**2.3. Experiment description and products characterization**

The starting materials used were 99,98+%-pure WO3 with an average particle size of 10-12 μm (commercially available material which is used in the production of hard alloys), P804-T furnace black less than 45 μm in particle size, and I.PF-1 magnesium powder (99.1+%) ranging from 0.25 to 0.50 mm in particle size.

To mix the components and grind the SHS products, we used ball mills with steel grinding media. Synthesis was carried out in a 30-l SHS reactor under argon atmosphere.

To prepare tungsten carbide, we used the exothermic reaction between tungsten oxide, carbon (black), and magnesium metal:

$$\text{WO} + \text{Mg} + \text{C} + \text{R} \rightarrow \text{WC} \cdot \text{MgO} \cdot \text{Mg} + \text{R'} + \text{Q} \tag{1}$$

where R is a regulating additive.

The temperature of this process exceeds 3000°C; it can cause decomposition of the forming tungsten carbide. To reduce the combustion temperature, we introduced different additives, inert or decomposing in the combustion wave to form gaseous products. The unstable additives also acted as dispersants ensuring a small particle size of the SHS products.

In addition to tungsten carbide and magnesia, formed in the oxidation-reduction reaction, X-ray diffraction revealed some amount of unreacted magnesium in the intermediate product and also intermediate compounds (magnesium carbides) formed in the synthesis (**Figure 1**).

According to the chemical analysis magnesium content in water-soluble compounds (it should be related to forming carbides) is 0.7 – 0.9 mass %, metal magnesium (unreacted) is 15-17 mass %. The study on the semiproduct microstructure has proved, that ultrafine crystallites of tungsten carbide appear to be embedded into the amorphous phase of the melts of magnesia and metal magnesium (**Figure 2**).

Self-Propagating High-Temperature Synthesis of Ultrafine Tungsten Carbide Powders 7

**Figure 1.** X-ray pattern of WC∙MgO∙Mg intermediate product.

6 Tungsten Carbide – Processing and Applications

submicron powders was of great practical interest.

ranging from 0.25 to 0.50 mm in particle size.

melts of magnesia and metal magnesium (**Figure 2**).

carbon (black), and magnesium metal:

where R is a regulating additive.

products.

(**Figure 1**).

**2.3. Experiment description and products characterization**

stage of magnesium reduction.

In [25] describes thoroughly the application of chemical dispersion for separating ultrafine and nanosized powders of boron nitride obtained by various methods under the SHS mode: from elements, with participation of boron and boron oxide, and from boron oxide with the

Possible production of tungsten carbide of ultrafine and nanosized structure by the SHS technology with a reducing stage with using chemical dispersion for separation of

This paper demonstrates the investigation results of the dependence of SHS tungsten carbide powder dispersion on the SHS process parameters and composition of the solutions used for chemical dispersion of the synthesis products and separation of the final product. The aim is producing single phase tungsten carbide with ultrafine and nanosized structure.

The starting materials used were 99,98+%-pure WO3 with an average particle size of 10-12 μm (commercially available material which is used in the production of hard alloys), P804-T furnace black less than 45 μm in particle size, and I.PF-1 magnesium powder (99.1+%)

To mix the components and grind the SHS products, we used ball mills with steel grinding

To prepare tungsten carbide, we used the exothermic reaction between tungsten oxide,

WО3 + Mg+ С + R → WC∙MgO∙Mg + R'+ Q (1)

The temperature of this process exceeds 3000°C; it can cause decomposition of the forming tungsten carbide. To reduce the combustion temperature, we introduced different additives, inert or decomposing in the combustion wave to form gaseous products. The unstable additives also acted as dispersants ensuring a small particle size of the SHS

In addition to tungsten carbide and magnesia, formed in the oxidation-reduction reaction, X-ray diffraction revealed some amount of unreacted magnesium in the intermediate product and also intermediate compounds (magnesium carbides) formed in the synthesis

According to the chemical analysis magnesium content in water-soluble compounds (it should be related to forming carbides) is 0.7 – 0.9 mass %, metal magnesium (unreacted) is 15-17 mass %. The study on the semiproduct microstructure has proved, that ultrafine crystallites of tungsten carbide appear to be embedded into the amorphous phase of the

media. Synthesis was carried out in a 30-l SHS reactor under argon atmosphere.

**Figure 2.** Microstructure of WC∙MgO∙Mg intermediate product.

The process of chemical dispersion in various solutions is necessary for separation of the target products from the cakes forming during SHS and their further purification from admixtures with simultaneous change in the obtained powder dispersion.

The milled cake was treated with water solutions of hydrochloric acid (1:1) or sulfuric acid (1:5) (acid enrichment) for tungsten carbide separation from the semiproduct. Unreacted metal magnesium and magnesium oxide which was formed during the synthesis process were dissolved.

