The Purpose and Intricacies of Grinding Mills

A grinding​ ​mills,​ ​an equipment apparatus​ ​used​ ​to​ ​break​ large, rock hard ​solid​ ​materials​ ​into​ ​smaller, finer and even powdery ​ ​pieces.

Before​ ​we​ ​get​ ​into​ ​the​ ​intricacies​ ​of​ ​Grinding​ ​Mills,​ ​it​ ​is​ ​a​ ​fair​ ​idea​ ​to​ ​first​ ​and​ ​foremost understand​  its​ ​meaning​ ​and​ ​purpose​ .​ ​In​ ​simple​ ​terms,​ ​is​ ​a​ ​type of​ ​a​ ​machine​ ​designed​ ​to​ ​break​ ​solid​ ​materials​ ​into​ ​smaller​ ​pieces.

union ironworks ball mill
Ball Mills grind rough material into quality powder.

The​ ​2​ ​key​ ​traits​ ​which​ ​distinguish​ ​a​ ​particular​ ​Grinding​ ​Mill​ ​from​ ​its​ ​counterpart​ ​are​ ​namely​ ​the type​ ​of​ ​material​ ​they​ ​grind​ ​and​ ​the​ ​process​ ​they​ ​follow​ ​to​ ​grind​ ​them.​ ​Each​ ​type​ ​is​ ​driven​ ​by​ ​the​ ​basic​ ​fundamental​ ​of​ ​maximizing​ ​the​ ​value​ ​from​ ​a​ ​raw​ ​material.​ ​The​ ​end​ ​product for​ ​developing​ ​Mills​ ​are​ ​developed​ ​to​ ​enable​ ​a​ ​crushed​ ​product​ ​to​ ​be​ ​directly​ ​used​ ​for​ ​industrial application.

There are ​several​ ​types​ ​including​ ​​ ​Ball​ ​Mills,​ ​Pellet​ ​Mills,​ ​Hammer​ ​Mills​ ​or​ ​for​ ​that matter​ ​Disk​ ​Mills.

Grinding​ ​Mills​ ​(External)

The purpose of this article is to explain its usage in a mineral processing operation and to enable buyers to understand the best suitable grinding mill as per their specific needs. Please do not hesitate to contact us for your requirement. We will be happy to assist you.

As is quite evident, the ultimate objective of processing a mineral is to extract the maximum value from a raw material. There are two end results that one is trying to achieve – either to get a crushed product that can be directly used for industrial application or to​ ​recover​ ​a​ ​metal​ ​from​ ​a​ ​complex​ ​ore.

The​ ​Top​ ​Brands​ ​for​ ​Grinding​ ​Mills

Marty wants you to go back in time and take a look at the major player Metso whose roots can be traced back to the 1930’s when Hardinge Cascade Mill was built. In the 1970’s and 1980’s, Hardringe (then Koppers) was competing with other market leaders namely Dominion Engineering and Allis Chalmers. In 2001, brands including Allis-Chalmers, Hewitt-Robins, Nordberg, Hardinge and Dominion were all combined under Metso Minerals Svedala range.

Minerals​ ​Processing​ ​Framework​ ​Diagram

Ore​ ​-​ ​Drilling​ ​​ ​​ ​-​ ​Crushing​ ​&​ ​Screening​ ​​ ​-​ ​​ ​​ ​Grinding​ ​-​ ​​ ​​ ​​ ​Flotation​ ​- Thickening​ ​- Dewatering Mineral​ ​Concentrate

Stage​ ​1​ ​–​ ​Drilling
This is the stage where it all begins. A metal cutting process, drilling sets the stage for the next steps in the mineral processing operation. Be it surface or underground deposits, we are mining our​ ​way​ ​to​ ​the​ ​next​ ​stage!

Stage​ ​2​ ​–​ ​Crushing​ ​&​ ​Screening
So now the size reduction of our ore begins! In terms of tonnage, this is the biggest process in the processing cycle. Different types of crushers are used as per the various established parameters of size, shape and strength. Jaw Crushers, Roll Crushers, Gyratory Crushers, Cone Crushers​ ​are​ ​some​ ​of​ ​the​ ​crushers​ ​that​ ​are​ ​used​ ​in​ ​this​ ​process.

