Adding a font awesome icon to a css class

I am not 100% understand your need but i was try to make something make close of your need.

HTML:

<span class="icon fa-id-card-o"></span>

CSS:

 .icon {
        display: inline-block;
        font: normal normal normal 14px/1 FontAwesome;
        font-size: inherit;
        text-rendering: auto;
        -webkit-font-smoothing: antialiased;
        -moz-osx-font-smoothing: grayscale;
        position: absolute;
        right: 0;
        color: red;
    }
    .icon:before {
        content: "\f2c3";
    }

.icon {
    display: inline-block;
    font: normal normal normal 14px/1 FontAwesome;
    font-size: inherit;
    text-rendering: auto;
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
    position: absolute;
    right: 0;
    color: red;
}
.icon:before {
    content: "\f2c3";
}

<link href="https://maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css" rel="stylesheet"/>
<span class="icon fa-id-card-o"></span>

ANOTHER EXAMPLE with a fixed div like a widget's

LIve view

HTML:

<div class="box">
  <span class="icon fa-id-card-o"></span>
</div>

CSS:

.icon {
    display: inline-block;
    font: normal normal normal 14px/1 FontAwesome;
    font-size: inherit;
    text-rendering: auto;
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
    color: red;
    float: right;
}
.icon:before {
    content: "\f2c3";
}
.element.style {
}
.box {
    background: green;
    margin: 100px;
    display: block;
    width: 70%;
    height: 300px;
    }

.icon {
    display: inline-block;
    font: normal normal normal 14px/1 FontAwesome;
    font-size: inherit;
    text-rendering: auto;
    -webkit-font-smoothing: antialiased;
    -moz-osx-font-smoothing: grayscale;
    color: red;
    float: right;
}
.icon:before {
    content: "\f2c3";
}
.element.style {
}
.box {
    background: green;
    margin: 100px;
    display: block;
    width: 70%;
    height: 300px;
    }

<link href="https://maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css" rel="stylesheet"/>
<div class="box">
  <span class="icon fa-id-card-o"></span>
</div>

Well, i'am understand something like this...

HTML

<div class="box">
  <div class="box-icon"></div>
</div>

CSS

.box {
   box-shadow: 0px 0px 10px #000;
   height: 5em;
   width: 5em;
}
.box-icon {
   position: relative;
}
.box-icon:before {
  content: "\f2da";
  display: inline-block;
  font-family: FontAwesome;
  position: absolute;
  right: -1.5em;
}

I hope can i a help you.