Ferrosilicon Grains: Deoxidation, Inoculation and Size Selection

What "Grains" Means in the Ferrosilicon Trade

In the ferroalloy trade, ferrosilicon is sold in three physical forms that are not interchangeable: lump, grain and powder. Lump is the coarse product taken straight from the casting bed, normally 10-100 mm, and it is what a steel plant charges into a ladle or a converter. Grain is the same alloy crushed and screened to a narrow band, most commonly 0.2-0.8 mm, 0.5-1.5 mm, 1-3 mm or 3-8 mm. Powder is ground below roughly 200 mesh and is used in welding consumables, in powder-metallurgy blends and in briquetting.

The distinction matters because the two dominant uses of ferrosilicon pull in opposite directions on particle size. For deoxidation in steelmaking, coarse material is preferred: a large particle dissolves slowly and sinks through the slag into the metal, where the silicon is available to react with dissolved oxygen over the whole tap-to-teeming interval. For inoculation of cast iron, fine grains are essential: the inoculant has to dissolve completely within seconds in a small stream of metal, and any undissolved particle becomes a hard spot or a slag inclusion in the casting.

Grain is therefore not a by-product of crushing. It is a graded product with its own size specification, its own chemistry bias and its own acceptance rules, and the buyer who treats it as "small ferrosilicon" usually ends up with either poor recovery or dirty castings.

Standard Grades and Chemistry Behind the Granules

Chinese ferrosilicon is graded by silicon content, and the grades that appear on most grain contracts are FeSi75, FeSi72 and FeSi65 under GB/T 2272. FeSi75, the workhorse grade, is specified at 74.0-80.0% Si. FeSi72 sits at 72.0-80.0% Si in some tables and at 72% nominal in others, and FeSi65 at 65.0-72.0% Si. Alongside silicon, the standard fixes limits on aluminium, calcium, manganese, chromium, phosphorus, sulphur and carbon, and the aluminium limit is the one that varies most between sub-grades because it is the element that most affects cleanliness in steel and nodularity in iron.

A typical FeSi75 grain certificate shows silicon at 74-78%, aluminium at 0.5% or below for a low-aluminium sub-grade and up to 1.5% for a general-purpose one, calcium at 1.0% maximum, manganese at 0.4% maximum, chromium at 0.3% maximum, phosphorus at 0.035% maximum, sulphur at 0.020% maximum and carbon at 0.10% maximum. Those figures are the practical ceiling for a 75% grade produced in a submerged-arc furnace from quartz, coke and steel scrap; a certificate claiming 0.1% aluminium on a standard FeSi75 should prompt a question about how the heat was made.

Two further points belong in a grain contract. First, fines content: a 0.2-0.8 mm grain that arrives with 15% below 0.1 mm behaves like powder, dusts out of the ladle and gives erratic recovery, so a minus-0.1 mm limit is normally written in. Second, moisture: ferrosilicon grains pick up surface moisture readily because of their high specific surface, and moisture carried into a ladle is a hydrogen source. A moisture limit of 0.5% is common, and 0.2% is achievable with dried, bagged material.

Grade	Si content	Al (typical)	C max	P max	S max
FeSi75 low-Al	74.0-80.0%	≤0.5%	0.10%	0.035%	0.020%
FeSi75 general	74.0-80.0%	≤1.5%	0.10%	0.035%	0.020%
FeSi72	72.0-80.0%	≤1.5%	0.20%	0.040%	0.020%
FeSi65	65.0-72.0%	≤2.0%	0.20%	0.040%	0.020%

Size Fractions, Screening and How They Are Measured

Grain sizes are quoted as a minimum-maximum pair, and the pair is meaningful only if the test method is named. Particle size of ferroalloys is determined by sieve analysis to GB/T 13247, and sampling and sample preparation follow GB/T 4010, with ISO 3713 providing the international equivalent for sampling precision. A contract that states "0.2-0.8 mm, 90% within band" without saying which sieve series and which sampling method apply leaves room for a dispute that is expensive on both sides.

The practical fractions and their uses are fairly settled. Material below 0.2 mm is powder, used in welding electrode coatings and in briquetting. The 0.2-0.8 mm band is the standard in-stream inoculant for ductile iron because it dissolves in the pouring stream before the metal reaches the mould. The 0.5-1.5 mm band suits ladle inoculation and small castings. The 1-3 mm band is used for ladle addition in medium-section iron castings and for tundish application. The 3-8 mm band is a general-purpose grain for ladle deoxidation in small steel heats and for addition to induction furnaces. Anything coarser than 10 mm is classed as lump rather than grain.