But wait! This is not an article on Crushing & Screening (coming soon!) so why are we talking about crushing? Well, crushing and grinding in various combinations are used to maximize the value in size reduction so we wanted to give you a little peek into this stage. But with no further ado,​ ​let’s​ ​move​ ​on​ ​to​ ​the​ ​third​ ​stage​ ​and​ ​the​ ​one​ ​that​ ​brought​ ​you​ ​here​ ​–​ ​Grinding!

Stage​ ​3​ ​–​ ​Grinding
Crushing has a size limitation so for further reduction, we now start the process. Therefore, following the first controlled size reduction stage of crushing, we want to further reduce our ore to obtain a mineral powder. There could be various reasons for reducing the size – we might want to free the individual minerals trapped in ores or simply expose the mineral to a leach solution.

It can be either Dry or Wet. Dry processing (Marty says that you would have guessed!) either does not require water for processing or no water is permitted in the processing of the ore. Wet is generally preferred because of lower power consumption and higher mill capacity. Dry grinding must be used where wet grinding is feared to change the physical​ ​or​ ​chemical​ ​structure​ ​or/and​ ​if​ ​higher​ ​%​ ​of​ ​fines​ ​is​ ​required.

How does grinding work?

What are the forces in play? Imagine (Marty says “wait see the diagrams below!”) a cylindrical steel vessel with an ore and a medium moving inside the mill, the rotating drum lifting this material and finally breaking the ore into a specified product. They can be categorized into three major types – rod mills, ball mills and SAG mills. So,​ ​which​ ​one​ ​is​ ​for​ ​you?​ ​Well,​ ​the​ ​answer​ ​essentially​ ​lies​ ​in​ ​the​ ​feed​ ​size​ ​and​ ​overall​ ​capacity.

Types​ ​of​ ​Tumbling​ ​Mills:

Rod Mills (Heavy Duty Grinding) – these rotating mills use rods for media causing friction and attrition between steel rods and ore particles. Rod mills can accept feed up to approximately 50 mm. (2 in.) and produces the product in the range of 3000 to 270 mm. More
suitable for breaking up large pieces of ore due to the heavy weight of the rods. Examples include large pieces of rock, coal, copper or other media. Product will be a fine aggregate for use in building materials and chemical industries. Also, desirable when grinding coarse grained ores.​ ​Capacity​ ​ranges​ ​from​ ​15​ ​tons​ ​per​ ​day​ ​to​ ​465​ ​tons​ ​per​ ​day.

Ball Mills – these use balls for media breaking the coarse feed materials by impact and attrition into extremely fine powder. Ball mills use a feed of about 15 mm (½ in.) or finer and product size is very fine with no actual grind limit indicated. Capacity ranges from 100 tons per day to 1000 tons per day. Examples include use in mineral dressing processes, paints, pyrotechnics, ceramics and selective laser sintering. Widely used in production lines for powders such as cement, silicates, refractory material, fertilizer, glass ceramics, etc. as well as for ore dressing of both ferrous and non-ferrous metals. Capacity ranges from 100 tons per day to​ ​1000​ ​tons​ ​per​ ​day.

Semi Autogenous Mills/ SAG Mills (Primary/Coarse Grinding) – these mills use balls with forces of attrition, impact and abrasion between grinding balls and ore particles that result in finer particles. Feed size range is 400 mm / 16 in. The product of the large SAG grinding is either a finished size ready of processing, or an immediate size for further grinding. Examples include use in gold, copper and platinum mines with applications also in the lead, zinc, silver, alumina​ ​and​ ​nickel​ ​industries.​ ​Capacity​ ​ranges​ ​from​ ​per​ ​day​ ​to​ ​per​ ​day.

More​ ​questions?​ ​Read​ ​on…
How long do Grinding Mills last? As a matter of fact, well maintained mills will give you dependable and hassle free operation year after year. Capacity or product size remain unaffected by any wear on the grinding surfaces. Very little upkeep is required and inactivity has negligible effect if any. (Marty says we have serviced 40-50 years old mills and received client’s feedback​ ​of​ ​“Excellent​ ​Condition!”​ ​each​ ​time).

Which parts commonly worn out? The wear liners and impellers are the ones to worn out first. At Savona Equipment, all our refurbished grinding mills are inspected and supplied ready for service. Still not sure which is the best fit for you? Contact us and we will be happy to assist you!

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