At first the powder was treated by chloride solutions since it is known that water solutions of haloid salts destroy metal magnesium. Magnesium, potassium and ammonium salts were

chosen. It was carried out in order to avoid active gas release when the milled cake was treated with diluted acid solutions (hydrogen release during the interaction of unreacted magnesium with acids) as well as to decrease acid consumption for acid enrichment of the synthesized product.

For decreasing acid consumption, the pulp, consisting of WC∙MgO∙Mg semiproduct and some amount of magnesium chloride as a catalyst, was saturated with carbon dioxide. During this treatment magnesium content in the solid residue was decreased and in the solution it was increased. Metal magnesium is supposed to transform to solution in the following way:

$$\text{Mg} + 2\text{HxO} \rightarrow \text{Mg(OH)} + \text{H}\_2\tag{2}$$

$$\text{H}\bullet \text{O} + \text{CO} \hookrightarrow \text{H}\bullet \text{O} \tag{3}$$

Self-Propagating High-Temperature Synthesis of Ultrafine Tungsten Carbide Powders 9

WC∙MgO∙Mg +KCl+H2O 292 16.0 3.1 12.3 WC∙MgO∙Mg+NH4Cl+H2O 405 14.8 1.4 7.2

Concentration of substance in gas phase, mg/m3 СН<sup>4</sup> С2Н<sup>2</sup> С3Н<sup>4</sup>

Reactive system Gas volume, cm3

**Table 1.** Gas release at WC∙MgO∙Mg treatment with salt solutions

**Figure 3.** WC∙C powder separated from WC∙MgO∙Mg semiproduct by acid enrichment

from acid enrichment were refined with chromium mixture.

dissolved in diluted alkaline solutions (**Figure 4**).

**Table 2**.

Microstructure analyses (**Figure 3**) have shown, that the tungsten carbide powders resulting from acid enrichment represented large accumulations of fine particles of the main product and unreacted (free) carbon. The chromium mixture (10 g K2Cr2O7 in 100 ml H2SO4) oxidizes graphite and amorphous carbon at T ≤ 180°C. Preliminary research showed that the treatment of tungsten carbide powder with chromium mixture solution at T ≤ 180°C allowed removing free carbon without dissolving the main product. The carbide powders resulting

As a result, the content of free carbon decreased from 1.0-5.0 to 0.02-0.2%, while the content of oxygen increased due to oxidation of tungsten carbide particle surface. Tungsten carbide particles appeared to be covered by acicular tungsten oxide crystals, which are easily

The changes in the phase and elemental composition of tungsten carbide powder as a result of chemical dispersion in chromic acid mixture and alkaline solutions are presented in

X-ray diffraction analysis proved that the final products contained only one phase of tungsten carbide. Chemical dispersion in various media caused the primary agglomerates to

disintegrate into finer structures of hexagonal tungsten carbide **(Figure 5).**

$$\text{Mg(OH)} + \text{HCHO} \rightarrow \text{Mg(HCO)} + 2\text{HNO} \tag{4}$$

At first the pulp is prepared. It is suspension of the treated powder in water. Then the required amount of the acid equal to the stoichiometric ratio is introduced. The addition of water to WC∙MgO∙Mg is followed by active gas release and the solution heating though distilled water is not supposed to affect metal magnesium greatly due to Mg(OH)2 film appeared on magnesium particle surface [29].

It is known, that at 500°C, MgC2 can be formed; this carbide is easily disintegrated by water to form acetylene. As the temperature grows from 500 to 600°C, carbon is separated from MgC2 and Mg2C3 appears; this carbide being typical for magnesium only. Methyl acetylene releases during Mg2C3 hydrolysis.

So the following reactions can occur in the water solutions:

$$\text{Mg} \text{C} \natural + \text{4H} \text{xO} \rightarrow \text{2Mg} (\text{OH}) \natural + \text{HC=C-CH} \text{y} \tag{5}$$

$$\text{MgCl} + \text{HO} \rightarrow \text{Mg(OH)} \text{2} + \text{CaH} \tag{6}$$

$$\text{Mg} + \text{HxO} \rightarrow \text{Mg(OH)} + \text{Hz} \tag{7}$$

Infrared spectroscopy was used to analyze the gases released in the reaction of WC∙MgO∙Mg intermediate product with chloride solutions (**Table 1**).

When the intermediate products are treated with potassium chloride and ammonium chloride solutions, a great amount of methane, acetylene, and methyl acetylene is released. It proves the supposition of magnesium carbide formation during SHS. Existence of some amount of methane in the gaseous mixture can be explained by hydrolysis occurring on tungsten carbide particle surface. More gas will be released if ammonium chloride solution is used due to the fact that ammonia is formed during hydrolytic decomposition.

The secondary compounds were removed completely due to the powder treatment with acid solutions


**Table 1.** Gas release at WC∙MgO∙Mg treatment with salt solutions

8 Tungsten Carbide – Processing and Applications

appeared on magnesium particle surface [29].