Screening quality shows up in the shape of the particles as much as in the size distribution. Crushed ferrosilicon has angular, sharp-edged grains that flow badly and bridge in hoppers; a grain produced with a controlled crushing and screening circuit has a more equiaxed shape and feeds steadily through a dispenser. For a foundry using automatic inoculation equipment, flow behaviour is as important as chemistry, and a trial batch through the actual dispenser is worth more than a sieve report.

Deoxidation of Molten Steel

Silicon is the second most widely used deoxidiser after aluminium, and its behaviour is fundamentally different. Aluminium reacts almost completely and produces solid alumina clusters that must be floated out; silicon reacts with dissolved oxygen to form silica, which is liquid at steelmaking temperatures and coalesces into a slag that is easier to remove but which also re-oxidises the steel if the slag is not controlled. Because silicon is a weaker deoxidiser, it is normally not used alone for fully killed steel: the common sequence is aluminium for final deoxidation followed by ferrosilicon for silicon alloying, or a silicon-manganese addition in which the two elements deoxidise cooperatively and produce a liquid manganese silicate inclusion that is globular and harmless.

Dosage depends on the aim silicon content rather than on a fixed rule. Silicon-killed structural and pressure-vessel steels are commonly specified at 0.15-0.35% Si, and with a recovery of 85-95% in the ladle that corresponds to roughly 2-4 kg of FeSi75 per tonne of steel. Rimmed and capped steels use far less, and electrical steels use much more, as described below. Recovery falls when the addition is made too early, when the slag is highly oxidising, when the grain is too fine to penetrate the slag layer, and when the addition is made without argon stirring.

The grain form has one specific advantage in this application: a screened 3-8 mm or 1-3 mm grain can be injected through a cored wire or fed by a vibratory feeder at a controlled rate, which makes the addition reproducible heat after heat. Lump charged by shovel into an open ladle gives a recovery that varies by ten points between operators, and that variation is what makes a steelmaker miss the silicon aim and rework the heat.

Inoculation of Ductile and Grey Iron

Inoculation is the largest single foundry use of ferrosilicon grain. The mechanism is not simply a silicon addition. Ferrosilicon particles dissolving in the melt provide a large number of local silicon-rich zones and, more importantly, they carry oxide and silicate nuclei that act as substrates for graphite nucleation. The result is more graphite nodules, a finer and more uniform nodule count, less chill and less carbide in thin sections, and a lower tendency to form exploded or irregular graphite.

For ductile iron the addition rate for a plain FeSi75 inoculant is typically 0.3-0.8% of the poured weight, with 0.5% a common starting point for medium sections and higher rates for thin-wall work. The grain size must match the pouring weight: 0.2-0.8 mm for in-stream inoculation of castings under about 20 kg, 0.5-1.5 mm for medium castings, and 1-3 mm for heavy sections where a finer grain would dissolve before the metal reaches the mould cavity. Inoculation fades with time, so the addition should be made as late as the process allows; a ladle addition that sits for five minutes before pouring loses most of its effect, which is why in-stream inoculation displaces ladle inoculation in thin-wall production.

For grey iron the same grain is used to control chill depth and to improve machinability, at rates of 0.2-0.5%. Here the risk is overtreatment: excess silicon raises the silicon content of the iron, promotes ferrite, lowers hardness and can cause graphite flotation in heavy sections. The foundry's aim chemistry, not the inoculant supplier's recommendation, should set the rate.

Plain FeSi75 is a modest inoculant. Calcium-bearing, zirconium-bearing and rare-earth-bearing ferrosilicon grades are used where the base iron has a high sulphur content, a long holding time or a strong chilling tendency, because those elements extend the time during which the nuclei remain active. A buyer should treat these as distinct products with their own certificates rather than as variations on the same grain.

Fraction	Ductile iron use	Grey iron use	Steel use
<0.2 mm	Not used, dusting risk	Not used, dusting risk	Welding consumables, briquetting
0.2-0.8 mm	In-stream inoculation, light castings	Late inoculation, thin sections	Rare, cored wire blends
0.5-1.5 mm	Ladle inoculation, medium castings	Ladle inoculation	Cored wire filling
1-3 mm	Heavy sections, tundish addition	Heavy sections	Tundish and mould addition
3-8 mm	Furnace addition	Furnace addition	Ladle deoxidation

Silicon Addition for Electrical Steel and Alloying

Silicon is the alloying element that makes electrical steel electrical steel. Dissolved silicon raises the electrical resistivity of iron substantially, and higher resistivity suppresses eddy-current flow, which is one of the two loss mechanisms that heat a laminated core under alternating flux. Grain-oriented and non-oriented electrical steels therefore carry 0.5% to 3.5% Si, with the higher silicon levels used where low core loss matters more than saturation flux density. The silicon is added as ferrosilicon during steelmaking, and the low-aluminium sub-grades are preferred because aluminium in electrical steel pins grain growth and degrades the magnetic texture.

