveyor weight in tonnes
• Mill weight in tonnes
• Mill speed in RMP
• Power being drawn in Amps
• Water wash rate

• Sound Pressure Level dB(A) or dB(f) where f = band pass filtered SPL

Hostile Environments...

As the microphone is typically located in a hostile environment, with mud, spray and other potential contaminants a constant hazard; NVMS have developed the NV100MT Hostile Microphone. This precision stainless steel device protects the delicate microphone diaphragm and pre-amplifier from damage and corrosion whilst providing a well-tuned ear to listen to your SAG or ball mill

How it works

The following chart shows process parameters on a mill undergoing commissioning.

In this example the feed rate of ore into the mill has been reduced in steps (1) in order to purge the mill of ore, leaving predominantly only steel balls. This lightens the mill (2) and due to the charge having little ore to dampen their impacts the mill has become noticeably louder (3). Once the mill is relatively empty the feed rate is increased (4) then ‘tweaked’ until mill stability was restored (5) During the ‘filling’ stage, as the ratio of Balls to Ore was brought back to normal levels, the dampened noise levels can clearly be seen (6).

Using Sound Pressure Level to regulate feed rate

With this solution operators can use the acquired data as a feedback control mechanism to regulate the amount of ore being fed into the mill. This can be very effective; if SPL increases feed rate increases which dampens ball / shell impact which reduces the SPL hence reducing the feed rate. This feedback control mechanism can allow the mill to stabilize with little user interaction. Any hence logically, by adjusting this set-point and with careful monitoring of waste and discharge; improved throughput can be achieved and repeated with ease.

NVMS have supplied over twenty such systems within WA alone.

Delivery and Commissioning

The solution is tailored to your particular installations requirements. NVMS will accompany your system to site and conduct a sequence of tests on your mill(s) which involve a partial grind-out and cycle. A full acoustic analysis of the process is analysed in 3D (below) and the configuration of both the NV100 and your interpretive DCS display is completed.

Why NVMS

NVMS specialise in Acoustics and Vibration and along with SVT-Engineering Consultants (our sister company) we have developed many highly specialized skills in our 25 years’ working with mine operators; helping improve their systems and processes. As technology partners with GE Energy and the Bently Nevada condition monitoring product portfolio we are optimally positioned to assist you.