So the following reactions can occur in the water solutions:

WC∙MgO∙Mg intermediate product with chloride solutions (**Table 1**).

releases during Mg2C3 hydrolysis.

acid solutions

synthesized product.

following way:

chosen. It was carried out in order to avoid active gas release when the milled cake was treated with diluted acid solutions (hydrogen release during the interaction of unreacted magnesium with acids) as well as to decrease acid consumption for acid enrichment of the

For decreasing acid consumption, the pulp, consisting of WC∙MgO∙Mg semiproduct and some amount of magnesium chloride as a catalyst, was saturated with carbon dioxide. During this treatment magnesium content in the solid residue was decreased and in the solution it was increased. Metal magnesium is supposed to transform to solution in the

H2O + CO 2 → H2CO3 (3)

 Mg(OH)2 + H2CO3 → Mg(HCO3)2 + 2H2O (4) At first the pulp is prepared. It is suspension of the treated powder in water. Then the required amount of the acid equal to the stoichiometric ratio is introduced. The addition of water to WC∙MgO∙Mg is followed by active gas release and the solution heating though distilled water is not supposed to affect metal magnesium greatly due to Mg(OH)2 film

It is known, that at 500°C, MgC2 can be formed; this carbide is easily disintegrated by water to form acetylene. As the temperature grows from 500 to 600°C, carbon is separated from MgC2 and Mg2C3 appears; this carbide being typical for magnesium only. Methyl acetylene

Mg2C3 + 4H2O → 2Mg(OH)2+ НС=С–СН3 (5)

MgC2 + H2O → Mg(OH)2 + C2H2 (6)

Infrared spectroscopy was used to analyze the gases released in the reaction of

When the intermediate products are treated with potassium chloride and ammonium chloride solutions, a great amount of methane, acetylene, and methyl acetylene is released. It proves the supposition of magnesium carbide formation during SHS. Existence of some amount of methane in the gaseous mixture can be explained by hydrolysis occurring on tungsten carbide particle surface. More gas will be released if ammonium chloride solution

The secondary compounds were removed completely due to the powder treatment with

is used due to the fact that ammonia is formed during hydrolytic decomposition.

Mg + 2H2O → Mg(OH)2 + H2 (2)

Mg + H2O → Mg(OH)2 + H2 (7)

**Figure 3.** WC∙C powder separated from WC∙MgO∙Mg semiproduct by acid enrichment

Microstructure analyses (**Figure 3**) have shown, that the tungsten carbide powders resulting from acid enrichment represented large accumulations of fine particles of the main product and unreacted (free) carbon. The chromium mixture (10 g K2Cr2O7 in 100 ml H2SO4) oxidizes graphite and amorphous carbon at T ≤ 180°C. Preliminary research showed that the treatment of tungsten carbide powder with chromium mixture solution at T ≤ 180°C allowed removing free carbon without dissolving the main product. The carbide powders resulting from acid enrichment were refined with chromium mixture.

As a result, the content of free carbon decreased from 1.0-5.0 to 0.02-0.2%, while the content of oxygen increased due to oxidation of tungsten carbide particle surface. Tungsten carbide particles appeared to be covered by acicular tungsten oxide crystals, which are easily dissolved in diluted alkaline solutions (**Figure 4**).

The changes in the phase and elemental composition of tungsten carbide powder as a result of chemical dispersion in chromic acid mixture and alkaline solutions are presented in **Table 2**.

X-ray diffraction analysis proved that the final products contained only one phase of tungsten carbide. Chemical dispersion in various media caused the primary agglomerates to disintegrate into finer structures of hexagonal tungsten carbide **(Figure 5).**

Self-Propagating High-Temperature Synthesis of Ultrafine Tungsten Carbide Powders 11

**Figure 5.** X-ray pattern (a) and microstructure (b) of purified tungsten carbide powder.

components in the green mixture:

it excess leads to a fine product (**Figure 6**).

The carbon content influenced the phase composition of the product (W2C content). The single phase product WC is formed in the case of the following ratio of the initial

(b)

(a)

33,6% WO3 + 23,0% Mg + 2,4% C + 40% (WC∙MgO∙Mg).

The content of magnesium in the starting mixture has a substantial effect on the size of carbide particles: the stoichiometric amount of magnesium results in coarse powders, while

**Figure 4.** Microstructure of oxidized tungsten carbide powder


Elemental analysis of WC∙MgO∙Mg semiproduct: Wtotal = 44.1 %; Ctotal = 4.1 %; Oxygen = 9.3 % Mgacid.sol. = 37.7 %; Mgmetal ~ 15.7 %; Mgwater sol. = 0.8 %

**Table 2.** Effect of chemical dispersion on the elemental composition of tungsten carbide powder