Outside electrical steel, ferrosilicon grain is the standard vehicle for silicon in spring steels, in silicon-manganese spring grades, in some stainless and heat-resisting grades, and in cast alloys where silicon improves fluidity and oxidation resistance. It is also used in the production of magnesium by the silicothermic route, where ferrosilicon acts as the reducing agent, and in the manufacture of silicon tetrachloride and other silicon chemicals. In those chemical uses, the aluminium and calcium content of the grain affects the purity of the downstream product, so the certificate has to be read against a specification rather than against a grade name.

One point is often missed by buyers comparing on silicon content alone: a 78% Si grain is not simply 4% better than a 74% grain. The higher silicon heat usually carries a lower aluminium and calcium level as well, and that combination is what a clean-steel or an electrical-steel customer is actually paying for. Comparing on price per kilogram of contained silicon is a useful discipline, but it must be done on the same grade and the same aluminium sub-grade.

Storage, Handling and Acceptance

Ferrosilicon grains are sold in 25 kg bags, in one-tonne big bags and in bulk, and the packaging choice drives the storage rules. The material must stay dry. Ferrosilicon in contact with water releases hydrogen, and damp ferrosilicon in a closed container or a confined space has caused explosions; the risk rises with fines content, because fines have the largest surface area. Storage should be under cover on a drained concrete floor, away from acids and alkalis, and a silo or a container that has held damp material should be ventilated and checked for gas before anyone enters. Bagged grain should be used within about six months, with the oldest stock issued first.

Acceptance testing for a grain lot follows the sampling standards rather than the eye. A composite sample taken to GB/T 4010 from at least ten positions in the lot is reduced and analysed for silicon, aluminium, calcium, carbon, phosphorus and sulphur; a sieve analysis to GB/T 13247 confirms the size band and the fines content; and a moisture determination on a sealed sample confirms the drying step. Weight verification matters as much as chemistry on a grain contract, because the difference between a 25 kg bag and a 24.5 kg bag is 2% of the invoice value and is invisible without a check weighing.

The delivery requirements of GB/T 3650 cover the packing, marking and lot documentation that make this verification possible, and a buyer should insist that each bag or big bag carries the heat number so that a failed sample can be traced to a single heat rather than to a whole shipment. Where the grain is destined for a foundry inoculant dispenser, a small trial through the actual equipment, checked against the sieve report, is the fastest way to confirm that the lot will feed as expected.

FAQ

Q: What is the difference between ferrosilicon grains and ferrosilicon lumps?
Lump is the coarse 10-100 mm product charged into a furnace or a ladle for bulk silicon addition. Grain is the same alloy crushed and screened to a narrow band such as 0.2-0.8 mm or 1-3 mm, and it is used where dissolution speed and reproducible feeding matter, mainly inoculation and cored-wire injection.

Q: What silicon content should inoculant-grade grains carry?
FeSi75 at 74.0-80.0% Si under GB/T 2272 is the normal base, with aluminium at 0.5% or below for demanding work, calcium at 1.0% maximum, and carbon at 0.10% maximum. Calcium-bearing and zirconium-bearing grades are separate products used where the base iron chills strongly or holds for a long time.

Q: Which size fraction suits ductile iron inoculation?
0.2-0.8 mm for in-stream inoculation of castings below about 20 kg, 0.5-1.5 mm for medium castings, and 1-3 mm for heavy sections. The fraction must be fine enough to dissolve in the pouring stream and coarse enough not to dust out before it reaches the metal.

Q: How much ferrosilicon is needed for steel deoxidation?
For a silicon-killed steel aimed at 0.15-0.35% Si, roughly 2-4 kg of FeSi75 per tonne of steel at 85-95% ladle recovery. The rate follows the aim chemistry and the oxidising state of the slag, not a fixed recipe.

Q: Does grain size affect silicon recovery?
Yes. A 3-8 mm grain penetrates the slag layer and dissolves over the tap-to-teeming interval, giving a steady recovery, while a very fine grain oxidises at the slag surface or dusts out. Injection through cored wire with a screened 0.5-1.5 mm grain gives the most reproducible recovery.

Q: How should ferrosilicon grains be stored?
Dry, under cover, on a drained floor, away from acids and alkalis, with stock rotated within about six months. Damp ferrosilicon releases hydrogen, and fines in a closed silo or container present the greatest risk, so ventilation and gas checks are required before entry.